THE GENETICS AND PHYSIOLOGY OP FOLIAR DIEBACK IN THE GREENHOUSE SNAPDRAGON, ANTIRRHINUM MAJUS L. By CHARLES E S' ACKERMAN A THESIS Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science In partial fulfillment of the requirements for the degree of DOCTOR OP PHILOSOPHY Department of Horticulture 1952 THE GENETICS AND PHYSIOLOGY OF FOLIAR DIEBACK IN THE GREENHOUSE SNAPDRAGON, ANTIRRHINUM MAJUS L. BY . (-..V i\ '< 1' CHARLES E f ACKERMAN An Abstract of A THESIS Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Horticulture 1952 THE GENETICS AND PHYSIOLOGY OP POLIAR LIEBACK IN THE GBEBNHQUSE SNAPDBAGON, ANTIBBEINDM MAJUS L. By e Charles E. Ackerman ABSTRACT Several light intensities, light qualities, photoperiods, air temperatures, and soil temperatures in various combinations were es­ tablished to determine the influence of these environmental factors upon the Incidence of foliar dieback in the greenhouse snapdragon. Antirrhinum madus L« this study, Several varieties of snapdragon were used in Poliar dieback consistently occurred on the entire sample population of an isolated strain of variety Lady Dorothy when exposed to an air temperature of hO° P, or lower for a period of eight hours. The Pg progenies of several crosses of which the isolated strain of Lady Dorothy was a parent showed dieback symptoms in the ratio of three normal to one dieback when exposed to a h0° P, air temperature for eight hours, Dieback occurred on the susceptible lines only when they were exposed to an air temperature of **0° P, or lower. The nature of the response of the susceptible genotypes indicates the expression of a single recessive gene under the influence of a low air temperature. TABLE OF CONTENTS Page Acknowledgments Introduction . • » Literature Be view • 1 ......................... Ur Procedures and Results . . . . . Discus8ion . . . . . . . . . ............. ............... • 9 24 SuHnaary . . . .. ............................ .. 33 Literature Cited 35 ....................... .. ACKNOWLEDGMENTS The author wishes to express his appreciation to Dr* W. J* Haney who suggested the problem and under whose supervision this study was pursued* The advice and kindly criticism given by the members of the Horticulture Department* Michigan State College* during the course of the investigation »r>d in preparation of the manuscript are sincerely appreciated by the author* THE GENETICS AND PHYSIOLOGY OF FOLIAR DIEBACK IN THE GREENHOUSE SNAPDRAGON, ANTIRRHINUM MAJUS L, Charles E. Ackerman INTRODUCTION Foliar dieback of the greenhouse snapdragon, Antirrhinum majus L « , occurs mainly on those leaves that have Just reached maturity, usually in the region of the middle third of the plant. However, its symptoms have Been observed in various stages of plant development by commercial growers. In the initial stage of development, the disturbance of normal leaf growth usually appears at the leaf apex as an acute isosceles tri­ angle of necrotic tissue with the apex of the triangle pointing toward the base of the leaf blade. The triangular area at the apical portion has a collapsed, water-soaked appearance which gradually dries to a light brown color. mittent manner. appearance. The injury advances basipetally, usually in an inter­ Each advance is characterized by a distinct banded Finally, under conditions favorable for the progressive development of foliar dieback, the stem is affected through the petiole of the affected leaf blade. Affected flowering stems, when in the bud stage, wilt and fall over. Complete recovery from the injury seldom occurs, and the flowering spike is rendered unmarketable. This leaf injury is prevalent only during occasional years from September to April, Short intermittent growth periods of favorable and unfavorable light conditions which tend to cause rapid and slow growth rateb respectively are invariably associated with the occurrence of foliar dieback. Varietal differences in susceptibility to foliar dieback are notice­ able. Susceptibility seems to indicate that the expression of the mal­ function is influenced by a, heritable factor (30). Haney (25) has isolated a strain of Lady Dorothy variety that shows leaf necrosis through­ out the entire population under certain cultural, conditions. The injury seems to be characteristic of a physiological malfunction similar in appearance to lettuce tip-burn (2), cucumber leaf scorch (52)• lily tip-burn (5^)» olive leaf dieback (53)» and mesophy11 collapse of citrus leaves (58)• In years when environmental conditions favor the expression of foliar dieback, commercial growers of greenhouse snapdragons may suffer great economic loss due to a partial or complete loss of the snapdragon crop. Occasionally, since 1930, several commercial growers have suffered complete loss of those snapdragon varieties showing susceptibility to foliar dieback. Past efforts of seedsmen to eliminate this undesirable trait have been ineffective. A technique serving to identify the origin of this trait could have utility in the seedsmen's program. The purpose of this investigation was to produce a controlled environ­ ment which would permit the development of foliar dieback in the greenhouse snapdragon, and to determine the genetic aspect of varietal differences in susceptibility. The knowledge of the interaction of the genotype with the environment may toe used as the toas is for the establishment of a breed­ ing technique that may toe used toy seed producers as a means of eliminating the undesirable trait of foliar dieback from snapdragon varieties* Literature Review Wilson (67)* in 19*43* described foliar dieback of snapdragon and suggested that high concentration of nitrate nitrogen accumulated by the plant during short photoperiods of low light intensity was the cause of foliar dieback. However, Howland (30) showed no correlation between high nitrate nitrogen and development of foliar dieback, and Wilson (66) found that high concentrations of nitrate nitrogen applied to the root system of the snapdragon caused a leaf tip burn and chlorosis of the young terminal leaves, but the leaf injury was not progressive. Spectro- graphic analysis of leaf tissue, reported by Howland (30) showed that injured leaves contained more in some cases, less in others, of lead, aluminum, barium, iron, and boron than uninjured tissue. He reported that none of the controlled environmental factors of nutrition, soil reaction, soil type, soil moisture, photoperiod, light intensity and air temperature used in his study influenced expression of dieback. A pathogen has never been associated with the expression of dieback, and the symptoms have never been experimentally produced, according to the literature. Several other higher plants have shown symptoms of leaf necrosis similar to foliar dieback. Anderson (2) obtained significant correlation between severity of tip-burn in lettuce and, differential ranges between air temperature and soil temperature. Schroeder (52) showed the importance of root temperature in greenhouse cucumber production. Kramer (37, 38) ani Baleigh (48) hare described the relationship between low water absorp­ tion by plants and low temperature of the growing media* Jones (35) has reported the effect of bright light* high temperature, and low relative humidity following a period of low light intensity* high temperature and high relative humidity upon the incidence of onion blast* Scott* et al« (53) were able to control leaf tip necrosis of the olive by treatment with borax* The role of light in the developmental pattern of the higher plants has been reviewed by Burkholder (9)* The effects of light intensity on plant growth were related primarily to the role of light in photosynthesis (l* 9 9 32* 46* 48 * 55)• Hibbard and Grigsby (28) found that as light in­ tensity decreased, the quantity of carbohydrate decreased, and Hibbard (2?) reported that a short photoperiod slows up the total salt absorption b y the root, and that more calcium, iron and potassium was absorbed by the plant under long photoperiod than under short photoperiod* Warington (62) found that plants grown on a boron^-deficient medium required a longer time to develop borom-deficiency symptoms under short photoperiod than similar plants grown under long photoperiod* Mac Vicar and Struckmeyer (43) and others (57) reported similar results on several plants* Alberda (l) stated that light was a primary factor in phosphorus absorption* and the rate of phosphorus absorption paralleled the growth rate of the entire plant* Pfeiffer (45) and others (9, 32, 33, 36* 46, 49, 55* 63) have reported the effects of light quality and photoperiod upon rate of photo­ synthesis, gross morphology of plant, and carbohydrate accumulation* Therm oper iodic response of plants was reviewed by Vent (65)« He showed that the expression of growth and development of some plants is better under conditions of fluctuating day and night temperature than under conditions of constant day and night temperature, and that the temperature optimum for plant growth shifts downward with increasing age* Another important aspect of the thermoperiodic response may be the favorable balance between the photosynthetic production of plant material during the day and the respiratory loss of plant material at night* Jones (35) found that air temperature had little effect upon gardenia chlorosis caused by low soil temperature* Ball (h) recommended a minimum temperature of 50° F* for the early plantings and h3°-*V5° greenhouse snapdragons* for the late planting of Shoot temperature apparently regulated rate of development, while root temperature influenced general growth pattern of plant tissue (2^4-). Temperatureb above maximum or below minimum require­ ments retarded growth or injured the root system, and better aeration of soil and solution— cultures was required at high temperatures than at low temperatures (38). Little knowledge is available concerning the extent to which the decrease in nutrient absorption related to low soil temper­ atures retarded plant growth; since low soil temperature retards growth in a number of ways, it was difficult to distinguish them from each other* (38). The hydrogexv-ion concentration or pH of the growing medium noticeably affects the manner of growth and type of development a plant will assume. While the pH of the medium has little effect on the process of mineral absorption, it greatly affects the degree of availability, chiefly through an influence upon solubility, of the various elements. Plants have been well grown in media ranging from pH iJ-.O to pH 9*0 providing the essential elements remained available (3)* Collander (11) has shown that single plant species were relatively rich in certain cations, and that other plant species were rich in other cations. He found that, in the majority of plants studied, the major essential elements (potassium, calcium, and magnesium) were abundantly accumulated in the order listed. The nutritional status of economic plants has recently aroused con­ siderable interest in the producers of such plants. The known relation­ ship between excess or shortage of one or several nutrient elements and normal plant growth has been described (31)* sud the physiological roles of some nutrient elements have been reviewed by Wadleigh. (61). The work of Leopold and Thimann (39) indicates the close relation­ ship of nutrient elements upon the formative action of growth-sub stance s. Formative action has been defined by Zimmerman (69) as Ma deviation from the normal pattern resulting from the genetic constitution of the plant under the Influence of a particular environmentN. This concept of the interaction of the gene with its environment is similar to that of Dobzhansky and Holz (1*0, but less precisely defined. They state that "Genes produce not characters, but physiological states which, through interactions with the physiological states produced by all other geneB of the organism and with the environmental influences, cause the develop­ ment to assume a definite course and the individual to display certain characters at a given stage of the developmental process"* Since 1935 when the role of auxins, the plant growth hormones, were described (6*0, considerable work has been directed toward the discovery of other growth substances, their centers of origin, and the effect of environmental factors upon their expression (26, bo, *4-1, hh, 8)* Bonner et al. (?) found that growth of the leaf mesophyll was controlled by a substance such as aienine, a leaf growth factor. Dead roots of certain plants have been found to have an inhibitory effect upon growth of other plants of the same species (**7, 5). while some plants were able to secrete toxic substances from their root systems which inhibit the growth o f plants of different species (22) or plants of the same species (6). Gray and Bonner (23) reported that the leaves of Encelia contained a water soluble sul>stance which showed the properties of a growth inhibitor upon other plant species. The influence of temperature, light, water, nutrition, and other plants, either alone or in combination, have been shown to play a most important role in the environmental complex which the plant inhabits. Each species of plant apparently has a characteristic range of tolerances and intolerances. To obtain optimum growth and development, two main avenues of endeavor may be pursued: the proper range of tolerance of the one or several environmental factors may be made available to the par­ ticular plant, or the genetic pattern of the particular plant may be modified to fit the available environment. PROCEDURES AMD RESULTS The varieties of snapdragons used in this investigation included primarily those on which symptoms of foliar dieback was known to occur frequently* They were Rooster*s White, Sykora*s White Wonder, Sykora* s White Cheviot, Peace, Lady Dorothy, Margaret, a variety that had never heen reported as expressing the symptoms, and Lady Dorothy db, a selected strain of Lady Dorothy not available in the commercial trade (32) which has frequently shown the symptoms of dieback when grown under accepted cultural practices* Also, the F^ end generations arising from crosses involving the variety strain Lady Dorothy db as both the male and female parent were used* All seedlings were from seed propagated in a seedbed of one-third peat, one-third Band, and one— third loam under accepted cultural practices* EXPERIMENT 1 , One hundred seedlings each of eight varieties (Table I), approximately three inches tall, were grown in 2^ inch clay pots containing a soil mixture of one-third peat, one-third sand, and one— third loam* Each variety was given a short-day of eight hours and a long—day of thirteen hours at three different air temperatures for a period of thirty days under prevailing light conditions* Three replicates were used during each of five different months, October, November, January, February, and March of 1951 and 1952* After a thirty-day period of growth, the plants grown under the *K>° F* temperature and under the 50° F* temperature were changed to 50° F* temper­ ature and h0° F* temperature, respectively, but remained under the former TABLE I COMPARATIVE RESPONSE OP VARIETIES TO PHOTOPERIOD AND AIR TEMPERATURE Dieback Symptoms in Percentage Photoperiod 13-hour 8-hour Degrees P. KW* SWW SWC P LD LDdb LDxLDdb(P?) PxLDdb(P1) uo 0 0 0 0 0 100 2h 0 50 0 0 0 0 0 0 0 0 60 0 0 0 0 0 0 0 0 ko 0 0 0 0 0 100 25 0 50 0 0 0 0 0 0 0 0 60 0 0 0 0 0 0 0 0 ID - Lady Dorothy SWW - Sykora1s White Wonder LDdb - Susceptible Lady Dorothy SWC - Sykora'8 White Cheviot LDxLDdb(P2 ) - Lady Dorothy i Lady Dorothy db page <»£> P - Peace To accoarpaay *IW - Koester’s White PxLDdb(P^) - Peace x Lady Dorothy db (P^) 9 photoperiods* BESULTS. The data presented in Table I are the results of the one replicate run during October* The results of each replicate in each month in each year were very nearly identical with each other* The symptoms of foliar dieback occurred only on those leaves that had not yet reached maturity. Figure I shows the manner of dieback development expressed by the susceptible strain Lady Dorothy db at 4-0° F, and Figure II shows normal development of Lady Dorothy db at 50° F* The initial symptoms of foliar dieback appeared as a sunken water-soaked area which occurred primarily at the leaf tip, but occurred frequently at var­ ious parts of the leaf edge and at various sections of the leaf blade from the apex to the middle section* The water— soaked area occurred on the third day following a four or eight hour exposure to a UO° F. air teznper&ture. This water-soaked area gradually dried to a light brown necrotic area. On the sixth day following the low temperature exposure, the necrotic area was distinct* When the affected plants were placed in a 50° F. minimum temper­ ature further advance of dieback was arrested, but when the affected plants were returned to a 40 o F. temperature, new advances of dieback occurred and were obviously separated from the previously developed dieback areas by distinct limits of necrotic tissue* ijjke ratios of dieback obtained in Lady Dorothy db and the Fg population of Lady Dorothy X Lady Dorothy db were precise. The Lady Dorothy db pop­ ulation showed 100 percent susceptibility when exposed to 40° F* The Lady Dorothy population did not show dieback at this temperature, while the Figure 1« Dieback on terminal leaves of Lady Dorothy db • o following exposure to a 40 F* air temperature* To accompany page 10 Figure II. Normal development of Lady Dorothy dh grown at a 50° y. minimum air temperature. To accompany page 10 progeny Lady Dorothy X Lady Dorothy dh showed a segregation ratio of three to one. None of the other varieties showed diehack under the conditions of this study. When all the plants which showed diehack in the h0° P. temperature at "both short and long photoperiod were moved into a 50° P. temperature under short and long photoperiod, development of diehack was arrested and new leaf growth developed normally. However, of all the plants changed from a 50° P. temperature to a 40° I1, temperature at the former photoperiods, diehack occurred on the top third region of all Lady Dorothy dh plants and in 25 percent of the Pg population of Lady Dorothy X Lady Dorothy dh. Occurrence of diehack in percentage of population is shown in Tahle I. EXPERIMENT 2. In a study of the effect of light intensity upon foliar development, one hundred seedlings each of variety Rooster's White, Sykora*s White Wonder, Sykora*s White Cheviot, Peace, Lady Dorothy, »ntl Lady Dorothy dh, approximately one inch tall were grown in inch clay pots containing a soil mixture of one— third peat, one— third sand, »rid one— third loam for ten days at a light intensity that was fifty percent of the prevailing light intensity. The reduced light intensity was established through the use of cheese-cloth and the intensity measured hy a Weston portable light-meter. temperature was approximately 60° P. The minimum One—half the number of each variety was then exposed to a natural light intensity of 8,000-10,000 foot dfendles for a period of six hours, and the other half of each variety was kept in the original light intensity. RESULTS. After a ten-day period Tinder a 5*000 foot candle illumination at a 60° F. minimum temperature, snapdragon seedlings exposed to a light intensity of 8,000-10,000 foot candles and a minimum temperature of 80° F. developed a type of leaf injury that gave the exposed leaves a silvery appearance. No diehack occurred, and the silvery appearance gradually dis­ appeared as the plants continued to grov in the prevailing full light intensity* The unexposed varieties did not develop diehack* EXPERIMENT 3 * One hundred seedlings each of Koester's White, Sykora1s White Wonder, Sykora*a White Cheviot, Peace, Lady Dorothy, and Lady Dorothy dh approximately one inch tall were grown in 2± inch clay pots containing a soil mixture of one— third peat, one— third sand, and one-third loam hy volume, for a period of ten days. On the eleventh day, one-fourth of each variety were placed into each of the following environments for ten days: A light intensity of 1,000-2,500 foot candles and one of 2,500-6,200 foot candles, each in a saturated atmosphere at 60°— 68° F* under a fourteen-hour photoperiod; a light intensity of 2,000—3,000 foot candles and one of 5*000—7,000 foot candles each in an atmosphere of 5o3&-?6$ relative humidity at 60°-70° F. under a fourteen-hour photoperiod* At the end of ten days, one-half of each treatment was exposed to a natural-light intensity of 6,000-12,000 foot candles, temperature of 8^°97° F* in an atmosphere of h2J&-5^$ relative humidity, and the other half of each lot was kept in the original environments for a period of ten days* RESULTS. 2,500 Plants which had been grown in light intensities of 1,000— foot candles at relative humidities of 98-100 percent 50—76 percent respectively, under a 1*1— hour photoperiod for a period of ten days, then exposed to a natural light intensity of 6,000-12,000 foot candles, temperature of 85°-97° a^d & relative humidity of UZf^-5^ for a period of eight hours under a I*!—hour photoperiod, caused a type of leaf injury that became evident on the day following exposure. The leaf portions that were exposed, to the high light intensity developed a silvery appearance on the exposed leaf surface. The Injury gradually disappeared from the plants at the end of eight days under the natural light conditions. The plants that were kept under the original conditions of light intensity and relative humidity continued to grow normally for a sixty-day period. No diehack symptoms occurred on any of the plants. EXPERIMENT h . A series of treatments in which the soil temperature was held constant at various ranges while the air temperature varied over a wide range was carried out under a 9—hour photoperiod and a 15—hour photoperiod for a period of fifty days. One hundred seedlings each of the eight varieties were planted in 3 inch clay pots in a soil mixture of one-third peat, one— third sand , and one— third loam. Thermostatically controlled air inlets and cold water baths controlled the air and soil temperatures. Tin foil was used to enclose the clay pots prior to immersion in the water bath. RESULTS. Dieback developed only at the **0° E. air temperature on the entire population of Lady Dorothy db and on approximately 2$ percent of the Eg populations of Lady Dorothy X Lady Dorothy db and Peace X Lady Dorothy db. Occurrence of dieback in percentage of population is shown in Table II. EXPERIMENT 5 . In a series of studies concerned with light quality and its influence upon the development of dieback symptoms on snapdragon seedlings, various filters were used. The wave length and the light in— To acconp&ny page 13 rCM Pm \r>>r\»n OOOOOOOOOCMCMCM O O O O O O O o O C V ) O O O O O O O O O N O O O O O O O O O O i C J ! _CM (^4 +» a BETWEEN SOIL AND AIR TEMPERATURES 8 u >or> 3 tensity transmitted by the filters are shown in Table III. energy transmitted was not determined. Total light One seedling, approximately one inch tall, of each of four varieties, Boaster's White, Sykora's White Wonder, Lady Dorothy db and Margaret was planted into each of twenty-four inch clay pots. Bach clay pot, containing four seedlings, was placed in a 2k x 12 x 3 inch metal container partially filled with neutral peat so that the clay pots were imbedded three-fourths of their depth. Atop this metal container, a light— sealed, wooden frame of filters was placed. Each filter covered four of the clay pots. The filter in each compartment was approximately five Inches above the top of the seedlings. temperature was 55° B. end the photoperiod was 9 hours. The minimum Seven days after the treatments were started, one pot of four seedlings from each light filter compartment was removed and placed in a 1+0°B. temperature at the prevailing light intensity and a 9—hour photoperiod. Serially, at seven- day intervals, one pot of four seedlings from each light-filter compartment was removed to a h0° B. temperature and a 9-hour photoperiod. A similar series of treatments was repeated using a 15—hour photoperiod. ’RESULTS. Plants grown under the various light filters expressed widely different patterns of growth. filter is expressed in Table IV. The manner of plant growth under each Only Lady Dorothy db showed foliar dieback five days after an 8—hour exposure to the Uo° B* air temperature regardless of light quality treatment. EXPERIMENT 6. In another series of experiments, two lots of 91 seed­ lings each of varieties Booster's White, Margaret, Sykora's White Wonder, Sykora's White Cheviot, Peace, Lady Dorothy, and Lady Dorothy db, and an TABLE III LIGHT QUALITY AND INTENSITY TRANSMITTED BY THE VARIOUS FILTERS ORDER 6,000-10,000 SOOT CANDLES SOURCE Wave Length Light Intensity Max* Min, 200 f c 1. Ordinary window glass 36op-76op b u 1330 f e 2. Lantern Blue filter 360.0-4000 H 1320 n 190 M 3. Signal green filter 4800-600.0 " 1350 ti 220 * 4. Signal yellow filter 526.0-580.0 M 1370 N 240 ■ 5860-6420 H 1360 « 230 * 640.0-760.0 n 1340 i» 210 w 5* H» 1, yellow filter 6. Signal red filter TABUS IV BELATIYB PLAHT GBOWTH PATTEHRS UHDEB VAEIOUS LIGH^PILTEES AT THE PHD OP 28 DATS OP 9-EOI® PHOTOPERIOD Leaves Stem Ordinary glass Considered normal Considered normal Considered normal Bins Dark green, copped abaxlally Short internodee Dwarf stooky Green Light green with leaf tip very light Very long internodes Very spindly Yellow Light green flat leaf Long intemodes Spindly H. E. Yellow Medium green, moderate abaxlally curling length­ wise Moderately long intemodes Spindly Bed Light green, extreme abaxlally curling length­ wise Moderately long intemodes Spindly Source Plant To accompany page lU 3*2 of Koester's White X Lady Dorothy, Peace X Booster's White, Armstrong's White X Booster's White, Peace X Armstrong's White, Lady Dorothy dh X Peace, and Lady Dorothy X Lady Dorothy dh were grown to a six-inch height. Ten plants of each variety were given one of the following treatments: 1. All the leaves were removed. 2, The top three pairs of leaves were removed. 3* The middle three pairs of leaves were removed. pairs of leaves were removed. 6. 5» The top six pairs of leaves were removed. The bottom six pairs of leaves were removed. the bottom three pairs of leaves were removed. removed. Jf. The bottom three 7. The top three pairs and 8. Hone of the leaves were Five plants of each treatment were grown for a period of fifty- five days in a 50° P. minimum temperature and the remaining five plants were grown in a *10° P. temperature with the prevailing light conditions of a 10-hour photoperiod. RESULTS. In the defoliation experiment, none of the plants grown at the 50° P. temperature exhibited symptoms of foliar dieback. Of the plants grown at 40° P. minimum temperature, symptoms of foliar dieback developed on the entire population of Lady Dorothy db, on the Pg of Lady Dorothy X Lady Dorothy db in the ratio of 67 normal to 2h dieback, and on the Lady Dorothy db X Peace in the ratio of 69 normal to 22 dieback. of Hone of the other varieties shoved dieback symptoms. EXPERIMENT 7 . A series of splice-grafts was made using two plants each of Margaret *>r»d Lady Dorothy db, each approximately eight inches in height. Each plant was well rooted in a 6 inch clay pot containing one part peat, one part sand, and one part clay-loam. Onto each of the two plants of variety Margaret, one four-inch terminal portion of Koester’s White, Lady Dorothy, and Lady Dorothy dh was splice— grafted* The grafted plants were grown for a period of ten days at a 60° I*, minimum temperature under the prevailing light conditions* One grafted plant of Margaret was then placed in a 40° F. minimum temperature under a 9-hour photoperiod, and one was placed in a 50° F* minimum temperature under a 9—hour photoperiod at the prevailing light intensity* 35 days* The duration of these treatments was Likewise, onto each of two plants of Lady Dorothy dh, one four- inch terminal of Koester's White, Lady Dorothy, and Margaret was splicegrafted* The grafted plants were grown at 60° F* for ten days. Then one plant placed in a ^0° F. and one placed in a 50° F* temperature for a period of 35 days* A repeat treatment of splice-grafts using the same combinations of varieties was carried out under a 15-hour photoperiod* RESULTS. In the splice— graft treatment with Margaret and Lady Dorothy dh serving as stock plants, the symptoms of diehack occurred only on the Lady Dorothy dh scions on stock plant Margaret, and only on the stock plant Lady Dorothy dh when the respective plants were exposed to a 40° F. temp­ erature under a 9-hour photoperiod during the 35 day treatment* Results are shown in Tahle V* The results of a repeat splice-graft experiment involving the same combinations of varieties hut under a 15-hour photoperiod, gave results identical with those which occurred under a 9-hour photoperiod. EXPERIMENT 8. One hundred two-inch seedlings of each of the varieties TABLE V RESPONSE Of GRAFTED PLANTS TO DIFFERENT AIR TEMPERATURES UNDER A NINE-HOUR FBOTOPERIQD Temperature Stock Plant Margaret Scion IW LD LDdb iio0 r . 0 0 0 ♦ * o 50 ? . 0 0 0 0 IW LD ♦ * 0 0 0 0 0 0 0 LDdb ko° y. Margaret • »r\ oo + indicates dieback symptoms KW - Roaster's White LD - Lady Dorothy LDdb • Lady Dorothy db of KoeBter's White, Sykora’s White Wonder, SykOra’s White Cheviot, Peace, Margaret, Lady Dorothy and Lady Dorothy dh, and the Pg of Lady Dorothy X Lady Dorothy dh and Peace X Lady Dorothy dh were transplanted into three different lots of flats. One lot contained a soil mixture of one-half unsterilized soil from a chrysanthemum hench and one-half chrysanthemum roots hy volume. Another lot contained a soil mixture of one-half im- sterilized soil from a snapdragon hench and one-half snapdragon roots hy volume. The third was a sterilized soil mixture composed of one-third peat, one— third sand, and one— third loam hy volume. All the plants in this treatment were exposed to a 9-hour photoperiod with prevailing light intensity ranging from 2,000 to 7,500 foot candles at two different air i o temperatures; one with a minimum of dO P. and one with a minimum of 50° P. The second study in this series of treatments was identical except that a 15-hour photoperiod was used, and the light intensity range was 2,800— 3,600 foot candles. Both series of treatments were continued under the respective environmental conditions for a period of seventy days, RESULTS, Diehack developed only at the d0° P, air temperature on the entire population of Lady Dorothy dh in the three soil fcixtures used, Diehack developed only at the d0° P, air temperature on approximately 25 percent of the populations of Lady Dorothy X Lady Dorothy dh and PEACE X Lady Dorothy dh in the three soil mixtures. Occurrence of diehack in percentage of population is shown in Tables VII and VIII, EXPERIMENT 9, In a series of nutritional studies, nutrient solutions were supplied by the constant level and subirrigation methods. coarposition of the nutrient solution used are shown in Table TI. Details of The com­ plete solution of macro-elements contained, in parts per million* 217 N* 200 Ca, 156 K, 60 Mg* h6 P* and 2h0 S0^. The salts were C.P* grade* Nutrient solutions of major elements were prepared from 0*5 molar stock solutions which were kept in soft glass containers* The supplementary solution used, one with boron and one without boron* was comparable to that suggested by Hoagland and A m o n (29)* Iron solution was added to the var­ ious prepared nutrient solutions when considered necessary for good plant growth* The solutions were adjusted to a pH of 6*2 with 1*0 N NaOH, and readjusted to that pH with additions of required amounts of 0*1 N NaOH every five days* This pH was within the range of that suggested by Spurway (56) for optimum growth of snapdragons* A Macbeth continuous recording pH meter was used to determine the hydrogen-ion concentration of the solutions* The first in the series of nutritional studies was established in which one two-inch plant of variety Booster's White* Margaret* and Lady Dorothy db were transplanted into 3 inch clay pots containing chemically clean white quarts sand* The plant roots were washed free of soil with tap water* then rinsed several times with distilled water prior to placing in the sand* paint* The clay pots were painted on the Inside with asphaltum The constant level method was used to supply nutrient solutions to the plants* Bobbins (50)• Table 71* Tha method was essentially the same as that described by The composition of the nutrient solutions are shown in The solution in each clay pot was maintained at a level considered TABLE VI COMPOSITION OP NUTBIEHT SOLUTIONS Molar concentration of Nutrient Salts Treatments » W 04 Complete 0.0015 Minas P m>3 Ca(N03)2 MgSO^ 0.0040 0.0050 0.0025 0.0040 0.0050 0.0025 0.0015 0.0050 0.0025 0.0005 Minas K 0.0015 Minus Ca 0.0015 0.0040 Minus Mg 0.0015 0.0020 0.0050 Minus B 0.0015 0.00^0 0.0050 (u^)2so^ 0.0015 0.0005 0.0025 Micro-Nutrients 2*50 F® 8* FeSOj^ 0.05 p.p.m. Zn as ZnSO^ 0.50 p.p.m. B as H^BO^ 0.02 p.p.m* Gu as CuSO^ 0*50 p.p.m. Mn as MnSQ^ 0.01 p.p.m. Mo as HgMoO^ m*(ik>3)2 X„S0. 2 4 0.0025 0.0010 RESPONSE OF PLANTS UNDER A 9-EOUR PHOTOPERIOD IN DIFFERENT SOIL MIXTURES Soil Mixture by volume Min* Temp* Degrees F* Varieties Showing Dieback in Percentage EW 1/2 snapdragon roots, l/2 un­ sterilized soil from snapdragon bench 1/2 chrysanthemum roots, l/2 uxh sterilised soil from chrysanthemum bench Sterilized, l/3 peat, l/3 sand, and 1/3 loam EW - Eoeater’e White P - Peace SWW SWC P Marg LD LDdb LDxLDdbF2 PxLDdbF2 40 0 0 0 0 0 0 100 26 25 50 0 0 0 0 0 0 0 0 0 ho 0 0 0 0 0 0 100 25 25 50 0 0 0 0 0 0 0 0 0 ho 0 0 0 0 0 0 100 25 25 50 0 0 0 0 0 0 0 0 0 SWW - Sykora’s White Wonder Marg - Margaret LDdh - Lady Dorothy susceptible SWC - Sykora's White Cheviot LD - Lady Dorothy LDxLDdbF^ - F^ of Lady Dorothy x Lady Dorothy susceptible PxLDdbF2 - F2 of ^oace x Lady Dorothy susceptible BESPONSE OP PLANTS GNDEE A 15-EOUE PHOTOPEBIOD IN DIFFEEENT SOIL MIXTOBES Soil Mixture by volume Mis. Temp Degrees P. l/2 snapdragon roots, l/2 WOrsterilizei soil from snapdragon bench l/2 chrysanthemum roots, l/2 un­ sterilised soil from chrysanthemum bench Sterilised l/3 peat, l/3 loam and l/3 sand KW - Koester’s White P - Peace Varieties Showing Dieback in Percentage KW SWW SWC P Marg LD LDdb LDxLDdbP^ ho 0 0 0 0 0 0 100 26 2h 50 0 0 0 0 0 0 0 0 0 ho 0 0 0 0 0 0 100 25 25 50 0 0 0 0 0 0 0 0 0 ho 0 0 0 0 0 0 100 25 25 50 0 0 0 0 0 0 0 0 0 SWW - Sykora’s White Wonder Marg - Margaret LDdb - Lady Dorothy susceptible PxLDdbPg SWO - Sykora’s White Cheviot LD - Lady Dorothy LDxLDdbFg - Eg °* Lady Dorothy x Lady Dorothy susceptible PxLDdbPg - Eg °* Pcace x Lady Dorothy susceptible 19. to be conducive to normal plant growth* Each treatment was used in each of three replicates under a 9—hour photoperiod and a light intensity range of 2,100-7*200 foot candles. RESULTS. The treatment was of 35 days duration. The response of Koester’s White, Margaret, and Lady Dorothy db to the nutritional treatments in which the constant level method was used are shown in Table IX which is the response of one replicate. Each of the other replicates showed no observable difference in the respective nutritional deficiency symptoms. Symptoms of foliar dieback did not develop under any of the treatments during this study. EXPERIMENT 10. The second in a series of nutritional studies was established in which sir-inch plants of variety Margaret X Junglewood Lemon F^» Helen Tobin X Armstrong White F^, and Witterstatter1s Orchid (originator’s strain) were grown in a soil mixture of one-third peat, onethird sand, and one-third loam. The roots of the plants were thoroughly cleansed of soil with tap water, then rinsed several times with distilled water. One plant of each variety was then transplanted into each of the chemically clean eight-inch glazed crocks containing chemically clean quartz sand. Nutrient solution was applied to each crock in sufficient amount to reach one-quarter inch above the surface of the sand at threehour intervals by the subirrigation method. same as that described by Davidson (13)* a period of 28 days. The method was essentially the Th© duration of the study was for The six nutritional treatments were exposed to four different light conditions: a 9-hour photoperiod with a high light in­ tensity of 6,000-11,000 foot candles and a low light intensity of 3,0005,500 foot candles respectively; a 1^-hour photoperiod with the same high TABLE II BESPONSE 0? PLMTS TO HUTRITIQNA1 THSATMEKT5 UKUEE A 9-HDUE PHDTOPERIOD Treatment Variety Complete XW Description of Plant Leaves Dieback Marg Lddb Dark green foliage, flat broad leaf considered normal t t t t t t t t H f l t t t t t t t t i t t t i t i i i t t t 0 0 0 Minus P XI Marg LDdb Dark bluish green, more linear than normal it tt tt it w it it tt M II II H It tt n II 0 0 0 Minus K KW Marg LDdb Yellow-green intervelnal area which first appears at apex « h it n n n u it 11 H tt II tt H It M 0 0 0 Minus Ca Or Marg LDdb Young expanding leaf is dwarfed brittle H II II II « II M n it ti ii n it ti chlorotic H « 0 0 0 Minus Mg XW Marg Basal leaves become chlorotic throughout 11 H ti H II H tt tt 0 0 Minus B EW‘ Marg LDdb KW - Xoester's White & All leaves abaxlally cupped, apex tends to roll under leaf H it n it h h n n H II it it ti it n It Marg - Margaret LDdb - Susceptible Lady Dorothy 0 0 0 and low light intensities, respectively. The experimental design for the second in the series of nutritional studies is shown in Table X, RESULTS. Noticeable differences were not observed between varieties in the same or in the different macro-element-deficiency solutions under the l^>hour photoperiod with low light intensity, the 9-hour photoperiod with low light intensity, and the 9-hour photoperiod with the high light intensity, respectively* Under the Ih-hour photoperiod with high light intensity, each variety responded similarly in the same deficiency treat­ ment, but the varieties within a deficiency treatment differed noticeably from those of other deficiency treatments* The varieties in the complete- solution treatment showed growth patterns considered to be normal. In the minus-phosphorus treatment, young terminal growth appeared stunted and bluish-green in color. In the minus-potassium treatment, the young term­ inal leaves developed light yellowish-green interveinal areas at the leaf apices which were abnormally small* The minus-calcium treatment produced chlorotic, dwarfed, teminal leaves which were extremely narrow. In the minus-magnesium treatment, the terminal growth was considered normal, except for a light green leaf color* In the minus-boron treatment, the terminal leaves developed an abaxially cup-ohaped appearance with an iridescent coloration developing near the leaf apices* Typical foliar dieback symptoms were not expressed in any variety in any treatment* EXPERIMENT 11* The third in the series of nutritional studies was designed to grow three varieties of plants each four-inches tall in a TABLE X LIGHT CONDITIONS, NUTRITIONAL TREATMENTS AND VARIETIES PHOTOPERIOD 9-hour Light Intensity in foot oandlea Nutrients 6,000-11,000 Complete -P -K -Ca -Mg -B 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3,000-5,500 Complete -P 0 0 0 0 0 0 0 0 0 0 0 Variety Shoving Diehack Symptoms MargxJL(P^) 6,000-11,000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -P 0 0 -K 0 -Ca -Mg -B 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -Ca -Mg -B Complete 0 20 Complete page 0 0 0 To accompany -Ca -Mg -B -P -E 3,000-5,500 VI 0 0 0 0 0 0 0 14-hour HTxAWCP^) complete nutrient solution for thirty days under a 15-hour photoperiod and two different light intensities method was used during this period. The subirrigation send culture On the thirty-first day, the sub­ irrigation system was detached and the nutrient solution rinsed from the sand with distilled water. The rinsing was done five times hy running two liters of distilled water through the sand each time at ^5-mihute intervals. A complete nutrient solution minus-horon was then added to the crocks at a level considered conducive to good plant growth. The constant level method was used for the remainder of the treatment. RESULTS. In the third of the series of nutritional studies, the plants developed rapidly and normally for thirty days in the complete nutrient solution. Thirty days after treatment with the minus-horon nutrient solutiop began, all the flower spikes showed color, hut symptoms of foliar diehack were not evident. The study was discontinued at this stage of plant development. EXPERIMENT 12. The fourth in a series of nutritional studies was designed with two replicates, one of which is shown in Tahle XI. Six varieties of snapdragons were used in each nutritional treatment: Rooster1s White, Sykora1s White Wonder, Margaret, Peace X Lady Dorothy dh, Helen Tohin X Armstrong's White Vitterstat ter'a Yellow. One-inch seedlings were taken from the seed-hed, the roots cleansed of soil with tap water, rinsed several times with distilled water, and placed in the chemically clean white quarts sand contained in the chemically clean eight-inch glazed crocks. A starter solution of complete minus boron TABLE XI COMPARATIVE RESPONSE OP PLANTS UNDER DIFFERENT LIGHT INTENSITIES AND NUTRITIONAL TREATMENT Light Nutrient Solution* Varieties Showing Diehack Symptoms** KW SWW Marg PxLDdh HTxAW^) WY Pull light (3*800-11,000 foot candles) Complete -B -B-P -3-1 -B-Ca —B-Mg 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 c 0 0 0 0 0 0 0 0 0 0 0 0 0 Shaded (60# of full light Complete -B -B-P -B-X -B-Ca -B-Mg 0 0 0 0 0 0 0 0 0 0 0 0 0 0 c 0 0 0 0 0 0 0 0 0 c 0 0 0 0 0 0 0 0 0 0 0 ♦Starter solution of complete minus 'coron wae used for first 7 days. XW - Chester's White SWW - Sykora’s White-Wonder Marg - Margaret PxLDdb «• Peace x Lady Dorothy dh EPxAW(P^) - Helen Tohin x Armstrong's White P^ WY - Witterstatter1s Yellow as shown in Table II was used for seven days* The constant level method was used and the starter solution maintained at a level considered sufficient for good plant growth* The seedlings were shaded with one thickness of cheese-cloth for four days* The plants were never permitted to suffer apparent need or excess of water* On the fifth day* all the leaves of all varieties showed a tendency to cup abaxlally* On the evening of the fifth day* each crock in each treatment in each replicate was drained of starter solution* then each crock m i filled with distilled water to a level of about one-quarter inch above the surface of the sand* then Immediately drained* peated four times at one-hour intervals* This rinsing procedure was re­ After the final rinse* the crocks were connected to the subirrigation system* The subirrigation system forced the nutrient solutions to a level approximately one— quarter inch above the surface of the sand* During the daylight hours* the system operated at two-hour intervals; during the hours of darkness* the system operated at four-hour intervals* The system was adjusted to reach the one-quarter inch level* then immediately drain* The nutrient solution of each treat- sent was totally renewed every sixteen days* The study was continued for fifty-two days* When the subirrigation method was put into use* one of the two replicates was shaded with tobaoco cloth which reduced the prevailing light intensity to sixty percent* The prevailing light intensity ranged from 3*800 to 11,000 foot candles* RESULTS. The fourth series of nutritional studies produced develop- mental patterns of growth, especially of the leaf, distinctly different from developmental patterns produced hy snapdragon plants under any of the other environmental treatments studied. The developmental patterns of the leaves were apparently normal for the first four days of growth in the starter solution of complete minus-horon salts. On the fifth day, all the leaves of all varieties showed a tendency to cup abaxlally, After the starter solution was thoroughly rinsed from the crocks in the treat­ ments and the respective nutrient solutions (Table XI) were stqpplied to the plants, the abaxial cupping of the leaves gradually became more pro­ nounced on those plants exposed to full light intensity. Ten days after this nutritional study began, some of the first true leaveb on all var­ ieties developed a small water-soaked area at the leaf apex which dried to a light, huff-colored Isosceles triangle-shaped area with the apex of the triangle pointing toward the base. At this time, the next oldest leaves developed an iridescent, metallic sheen from the apex to about the middle of the leaf blade. Within this region, areas developed which had the appearance of having been scraped with some hard object. Pro­ gressively, all the first true leaves of all the plants in this study showed this developmental leaf pattern, except the treatments* which were returned to the complete solution. At this stage of growth, further development of the described leaf pattern did not appear. plants were permitted to grow to the flowering stage. However, the The symptoms of foliar dieback did not occur, but leaf necroses shown in Pig. Ill occurred in three of the six treatments of each replicate, Pig, IV shows growth of snapdragons at the termination of the 52—day study. Vigor* XXI. Imwtmm from 5th haaal mod* of plant• crown In sand culture*. Lear** ara 2/3 X natural •Isa. L#ft to right: -B, -B-Mg, -B-Ca, -B~E, -B-P, complete To a c c o m p a n y p a g e 23 Pigur* IT. $ Z ~ & . a y growth of snapdragons in sand culture. Loft to rightt —B* -B-Mg, -3-Cft, —B—X.* —B-P* complete. Figure IV Discussion Foliar dieback symptoms in the greenhouse snapdragon, Antirrhinum inajua L . , may occur frequently during short photoperiods of intermittent high and low light Intensities (30, 67) , ^ be more susceptible than others (30)* different genetic patterns (lh)* some varieties appear to This Indicates the expression of Snapdragon varieties used in this in­ vestigation include those on which symptoms have, and have not, appeared under accepted cultural practices* The appearance of similar leaf necrosis patterns in various crop plants and attributed to one or more different environmental factors, as reported by several workers (2, 3^, 35* 51* 52 , 53* 5^t 58), has suggested experimentation with a series of similar factors and various combinations of them* The factors included light, temperature, and mineral nutrition, primarily* The first five experiments were concerned with photoperiod, light intensity, light quality, air temperature and soil temperature* It a found that a selected strain of variety lady Dorothy, known to sfr foliar dieback under certain cultural conditions, and used in t^xs work under the name Lady Dorothy db, consistently showed foliar dieback when exposed to a 40° F* air temperature, but not at 50° F* or higher. Foliar dieback occurred on this selected strain under various conditions of photo­ period, light intensity, and soil temperature, but only when the air temperature was not above h0° F* Light quality was not a factor in the expression of leaf dieback on Lady Dorothy db* The occurrence of foliar dieback in the progeny of Lady Dorothy db was one hundred percent* In the ?2 progenies of Peace X Lady Dorothy db and of Lady Dorothy X Lady Dorothy dh, diehack occurred in the ratio of three plants haring normal appearing leares to one haring foliar diehack, when these plants were exposed to a h0° IT* air temperature for eight hours, subsequent to fire different light quality exposures at four different time exposures. The manner of response of these plants to a h0° 7. air temperature indicates that a single factor pair of genes is responsible for the expression of foliar diehack (Figure 1) occurring in the progeny of Lady Dorothy dh, and in the 7^ progenies of Peace X Lady Dorothy dh and Lady Dorothy X Lady Dorothy dh* The nature of the response of these progenies to a 40° 7, air temperature indicates that Lady Dorothy dh is a homosygous recessive and one-fourth of the individuals of the 7g progenies of Peace X Lady Dorothy dh and Lady Dorothy X Lady Dorothy dh which showed foliar diehack are homozygous recessive* The 7^ of Peace X Lady Dorothy dh and of Lady Dorothy X Lady Dorothy dh never expressed symptoms of foliar diehack when exposed to a h0° 7* air temperature. Differential soil and air temperatures have heen reported (2, 3h, 48, 51) as causes of leaf tip necrosis in certain plants and the effects of the relationship have heen discussed (37* 33) • Differential soil and air temperature apparently had no Influence upon the incidence of foliar diehack, Foliar diehack, however, occurred at various soil temperatures only when the air temperature was h0° 7, The correlation between light intensities, relative humidities, and temperatures has heen shown hy Jones (35) in. the expression of onion blast* However, the response of snapdragons to similar treatments resulted in a type of leaf injury that was temporary. The injury occurred as a silvery appearance on the upper 8-orface of the leaf. Continued exposure of the Injured plants to Intense natural light, low humidity and high temperature for eight days permitted the injured leaves to assume normal appearance* Vo diehack symptoms developed on any of the plants in this study. Jones (3*0 found that air temperature had little effect upon gardenia chlorosis caused hy low soil temperature. While root temperatures generally in­ fluence growth patterns of plant tissue (2*0, air temperatures apparently Influence developmental patterns of leaf tissue in some varieties of snap­ dragon. Low temperatures recommended (4) for late planting of greenhouse snapdragons is in agreement with Went (65) • hut reduction of leaf temp­ erature due to radiation (12) at low temperatures may he sufficient to cause leaf diehack in some varieties of snapdragons if the heterozygous condition of the single factor responsible for diehack in Lady Dorothy dh exists in the lines of snapdragons used in breeding work. Xteeent results reported (5* 6, 7* 8, 22, 23» 40, 44, 47, 63) in the field of phytohormones have suggested a series of experiments concerned with the influence on developmental patterns of certain plant parts, and the inhibitory nature, or toxic effect, of auxins of certain plants and plant parts* The Influence of defoliation, grafting, and plant roots of the same or different crops, upon the incidence of foliar diehack in greenhouse snapdragons were studied* Ee suits were negative* The influence of light end temperature upon the synthesis (7, 40, 60, 63), and movement (8, 44) of plant hormones has suggested a study of defoliation of snapdragons and its Influence upon the incidence of 27. foliar dleback* Defoliation of apical, middle, basal regions of the plant, of various combinations, and complete defoliation had no apparent influence upon the incidence of foliar dlebaok at either a 50° 7. or a ho° 7. air temperature during the 55-*day growing period following the treatment. However, under both short and long day treatment, the entire population of Lady Dorothy db showed leaf dleback on the young developing leaves six days after exposure to b0° 7. air temperature, Also, the progeny of Lady Dorothy X Lady Dorothy db showed leaf dleback in the ratio of 6? normal to 2 U dleback and the 7^ progeny of Peace X Lady Dorothy db developed leaf dleback In the ratio of 69 normal to 22 dleback six days after exposure to a h0° 7. air temperature. The 7^ progeny of Lady Dorothy X Lady Dorothy db and of Peace X Lady Dorothy db developed no syriptoms of leaf dleback. back* Hone of the other varieties, at *K)° 7, developed leaf die- Hone of the varieties used in this study developed foliar dleback at the 50° 7, air temperature. Apparently, leaves of different ages neither influence nor inhibit the expression of leaf dleback in those progenies that are susceptible to leaf dleback. Differential defoliation had no apparent influence upon the incidence of foliar dleback, which suggests that a leaf-produced agent, inhibitory or toxic to normal leaf development under either short or long day, had no role in the development of foliar dleback* Since certain phytohormones are dlffusable through a graft union (26, 63), splice graft studies were conducted to determine whether the causal agent of foliar dleback is Lady Dorothy db, when exposed to a hO° 7. air temperature, would pass a graft union* 7oliar dleback developed only on the young developing leaves of stock plant and scion of Lady Dorothy dh five days after exposure to the h0° 7. air temperature. None of the other varieties showed diehack during the 35—day exposure to a bO° F. air temp­ erature. The results of this study indicate that the causal nature of foliar diehack is not diffusahle through a graft union. Foliar diehack did not occur on any of the varieties used during this study at the 50° 7. air temperature. The accepted practice of growers of placing snapdragons in the same "bench used to grow chrysanthemums or snapdragons, immediately after the removal of those crops, has suggested a study concerned with various soil mixtures containing chrysanthemum roots and snapdragon roots influence upon the incidence of foliar diehack. their Under a 9-hour photoperiod and a 15-hour photoperiod, each at a minimum air temperature of bO° F., and 50° 7. respectively, the three kinds of soil mixtures described in Tables Til and Till, apparently had no effect upon the incidence of foliar diehack. However, varieties Lady Dorothy dh, and the 7^ progenies of Lady Dorothy X Lady Dorothy dh and Peace X Lady Dorothy dh, when ex— posed to a ho o 7. air temperature gave results, shown in Cables 711 and Till, which indicate the expression of a single factor pair of genes under the influence of a low air temperature exposure. Foliar diehack did not o occur on any variety in any soil mixture at 50 F. air temperature. The nutritional studies were conducted mainly during June-August of 1952. The prevailing light Intensity ranged from 3*600 to 11,000 foot candles. The temperature ranged from 6 6 ° F. to 10^° F. Deficiency 29 symptoms of tho major elements developed similarly in all varieties of snapdragons used, "but occurrence of symptoms were not simultaneous and the degree of severity was not the same in every variety used. Each variety responded similarly to each treatment "but not in the same time sequence. These results show that some varieties of snapdragons respond more quickly to nutrient deficiencies than do other varieties. The leaf development of all the varieties grown in treatments deficient in P, X, Ca, **nd Mg respectively, showed symptoms similar to those which are considered typical in some other crop plants (31). In the P—deficient treatment, the different varieties developed leaves that were progressively smaller from the haae to the apex. The leaves were a dull bluish-green and relatively mors narrow than the leaves of plants grown in the complete solution. very wiry. The stems were thin but In the variety Witterstatter* s Orchid, the lower leaves be­ came intensely red on the abaxlal surfacts. 3his coloration did not occur on the same variety in any of the other nutrient treatises'' statter’s Orchid could serve as a good indicator plant Witter- «>r P-deficiency in the growing medium. In the K«deficlency treatment, the young leaves became chlorotle between the veins near the leaf apex and progressed to the leaf base. Th«* leaf margins became necrotic. The necrosis advanced from the apex toward the center and base of the leaf. In Wit ter atatter’s Orchid, the lntervelnal areas of the leaves became chlorotic seven days before any of the other varieties showed similar symptoms. All the varieties were dwarfed with stunted axillary shoots. The sterna axti. leaves gradually became brittle. In the Ca-deficiency treatment, the varieties quickly showed chlorotlo spots at the juncture of the leaf-blade » * A petiole near the main vein of the young dare loping leaves. These amall spots gradually enlarged Into light brown necrotic areas which ad-ranced to leaf apex and to base of the petiole* The atem developed necrotic spots and the terminal bud became necrotic and died. The lower leaves remained green and healthy In appear­ ance , but the axillary shoots were chlorotic, stunted and brittle, and the terminals of the shoots died before the second or third node could be recognized. In Wltterstatter1s Orchid, the apex of the main shoot died five days sooner than that of any other variety. In the Mg-deficiency treatment, all the leaves progressively became pale green with chlorotic areas occurring throughout the leaf blades. The chlorotic areas became white then turned brown. areas enlarged slowly* The brown necrotic Leaves of the developing axillary shoots showed the same pattern of chlorosis and necrosis. Wit ter stutter* s Orchid de­ veloped these symptoms six days sooner than any other variety. In the minus—B treatments, the snapdragon varieties used, developed leaves which were dark green, thickened, leathery, more lanceolate than normal* and cupped abaxially. The axillary shoots were stunted, and the apices of all shoots were rosette-like in appearance* The flower spike, in the bud stage, became chlorotic, and the flower buds never expanded into full bloom. These developmental patterns were more pro- nounced In. h ig h than in low l i g h t i n t e n s i t y . L eaves o f p la n t s w hich were grown i n sand c u lt u r e are shown in F ig . 3* A b axial cu p p in g o f snapdragon le a v e s a ls o occurred under hlwe l i g h t a t low i n t e n s i t y and under n a tu r a l l i g h t a t h ig h i n t e n s i t y . The b lu e r e g io n o f th e v i s i b l e spectrum i s co n sid er ed in h ib it o r y to th e p ro d u c tio n o f grow th hormones (64), and h ig h l i g h t I n t e n s it y ten d s to in a c t iv a t e or d e s t r o y a u x in ( 6 0 ) , w h ile low w ater a b so r p tio n p a r a l l e l s low a u x in con­ t e n t in p la n t s (51 )• S in ce a b a x la l cupping o f le a th e r y le a v e s occurred in snapdragon v a r i e t i e s grown in m inus-boron trea tm en ts a t h ig h and low l i g h t i n t e n s i t y , and a t h ig h l i g h t I n t e n s it y and under b lu e l i g h t w ith c om plete n u t r ie n t tr e a tm e n ts, i t i s h y p o th e s iz e d , by way o f in fe r e n c e , th a t boron h as an im portant f u n c t io n a l r o le in th e foxm atlon o f growth su b sta n c e s r e s p o n s ib le f o r normal l e a f developm ent in the snapdragon. Eaton (16) con clu d ed th a t boron i s e s s e n t i a l f o r the fo rm a tio n o f the a u x in r e s p o n s ib le f o r normal l e a f ex p a n sio n in th e c o tto n p la n t . In th e compound d e f ic ie n c y trea tm en ts o f m inus-boron and minus one o f th e major e s s e n t i a l e le m e n ts , th e d e f ic ie n c y symptoms d ev elo p ed more q u ic k ly and w ere more s e v e r e than when a m ajor e s s e n t i a l elem en t a lo n e was d e f i c i e n t . A b axial cupping o f le a v e s occu rred in a l l v a r i e t i e s in a l l compound—d e f i c i e n c y tr e a tm e n ts. Under long photopexiod used in these nutritional studies which is similar to that reported as favorable for the absorption of phosphorus (10, 19, 20, 21, 42), potassium (15. 17. 2?, 28), calcium (27, 28, 49), and boron (25, 43, 57* 68) as compared with short photoperiod, deficiency ■ymptomB developed earlier and were more severe in the snapdragon var­ ieties grown under a 14—hour photoperiod than in the same varieties grown under a 9-hour photoperiod. Also, deficiency symptoms occurred earlier and were more severe on the varieties grown in the fall in­ tensity of the summer sun than on the same varieties grown in 60% of the prevailing summer light intensity* 3he results of these experiments indicate that foliar diehack occurred in the genetically susceptible line of snapdragons only at or below a *K)° F. sir temperature. susceptible line and its dicates The manner of response of the genetically progenies to a Uo° F* air temperature in~ that susceptibility is due to a single pair of recessive genes. The recessive nature of susceptibility complicates the seedsman's task of eliminating the undesirable trait of foliar dieback* Even though all individuals showing the trait in a population were eliminated, a number twice as great might be expected to carry the recessive factor and to produce a progeny in which one-fourth would be susceptible* samples from the varieties previously observed to be susceptible did not produce symptoms of foliar dieback* The small Besults were consistent, however, in the susceptible strain Isolated for the purpose of this study* Additional complication from the seedsman's viewpoint arises from the fact that environmental conditions favorable to expression of this defect are not realised in every season's growth* Should it be the seedsman* s misfortune to continue the line through a susceptible individual not showing the defect because of environmental masking, future generations would be susceptible* Effective control will require progeny testing at low temperatures and elimination of all individuals of the segregating populations ex­ pressing the trait* Summary 1* Several controlled environmental factors, singly and in various combinations, were used in an attempt to produce foliar dleback of green­ house snapdragons experimentally* 2* Foliar dieback was produced experimentally in an isolated strain of variety Lady Dorothy and in the Fg progeny of several crosses of which the Isolated strain of variety Lady Dorothy was a parent* 3* She expression of foliar dieback was the response of the genetically susceptible lines to an air temperature of 40° F* or below* h* The nature of the response indicates that foliar dieback in the greenhouse snapdragon is the response of a single homosygous recessive gene to a low temperature* 5* Foliar dleback did not occur on resistant genotypes under any of the environmental conditions established in this series of studies* 6* Several kinds of leaf necrosis which developed on plants grown in sand cultures are described* 7* The leaf necrosis patterns produced on resistant lines in sand cultures are different from foliar dieback* 3*fc 8. The similarity of leaf patterns which developed in the snapdragon varieties used in these studies tinder conditions of high light intensity, the blue region of the visible spectrun, and in boron-deficient sand cultures has suggested the hypothesis that boron has an important functional role in the development of the normal leaf pattern of these varieties. Literature Cited ATber&a. T* The influenoe of some external factors on growth and phosphate uptake of maize plants of different salt conditions* Bee* trav* ho tan* neerland* 4 l *541-601. 1946* Andersen, E* M* T i p b u m of lettuce* 829, 1^14. 1946* Cornell Agr* Exp* Sta* Bui* Arnon, D* I*, and C* M. Johnson* Influence of hydrogen ion con­ centration on the growth of higher plants under controlled conditions* Plant Physiol. 17*525-539. 1942. Ball, 0. J. Ball Bed Book* Benedict, H* M. of brome grass* 7th ed. 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