.«vV—r —v——1-A I I... . .1 . ... mmmmn——4" ' .' " u I ' A A A ~ A _ \ .4. - ‘ , h , I THE EFFECT OF DROUGHT STRESS ON THE GROWTH, DEVELOPMENT, PHOTOSYNTHESIS AND TRANSPIRATION 7 I OF SIX SPECIES OF NORTH AMERICAN SPRUCE ' Dissertation for the Degree of Ph. Df MICHIGAN STATE UNIVERSITY BRUCE ALLAN ROTTINK 197-4 This is to certify that the thesis entitled The Effect of Drought Stress on the Groéth, Development, Photosynthesis and Transpiration of Six Species of North American Spruce presented by Bruce Allan Rottink has been accepted towards fulfillment of the requirements for Ph. D. degree in Forestry \ O Major professor Date April 16, 1974 0-7639 ABSTRACT THE EFFECT OF DROUGHT STRESS ON THE GROWTH, DEVELOPMENT, PHOTOSYNTHESIS AND TRANSPIRATION OF SIX SPECIES OF NORTH AMERICAN SPRUCE BY Bruce Allan Rottink Drought stress influenced numerous parameters of size in six-month old greenhouse grown seedlings, as well as survival. In any seedlot the percentage of seedlings which formed dormant apical buds was in most cases independ- ent of water treatment level. The net photosynthetic rate of blue spruce (Eiggg pungens Engelm.), a species native to arid areas, is not depressed as much by a reduction in soil water potential as is the net photosynthetic rate of Sitka spruce (B. sitchensis [Bong.] Carr.), a species native to moist areas. The net photosynthetic rate of Sitka spruce seedlings also recovered less completely and more slowly following a post- drought watering than species native to more arid areas. Seedlots which are most drought resistant tend to alter their transpiration rates more in response to changes in 1' ‘ x ,. 1\ ' .3 Bruce Allan Rottink soil water potential than seedlots which are less drought resistant. Drought stress appeared to have little influence on the development of needle surface waxes. The few changes that were observed were not consistent within a species. There were no obvious differences in needle surface waxes between the species of spruce. In some instances, the relative sizes of the plant organs varied with water treatment. Plants growing under dry conditions tended to have heavier stems, at the expense of needle production. As needles are the principle water- losing organs, this change could have adaptive significance. THE EFFECT OF DROUGHT STRESS ON THE GROWTH, DEVELOPMENT, PHOTOSYNTHESIS AND TRANSPIRATION OF SIX SPECIES OF NORTH AMERICAN SPRUCE BY Bruce Allan Rottink A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Forestry 1974 ACKNOWLEDGMENTS I would like to express my thanks and appreciation to the many people who have assisted me in this study. First and foremost is my committee chairman Dr. James W. Hanover, Department of Forestry. The other committee members, Dr. Peter Murphy, Dr. Donald Penner, and Dr. S. K. Ries, have likewise been very helpful. Thanks are also due to the many people who have given generously of their time and effort in making collections of seed, from which the trees used in this study were grown. Finally I would like to express my thanks to my wife, Karen, for: (a) helping cokat seeds, (b) assisting with various measurements of the seedlings, (c) typing and proofreading, (d) keeping me sane, and (e) putting up with me during what has seemed to be the endless task of preparing this dissertation. ii TABLE OF CONTENTS Page ACKNOWLEDGMENTS . . . . . . . . . . . . . ii LIST OF TABLES . . . . . . . . . . . . . . iv LIST OF FIGURES . . . . . . . . . . . . . Vi INTRODUCTION 0 O O O O O O O O O O C O O Vii Chapter I. EFFECT OF DROUGHT STRESS ON GROWTH AND DEVELOPMENT OF SIX SPECIES OF NORTH AMERICAN SPRUCE O O O O I O O O O I O O O O 1 Materials and Methods . . . . . . . . 1 Results and Discussion . . . . . . . . 7 Summary . . . . . . . . . . . . . 25 II. THE EFFECT OF DROUGHT STRESS ON PHOTOSYNTHESIS AND TRANSPIRATION OF SPRUCE SEEDLINGS . . . . 27 Materials and Methods . . . . . . . . 27 Results . . . . . . . . . . . . . 30 Photosynthesis . . . . . . . . . . 30 Transpiration . . . . . . . . . . 30 Discussion . . . . . . . . . . . . 35 III. THE EFFECT OF DROUGHT STRESS ON SURFACE WAX DEVELOPMENT IN SIX NORTH AMERICAN SPECIES OF SPRUCE O C O O O O O O O O O O 0 40 Materials and Methods . . . . . . . . 40 Results . . . . . . . . . . . . . 41 DiscuSSion O O O O O O O O O O O .\ 47 IV. CONCLUSIONS AND RECOMMENDATIONS . . . . . . 51 BIBLIOGRAPHY . . . . . . . . . . . . . . 54 iii 10. ll. 12. 13. LIST OF TABLES Seedlots of Picea used in this study . . . . Number of days from planting required for emergence of 50% of seedlings . . . . . . Percent of seedlings surviving after six months . Percent of seedlings surviving after six months under three watering regimes . . . . . . Percentages of seedlings in each seedlot which did not possess an apical bud six months after SOWing O O O I O I O O O O O O O O Percentages of seedlings not possessing apical buds six months after sowing . . . . . . Significance levels for differences between various measures of growth for ten seedlots of spruce at three watering levels . . . . Stem diameters of spruce seedlings grown under three watering regimes . . . . . . . . Shoot heights of spruce seedlings grown under three watering regimes . . . . . . . . Mean needle lengths of spruce seedlings grown under three watering regimes . . . . . . Mean root lengths of spruce seedlings grown under three watering regimes . ., . . . . . . Regression equations for water-level X Seedlot treatment combinations in which the slopes of the regressions are different for seedlings possessing an apical bud and those without an apical bud . . . . . . . . . . . . Regression coefficients for the regression of loglo needle dry weight on 10910 total dry weight . . . . . . . . . . . . . iv Page 10 11 11 16 l6 17 17 19 20 Table Page 14. Regression coefficients for the regression of 10910 stem dry weight on 10910 total dry weight 0 O O O O O O O O O O O O O 21 15. Regression coefficients for the regression of 10910 root dry weight on 10910 total dry weight . . . . . . . . . . . . . . 22 16. Characteristics of post-drought recovery of photosynthetic rate . . . .. . . . . . 33 17. Transpiration rate in grams of water lost per gram dry weight of needles per hour when soil moisture level was between saturation and field capacity . . . . . . . . . . . 34 18. Regression coefficients for the regression of 10910 transpiration rate (grams water lost per gram needle dry weight per hour) on loglo (-soil water potential) . . . . . . . . 34 LIST OF FIGURES Figure Page 1. Mean total dry weights of seedlings of twelve seedlots of spruce grown under three watering regimes o o o o o o o o o o o o o 12 2. 'Relationship between soil water potential and photosynthetic rate in blue spruce, white spruce and Sitka spruce . . . . . . . . 31 3. Surface waxes of spruce foliage, Part I . . . 43 4. Surface waxes of spruce foliage, Part II . . . 45 vi INTRODUCTION Seven species of spruce including white spruce (Picea glauca [Moench] Voss), black spruce (P. mariana [Mill.] B. S. P.), red spruce (P. rubens Sarg.), blue spruce (P. pungens Engelm.), Engelmann spruce (P. engelmannii Parry), Sitka spruce (P. sitchensis [Bong.] Carr.) and Brewer spruce (P. breweriana S. Wats.) are native to the United States and/or Canada (Harlow and Harrar, 1958). All species except Brewer spruce, which has an extremely limited distribution, are of commercial importance. The habitats occupied by the various species differ greatly, especially in degree of aridity. This study was undertaken to determine the responses of the various species to drought stress and, by comparing their responses to determine ways in which the various species have adapted to the water regime of their native habitat. The responses studied were: 1. Growth and development of the seedlings. 2. Exchange of carbon dioxide and water vapor between the plant and the atmosphere. 3. Development of epicuticular waxes on leaf sur- faces. vii Data on rates of seedling emergence were also collected to determine if this characteristic is of adaptive significance. This study was conducted entirely with seedlings because: 1. A variety of species can be assembled for study at a single location with a minimum of preconditioning effects if seed is collected and the plants grown at a single location. 2. The seedling stage has been widely recognized as the most crucial stage in the development of a plant (Bates, 1924; Clark, 1961; Cleary and Waring, 1969). The results of this study are of potential interest to: l. Phytogeographers seeking information about factors which may influence the distribution of the North American spruces. 2. Physiologists concerned with changes in the plant caused by drought stress. 3. Geneticists and tree breeders who desire to understand the variability within the genus Picea. viii CHAPTER I EFFECT OF DROUGHT STRESS ON GROWTH AND DEVELOPMENT OF SIX SPECIES OF NORTH AMERICAN SPRUCE Included among the many ways in which a species of plant might l. The l. adapt to drought stress are the following: Swift passage through drought sensitive stages of development. Alteration of form to a more drought resistant type. purpose of this experiment was to: Determine which species of the genus Picea (spruce) studied were drought resistant and which were not. Determine the rates of seedling emergence. Compare seedling form to determine if this was altered by drought to a more drought resistant form, or if differences in form which have adaptive significance exist between seedlots. Materials and Methods Seed of six species of spruce were collected from native stands. Two collections were made for each species from geographically diverse areas (Table 1). In each TABLE 1. Seedlots of Picea used in this Study. Shade MSJIkresskxx Seedlot Species ,Origin weight Number Blu-SS P. pungens Steamboat Springs, 4(hng 8429 Cohmxflb BlurGS P. pungens Glenwood Springs, 5.1mg 8181 Cdkxado Eng-Col P. engelmannii Steamboat Springs, 3.5mg 8425 Cobmmmb EngeMont P. engelmannii Selway-Bitterroot 2.7mg 8589 Vfildanmss,ubnuxa WheAlas P. glauca Fairbanks,.Alaska 2.&ng 8517-23 Wh=Mich P. glauca Barbeau, Michigan Zihng 8550-54 SitsAlas P. sitchensis Juneau, Alaska 2ghng 8555 Sit-Ore P. sitchensis Otis, Oregon 2.6a; 8562-66 Bla-BC P. mariana Ft. Nelson, 1.2ng 8573-77 Brfljshcxdmmna BlaeMich P. mariana Barbeau, Michigan 1.3mg 8583-87 Reerue P. rubens Portneuf County, 3.¢ng 8603 Qwaxn Redrwva P. rubens Marlinton,‘west 4.9mg 8597 \firghfia stand, seed was collected from four or more trees, mixed, and weighed before sowing. About 40 seeds were sown to a uniform depth in plastic pots 22 cm tall and 16 cm in diameter in a glass- house in June 1972. They were grown in a soil mixture of shredded peat and sandy loam. The following equation relating percent of moisture in the soil to soil water potential was determined experimentally using a soil pres- sure membrane apparatus: loglO (% soil moisture) = 1.847 - .207 (log10 [-soil water potential (lbs.)]). Continuousfluorescentlighting supplemented natural day- light. Seedlings were thinned to 14 per pot 2 to 5 days after emergence, and to 7 per pot two weeks later. One blue spruce seedlot and one red spruce seedlot germinated very poorly. Seedlings in these lots were pulled out and seed from lots Blu-GS and Red-WVa were planted as replacements two weeks after the other lots were sown. For this reason, these two seedlots are treated separately in most of the analyses. The soil was kept continually moist for the first month after sowing, at which time watering was halted to allow establishment of different water regimes. Because the seedlings began to die rapidly, watering was resumed 10 days later and continued for an additional two weeks. At this point, trees were thinned to three uniformly spaced seedlings per pot. The pots were then allowed to begin drying out again, and three watering regimes were established. The pots were weighed daily and sufficient water was added to saturate the soil when the moisture content of the soil declined to 45%, 32% and 25% for the wet, medium and dry treatments, respectively. .These treatments are the equiva- lent of -.6, -3.4 and -8.3 atmospheres soil water potential. These levels were selected on the basis of results from a preliminary glasshouse test to determine maximum tolerable levels of drought stress and a review of previously reported experiments (Babaloa, Boersma and Youngberg, 1968; Boyer, 1965; Jarvis and Jarvis, 1963; Larson and Schubert, 1969; Larson and Whitmore, 1970; Sands and Rutter, 1959; Stransky and Wilson, 1964). This schedule resulted in the "wet" treatment being watered once every two or three weeks when the seedlings were small. Obviously all treatments were quite dry. There were six pots in each seedlot-water level treatment combination. Six months after sowing, the trees were harvested. Measurements taken on the seedlings included: stem diameter immediately below the cotyledons (calipered to the nearest .01 cm), total height (to the nearest .1 cm), fresh and dry weight of the root system, fresh weight of the shoot, dry weight of needles, dry weight of stem plus branches (all weights to nearest .001 grams), length of the longest root from the groundline to the tip (to the nearest .5 cm), and the mean length of five needles picked from the stem 0 at uniform intervals (to the nearest .1 cm). Simple cor- relations were calculated for all possible combinations of the measured parameters. The preSence or absence of a dormant apical bud at the time of harvest was also noted. Data on seedling size were analyzed by a factorial design analysis of variance with seedlots and water levels used as factors, pots as replicates and seedlings within pots as subsamples. Significance of differences between means of various seedlot-water level treatment combinations was determined using Tukey's HSD test (Steel and Torrie, 1960). The logarithms of the dry weights of the needles, stems, and roots were individually regressed on the logarithm of the total dry weight of the seedling for each seedlot- water level treatment combination. This was done separately for those seedlings which had apical buds and those which did not. Differences in the regression coefficients were tested by the method described in Steel and Torrie (1960, p. 173). Seedling emergence was determined by taking a daily count of emerged seedlings in each pot. Counts continued for ten days after the emergence of the last seedling. "Days to 50% emergence" was that day on which 50% of all seedlings in a pot had emerged. A one-way analysis of variance was performed on these data and differences between seedlots were tested by Tukey's HSD test. Survival of seedlings in each pot was calculated at harvest as a percentage of the trees alive when differ- ential watering was initiated. These data were analyzed by a factorial design analysis of variance, with seedlots and water levels as factors, and pots as replicates. Differences in percentages of trees having a dormant apical bud were analyzed by computing Chi-squared values in a contingency table for all possible pairs of seedlots and water levels. Results and Discussion Species considered to be from moist habitats (Sitka and red spruce) emerged significantly more slowly than species considered to be from more xeric areas (blue and Engelmann spruce). In every instance, the more southerly of the two seedlots of a species emerged more slowly than did the northern-most seedlot (Table 2). These results are consistent with the results of previous studies which have indicated that blue spruce germinates more rapidly than white spruce (Hanover and Wilkinson, 1969), and that red and Sitka spruce germinate more slowly than white, blue, black or Engelmann spruce (Heit, 1961). A correlation between rate of emergence and seed weight was found to be nonsignificant. In xeric areas it would seem advantageous for a seed to germinate and become established as rapidly as TABLE 2. Number of days from planting required for emergence of 50% of seedlings. Days to 50% Seedlot I Emergence Sit—Ore 141a Red—Que 13ab Sit-Alas 12bc Wh-Mich 12bc Bla-Mich llcd Wh-Alas llcd Bla-BC llcd Eng-Col lOde Blu-SS2 lOde Eng-Mont 9e 1Each entry is the mean of 18 pots, 20 to 40 seedlings per pot. Numbers not followed by a cannon letter are statistically different at the 5% level as determined by Tukey's HSD test. 2Seedlot BlurGS was sown slightly later than these seedlots and so germinated under somewhat different conditions. Days to 50% emergence forfflu4§3wws9. possible following the onset of favorable conditions, as these conditions might be of short duration. Both seedlot (Table 3) and water treatment level (Table 4) are significant factors determining survival of seedlings. The interaction of seedlot and water treatment level is not significant at the 5% level. Of the two seedlots with significantly lower survival rates, Sit-Alas had a very low survival rate in the dry treatment, thus TABLE 3. Percent of seedlings surviving after six months. Seedlot ' Percent Survival Wh-Alas 94.4a1 Wh-Mich 92.6ab Eng-Col 90.7abc Red-Que 88.1abc Eng—Mont 80.9abc Sit-Ore 76.9abc Blu-SS 75.9abc Bla-Mich 68.5abc Sit-Alas 66.7bc Bla-BC 64.30 lPercentages not followed by a common letter are statistically different at the 5% level as determined by Tukey's HSD test. TABLE 4. Percent of seedlings surviving after six months under three watering regimes. Water Regime Percent Survivall Wet 85.3a2 Medium 83.7ab Dry 71.3b 1Average of all seedlots except Blu-GS and Red-WVa. 2Percentages followed by a common letter are not statistically different at the 5% level as determined by Tukey's HSD test. lowering overall seedlot survival percentage, while seedlot Bla-BC had poor survival rates at all water treatment levels. Six months after sowing, the percentages of seedlings possessing dormant apical buds varied significantly between seedlots (Table 5), but not between water levels averaged over all seedlots (Table 6). Testing all possible pairs of water treatment levels by individual seedlots, the only significant difference found was in seedlot Wh-Mich, where 53% of the seedlings in the dry treatment had apical buds, but only 6.2% of the seedlings in the medium treatment had apical buds. Taken as a whole, the data indicate that drought stress does not have a strong influence on the formation of dormant apical buds. The formation of an apical bud, with the attendant halt in height growth, may, however, be of some significance in survival under drought conditions. The two seedlots possessing the largest percentage of seedlings with apical buds also had the highest survival rate. Within any given seedlot-water level treatment combination all measures of seedling weight were consist- ently and highly correlated with total dry weight of the seedling (r2 3 .85). Therefore, of all the measures of seedling weight, only total dry weight is presented (Figure l). The dry weights of seedlots Wh-Alas, Wh-Mich, Eng-Mont, Red-WVa, Sit-Alas and Sit-Ore are averaged and presented as 10 TABLE 5.--Percentages of seedlings in each seedlot which did not possess an apical bud six months after sowing. % Not Possessing Seedlot An Apical Bud Bla-Mich 100.0a1 Sit-Ore 100.0a Blu-ss2 95.1a Eng-Mont 92.3a Eng-Col 91.8a Red-Que2 90.2ab Sit-Alas 88.2ab Bla-BC 83.3ab Wh-Mich 66.0ab Wh-Alas 17.6b lAny values followed by a common letter are not statistically different at the 5% level when tested in a contingency table with the Chi-squared statistic. 2Seedlots Blu-GS and Red-WVa were planted slightly later than these seedlots. Values for Blu-GS and Red-WVa are 94.3% and 94.4%,respectively. 11 TABLE 6. Percentages of seedlings not possessing apical buds six months after sowing. % Not Possessing Water Regime ' An Apical Bud Wet 82.4l Medium 83.3 Dry 72.1 1None of the values are significantly different at the 5% level when tested in a contingency table with the Chi- squared statistic. TABLE 7. Significance levels for differences between various measures of growth for ten seedlots of spruce at three watering levels. Seedlot X Parameter Water Level Seedlot Water Level Total dry weight *** *** *** Stem diameter *** *** N.S. Shoot height *** *** *** Needle length * *** *** Root length ** *** ** * p < .05 **p < .01 *** p < .001 N'S°Non significant at the 5% level. UJRTQA-‘s OUT Foe LHP'S teem HERE 0 k) -> 12 Figure l.--Mean total dry weights of seedlings of twelve seedlots of spruce grown under three watering regimes. Differences in seedling dry weight significant at the 5% level include: Blu-SS wet > Blu—SS dry. Blu-GS wet > Blu-GS dry and medium. Bla-Mich wet > Bla—Mich dry and medium. Within the: Wet Treatment Blu-GS>Eng-Col, Eng—Mont, Wh-Alas, Wh-Mich, Sit-Alas, Sit-Ore, Bla-BC. Bla-Mich>Eng-Col, Eng-Mont, Wh-Alas, Wh-Mich, Sit-Alas, Sit-Ore. Red-Que>Sit-A1as, Sit-Ore. Medium Treatment Blu-GS>Wh-Alas, Sit-Alas, Bla-BC. Dry Treatment No significant differences. SEEDLING DRY WEIGHT (a) LG 0.8 0.6 0.4 0.2 13 /Blu-SS ./. )Blo -Mich / l/j-eIu-Gs / 4’ Red fQue .’ BIG-BC "7‘". Eng -Co| /-Composite DRY MEDIUM WET WATER REGIME Figure 1 14 a composite line because they responded nearly identically to all treatments. Judging from the climatic conditions at the point of origin of the various seedlots, other experiments (see Chapter II), and the growth of seedlings under "normal greenhouse conditions," it is evident that the drought resistant seedlots were those in which the size of the seedlings varied with water treatment levels. Apparently, the seedlings from wet habitats were so stressed at all water treatment levels that little growth could occur. It should be noted that some of the difference be- tween total dry weight of the seedlings from the various seedlots was due to differences in seed weight. The corre— lation of mean seedling dry weight for each seedlot and seed weight was significant at the 5% level (with a corre- lation coefficient of .61) for the dry treatment. The corre- lation of seedling dry weight and seed weight was not signi- ficant for the medium and wet treatments, however. Corre- lation of the other size parameters with seed weight indicated that with the exception of needle length, there was no signi— ficant correlation between any parameter of seedling size and seed weight at the wet treatment level. At the medium treatment level, only needle length and stem diameter were correlated with seed weight. At the dry treatment level all size parameters were significantly correlated with seed weight. Based on the number of significant differences with- in a seedlot caused by water treatment level, the parameters 15 of growth in decreasing sensitivity to drought stress are: stem diameter > shoot height > total dry weight > root length = needle length (Tables 8 through 11). The impact of drought stress on needle length is much more dramatic than Table 10 indicates. Visual obser- vation indicated that the needles elongating and/or forming immediately after a pot was watered were long, straight and "normal" in appearance. Needles elongating and/or forming while the plant was under drought stress were considerably shorter and oftentimes twisted in a helical coil around the plant stem. Following watering, these twisted and coiled needles did not alter their appearance. Examples of this coiling behavior were observed in seedlots Blu-SS, Blu-GS, Eng-Col and Wh-Mich. The technique of averaging the lengths of five needles per tree doubtless helped to obscure these differences, by including needles formed under both wet and dry conditions. The depth of the soil the seedlings were growing in was about 20 cm, and it is evident that in most cases the roots extended to the bottom of the pot. It is possible that when the root tip encountered the bottom of the pot, the growth of the root may have been altered in an un- natural manner, and for this reason the root lengths are of dubious significance. 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Imam I95 I53 Imam uoHooom .moEHmmu mcfiuouMB wounu Horas c30uo mocfiavoom oonuAm mo mgumcoa uoou cum: .Ha mqmdh .poHcoom oEmm ms» mo ucoEumouu uoz oru coco Ho>ma mm ocu um uconMWHU >HocooHMHcmHm I .oEHu mcflzom cfi mooconommfip mo omsmooa Ho>oa Houo3 oEdm on» us muofipoom nonuo umcflmms powwow uOZI P. .Ho>oa wm onu um ucouowwflp haucmoamficowm uoc mum uouuoa coufioo m an Um3OHH0m any n casuaz mmuav>a moa mm mm mm mm mm mm Boa om moa «.IMH «00M mnn mm om om om om om ohm onoa onoa QBHM mma one Edapoz NHH QHH onoa oooa um um ohoa naa UQOH ULOH aha I.HI to; n r IlllllllllIIIIIIIIIIIIIIIIEDIIllllllllllllllllllllll m>3 moo noflz um who mafia coaz mafia ucoz H0O O mm was ox poop: Icmm Icom ImHm Imam Iuam Iuflm Izz ch Imam Imcm Izam Inam noflemmm H .moeflmou mafinoums wounu Moons ckoum mmcflaooom monumm mo mzumcoa canon: coma .OM mumda 18 organs has been shown to be the most meaningful way to study the distribution of dry matter in plants (Ledig, Bormann and Wenger, 1970). The smaller the regression coefficient of the regression of the logarithm of the dry weight of a plant organ on the logarithm of the total dry weight of the plant, the slower the organ is growing compared to the rate at which the whole plant is growing. In this study, equations of the form: loglo(dry weight of an organ) = a + b (log10[total dry weight of seedling]) related the two dry weights in such a way that the regres- 2>.85 sion was usually significant at the 0.1% level, with r (Tables 13 through 15). Seedlot-water level treatment combinations contain- ing sufficient seedlings possessing apical buds to construct regression equations and allow comparison with regression equations for seedlings which did not possess apical buds were: Blu-SS (dry), Wh-Alas (dry, medium and wet), Wh-Mich (wet and dry) (Table 12). The equations in Table 12 indicate that changes in the relative growth rate of the organs of a plant sometimes occur when the plant produces an apical bud and stOps height-growth. Seedlots within a water treatment level which had significantly different regression coefficients for the HQ. \. L 19 u 1 no. v L $ unefloB mum noon CHOOH u w W «. xmma. I os~.HI u » xeeo. + oae.- u » spasms sue Anson thoH u x was SIHzIa; uanmB mum woozm OHOOH u y a x00». + mmm.I n w thm.a + mmo.l n w unmflo3 amp Hmuou CHmOH u x uvz mmwwIcs BLUHOB xup mammoc onOH H w a xmoo. + mmo.I u w xoma.a + mma.l u » ucmflo3 hum Hmuou oamoH u x Edaooz .mqum ao>oq mcsm Hmoflmd mmzm Hmofimd mcHLMAHm> ofiqmtd v;wooue OUCMUHchon swag mmcflacmom usonuflx mmcflapoom Loumz I III I IIIII II: I, II II IIIII..I1I. I III. IIIII Isl. IIIIstIIII, III I, III! III. 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Mmom.a onvm. m>3lpom novmm. vah. womH.H maeloom mmmoé smMH.H wwma.a QUHSImam Mmma.a nomm.a MmmH.H omImam omvo.H onno.a ommm. OHOIuflm mNmm. ommm. omom. mmaéIuHm mhma.a sumo.a mmvo.a SUHSInz II maam.a wham.a mmamlnz ommo.a ommo.a omvm.a ucozImcm MmNH.H omm0.H omma.a HOOImcm MNHN.H mwma.a ome.H mOIDHm awnmo.a Qmao.a Homam.a mmlsam who Esflpmz umB uoapmom usmflm3 who Eoum camoH mo 2 .unmfloz xup Hmuou camoa co oammoumou on» MOM mucoHOmeooo coflmmoumom .wa mam<9 22 .Hm>ma wm was an ucmummmfla MHDCMUAMHcmwm no: mum Houuoa coEEoo m an woBOHHom 30H m ca mosam> >c¢ a comm.a amen. ammso.a m>zIemm nmama.a nmmm.a mmmm. msoIemm amamm. nmmm. maao.a BUHzImHm _nmm~o.H sane. ammH.H omImHm mamm.a ammo.a ammo.a . mHOInHm ammo.a mamo.a mmmm.H mmaaIuHm ammm. Moms. «new. aonInz II amen. mmam. mmHBlu-SS, Bla-BC, Bla—Mich. Eng-Col, Red-Que>Bla-BC. Medium Treatment Blu-SS, Blu-GS, Eng-Mont>Red-Que. Dry Treatment Blu-GS>Eng-Col, Eng-Mont, Red WVa. Sit-Alas, Bla-Mich>Red-WVa. Bla-Mich>Eng-Mont. Seedlots within a water treatment level which had significantly different regression coefficients for the regression of logl0 stem dry weight on log10 total dry weight were: Wet Treatment Blu-GS, Blu-SS, Eng-Col, Eng-Mont, Wh-Alas, Bla-BC, Bla-Mich, Red-Que>Sit-Alas. Blu-SS, Red-Que>Sit-Ore. Medium Treatment Bla-BC>Blu-SS, Blu-GS, Eng-Col, Wh-Mich, Sit-Alas, Sit-Ore, Bla-Mich, Red-Que. Blu-GS, Wh-Alas, Bla-Mich>Sit-Ore, Red-Que. Wh-Alas>Eng-Col>Red-Que. Dry Treatment Blu-GS>Eng-Col, Eng-Mont, Red-WVa. Sit-Ore, B1a-Mich>Red-Que. B1a-Mich>Eng-Mont. Seedlots within a water treatment level which had significantly different regression coefficients for the 24 regression of log10 root dry weight on log10 total dry weight were: Wet Treatment Sit-Alas, Sit-Ore, Bla-BC, B1a-Mich>Blu-GS, Eng-Col, Wh-Mich, Red-Que. Blu-SS, Wh-Alas>Wh-Mich. Sit-Alas, Bla-BC>Blu-SS. Sit-Ore, Bla-BC>Wh-Alas. Medium Treatment Red-Que>Blu-SS, Blu-GS, Eng-Col, Wh-Alas, Bla-BC, Bla-Mich. ' Eng-Col, Sit-Alas, Sit-Ore>B1u-GS. Sit-Ore>Wh-Alas. Dry Treatment Red-WVa>Blu-SS, Blu-GS, Eng-Mont, Wh-Mich, Sit-Alas, Eng-Col, Eng-Mont>B1u-GS, Bla-Mich. For simplification, four seedlots are selected for direct comparison of responsive and non-responsive seedlots. Of the ten seedlots sown at the same time, Bla-Mich and Blu-SS are responsive seedlots, while Wh-Mich and Sit-Alas were selected to represent non-responsive seedlots. The selection of Wh-Mich and Sit-Alas were made on the basis of their having the lowest ratio of dry weight of seedlings grown in the wet treatment to dry weight of seedlings grown under the dry treatment. Seedlot Eng-Col was ignored in View of the fact that the dry weight of the seedlings varied in an atypical manner with water treatment level. Comparisons were made at the wet treatment level only because it is at this level that the greatest differences 25 in total dry weight were found. The responsive seedlings (those adapted to drought stress) apportioned equal or smaller quantities of dry matter into needles, and equal or larger quantities of dry matter into stems than did the seedlings adapted to moist conditions. There are no clearcut trends with regards to dry matter apportioned into root production. It has long been argued that the relative sizes of the water absorbing organs (roots) and the water losing organs (leaves and possibly stems) are of great importance in determining the water balance of the plant. The data presented here indicate that by apportioning lesser quanti- ties of material into needles, the plants adapted to drought stress may be adapting to the conditions of the environment in which they live. The recent recognition of the ontogenetic shift in relative sizes of plant organs has led to a re-examination of old data (Ledig and Perry, 1965) and the construction of new experiments (Ledig, Bormann and Wenger, 1970), all of which have failed to demonstrate the ability of drought stress to alter the relative sizes of plant organs. Perhaps this failure was the result of not using severe enough drought conditions. Summary The speed of seedling emergence of the spruce seedlots appears to be of adaptive significance in that 26 seedlots from arid areas emerge faster than seedlots from moist areas. In arid areas, this might be advantageous in that favorable conditions might be of short duration. Drought stress significantly reduced seedling survival, but appeared not to affect the tendency for a seedling to produce a dormant apical bud. Drought stress also reduced growth of the seedlings. Drought stress also proved effective in causing a change in the relative size of some plant organs in certain seedlots. These differences were largely in an increase in stem weight at the expense of increased needle production. As needles, but not stems, are water losing organs, the changes could have adaptive significance. CHAPTER II THE EFFECT OF DROUGHT STRESS ON PHOTOSYNTHESIS AND TRANSPIRATION OF SPRUCE SEEDLINGS Drought stress affects several physiological proc- esses within plants; including decreasing net photosynthesis, growth, and transpiration (Sands and Rutter, 1959; Jarvis and and Jarvis,Il965; Pallas, Michel and Harris, 1967; Babaloa, Boersma and Youngberg, 1968). Aside from a study on two ecotypes of Douglas-fir done by Zavitkovski and Ferrell (1968), few studies have compared the impact of drought stress on these physiological processes using closely related species of trees. This study was designed to measure and compare the effect of an imposed drought on rates of photosynthesis and transpiration in several species of spruce native to North America. Materials and Methods Two seedlots of each of six species of spruce as previously described in Table l were used in this study. Trees were grown from seed in plastic pots 16 cm in diameter and 22 cm tall. The soil was 91% sand and 9% 27 28 of a standard greenhouse mixture of sandy loam and shredded peat. The following equation was derived experimentally using a soil pressure plate membrane apparatus which relates soil moisture content to soil water potential: loglo(% soil moisture) = .630 - .174 (loglO[-soil water potentia1][lbs.]). This equation was derived to allow the results to be pre- sented in the more useful terms of soil water potential. Thirty to forty seeds were sown per pot and one to two weeks after germination the seedlings were thinned to five uniform trees equally spaced in each pot. Trees were grown under continuous artificial light provided by 40 watt cool white VHO flourescent bulbs at an intensity at soil level of 200 microeinsteins In"2 sec-1. The pots were watered daily so that the soil water level never dropped below field capacity. Five and a half months after sowing, when the trees were actively growing, the soil surface was sealed with wax, and all watering was discontinued. All pots were weighed daily from the time of sealing until a pot stopped losing weight. When this point was reached the trees were removed from the pot and the weight of the pot and the wax determined. The soil was oven-dried and weighed. The trees were oven dried and the needles 29 stripped off and weighed. The loss of water through un- sealed drainage holes in the bottom of the pot was cor- rected for by noting weight losses of wax Sealed pots containing no trees. Transpiration was then computed on the basis of water loss per unit of dry weight of needle per day. Three seedlots, Blu-SS, Wh-Alas, and Sit-Alas were simultaneously used to determine photosynthetic rates as the soil in the pots dried. The net photosynthetic rate of the seedlings in each of four pots for each seedlot was determined either daily or on alternate days, depending on the rate of change of the photosynthetic rate. Measurements were made in a closed plexiglass chamber system connected with a Beckman C02 infrared gas analyzer equipped with a strip chart recorder. Net photosynthetic rates were deter- mined from the rate of depletion of CO2 in the system from 330 to 270 ppm. Illumination was provided by a 400 watt Mercury lamp, giving an intensity of 600 microeinsteins m-2 sec"1 at the top of the seedlings. Temperature in the chamber was 22° i 1.5°C. Photosynthetic rates were expressed on the basis of dry weight of the needles. Photosynthetic measurements were repeated until the rate reached zero or less. At that time, the wax seal was punctured, the soil thoroughly watered and the pot resealed. Measurements were then resumed until two conditions were satisfied. First, the photosynthetic rate had to demonstrate an increase from 30 the previous day. After that, measurements were continued until the net photosynthetic rate declined for two consecu- tive days. Results Photosynthesis From the time that excess water had drained from the pot after the final pre-sealing watering until the soil had dried to field capacity, the photosynthetic rate was relatively constant. This constant rate was used as a "basal rate" of photosynthesis under conditions of no water stress, and all subsequent rates are expressed as percentages of this "basal rate" (Figure 2). The slope differences between the Blu-SS and Sit- Alas seedlots are significant at the 2% level. No other differences are significant at the 5% level. Seedlings in lot Blu-SS recovered to their maximum rate of post-drought photosynthesis significantly faster than Sit-Alas seedlings, while Wh-Alas seedlings recovered to a significantly higher percentage of their "basal rate" of photosynthesis than did Sit-Alas seedlings (Table 16). Transpiration Like photosynthesis, transpiration per unit dry weight of needle was relatively constant for a given pot of seedlings from the time the pot ceased draining after 31 Figure 2.-- Relationship between soil water potential and photosynthetic rate in blue spruce, white spruce and Sitka spruce. u 2:9... 28.: 34:230.. «LE; .__om 32 ‘ O. —--‘ 3 Q 00.: Ont OT DI 0.... _ A . d . I o _ .0 _ 0va _ I 8 30 _ wcxv‘ _ 1&3 n I o¢ mmI$Q “ 1 om _ .1 om .00. _ u I a. _ _ 3.1th 0|13HlNASOlOI-ld 'IVSVS :IO °lo 33 TABLE 16. Characteristics of post-drought recovery of photosynthetic rate. Mean Days to Highest - Recovery of Photo— Best Post-Drought Rate Seedlot synthetic Rate Following of Photosynthesis as an Imposed Drought Percent of Basal Rate Blu-SS 2.0al 30.8ab Wh-Alas 3.7ab 85.8a Sit-Alas 5.0b 2.8b lAny values in a column followed by a common letter are not significantly different at the 5% level as determined by Tukey's HSD test. the final pre-sealing watering until it reached field capaci- ty (Table 17). Significant differences were found in the regression coefficients for different seedlots when the log10 of the transpiration rate was regressed on the loglo (-soil water potential) for values of soil water potential between field capacity and permanent wilting point (Table 18). Although in certain seedlots a relationship other than the log-log relationship between transpiration and soil water potential gave a slightly higher r2 value, the log-log relationship presented here was in general superior. A rank correlation value significant at the 1% level was found which indicates that the more a seedlot transpired per unit dry weight of needles under conditions of no drought stress, the more it altered its transpiration rate in response to decreasing soil water potential. A illiitlllilllll 34 TABLE 17. Transpiration rate in grams of water lost per gram dry weight of needles per hour when soil moisture level was between saturation and field capacity. Seedlot _ Transpiration Rate Blu-GS .60a1 Eng-Col .46ab Bla—BC .44ab Eng-Mont .43ab Bla-Mich .40b Wh-Alas .40b Sit-Alas .38b Wh-Mich .34b Blu-SS .32b Sit-Ore .30b 1 Transpiration rates followed by a common letter are not significantly different at the 5% level as determined by Tukey's HSD test. TABLE 18. Regression coefficients for the regression of loglo transpiration rate (grams water lost per gram needle dry weight per hour) on loglo (-soil water potential). Seedlot Regression Coefficient Blu-GS —.62a1 Eng-Col -.59ab Bla-Mich -.59ab Bla-BC -.56b Eng-Mont -.46c Sit-Alas -.4lcd Red-Que -.39cd Red-WVa -.36cd Wh-Mich -.34d Wh-Alas -.34d Sit-Ore -.33d Blu-SS -.28d 1 Regression coefficients followed by a common letter are not significantly different at the 5% level. 35 rank correlation value significant at the 1% level was also found between those seedlots having the highest rate of transpiration under conditions of no water stress and those seedlots having the lowest rates of transpiration when the soil water potential reached —15 atmospheres. Discussion Blue spruce is native to areas that appear to be quite dry, although the species mostly occurs near streams and other moist microclimates (J. W. Hanover, personal communication). Sitka spruce is native to moist West coast areas. White spruce is native to areas that are inter- mediate in terms of aridity. This study indicates that there are at least three ways in which a species adapts to drought conditions. First, its photosynthetic rate can be less sensitive to changes in soil water potential, secondly it can recover rapidly from a drought stress, or thirdly, it can recover, after a drought stress, to a photosynthetic rate nearly equal to its pre-drought photo- synthetic rate. From the results presented here, white spruce and blue spruce are not significantly different in their adaptation to drought. However, earlier tests (see Chapter I) indicated blue spruce is much better adapted to drought stress than white spruce. The most reasonable explanation of this is that in the earlier tests all seedlings were Illllll. I Iii 1! 36 subjected to an equal level of stress before rewatering. Speed of recovery and extent of recovery may well be dependent on the soil water potential at which the trees are rewatered. A different response might occur if all of the seedlings in this study were rewatered at a constant soil moisture content rather than when the net photosynthetic rate of the seedlings in a pot fell to zero. The seedlots whose seedlings alter their transpi- ration rates most in response to changes in soil water potential, Blu-GS, Eng-Col, Bla-Mich and Bla-BC, appear to be the most drought resistant of the twelve seedlots tested (see Chapter I). Two exceptions to this pattern are: (l) Seedlot Red-Que, which appeared to be somewhat drought resistant in earlier tests, but which alters its transpiration rate only slightly in response to lower soil water potentials. However, the transpiration data for this seedlot are based on the results of a single pot, rather than on four pots as in the other seedlots. The relia- bility of this measurement may be low. (2) Seedlot Blu- SS which changes its transpiration rate least in response to changes in soil water potential, but appears to be the most drought resistant of all seedlots. Perhaps it is so well adjusted to drought that its transpiration rate was never restricted. It seems reasonable that drought resistant trees should be able to alter their transpiration rates in 37 response to soil water potential. If drought resistant trees had continually high transpiration rates, drought conditions would probably be fatal as the foliar tissues would dessicate and die. As both CD2 and water vapor are exchanged through the stomates, continuously low transpi- ration rates would probably be coupled with continuously low photosynthetic rates. If this were the case, the plants would be unable to take advantage of periods of adequate moisture, and this would be detrimental in their competition with other species. The suppression of photosynthesis by soil moisture levels above field capacity has been noted in some studies (Clark, 1961; Pallas, Michel and Harris, 1967; Schneider and Childers, 1941). Another study failed to find this suppression (Zavitkovski and Ferrell, 1968). It has been suggested by Clark (1961) that this suppression is due to poor soil aeration at very high soil moisture levels. A very sandy soil, such as that used in this study, might allow for better aeration at high soil water potentials than would a heavy soil (Buckman and Brady, 1967). It appears that in cases where a sandy soil was used no suppression of photosynthesis by soil moisture levels above field capacity was observed, while experiments using heavier soils consistently demonstrated suppression of photosynthesis when soil moisture contents were above or at field capacity. 38 The extent and speed of recovery of the net photo- synthetic rate following relief from drought stress reported here for white spruce agrees quite well with values previ- ously reported (Clark, 1961). This study indicates that species native to dry areas differ significantly from a species native to a wet habitat in at least three ways: (1) The net photosynthetic rate is less sensitive to changes in soil water potential. (2) Photosynthetic rates of species native to xeric areas recover faster following relief from a drought stress than a species native to a moist habitat. (3) Species adapted to dry areas recover a greater proportion of their pre- drought photosynthetic rate than species adapted to wet areas. Transpiration rates are apparently not correlated with drought resistance. However, those species having the highest rates of transpiration under conditions of no water stress tend to have the lowest rates of transpiration under conditions of high water stress. CHAPTER III THE EFFECTS OF DROUGHT STRESS ON SURFACE WAX DEVELOPMENT IN SIX NORTH AMERICAN SPECIES OF SPRUCE The plant cuticle, located at the interface of the leaf and the atmosphere, plays an important role in the regulation of plant water relations. Scanning electron microscopy allows the easy examination of the cuticular surface for studying the adaptive characteristics of various cuticular features. The leaf surfaces of many plants including a wide variety of conifers are covered with epicuticular waxes, at least some of which take the form of rods, fibers, spheres or plates. The epistomatal chamber appears to be at least partially plugged with wax in many conifers (Hanover and Reicosky, 1972). The development of this structural epicuticular wax (or bloom) which contributes to the "blue" coloration of leaves is apparently under both genetic and environ- mental control (Denna, 1970; Cameron, 1970; Hallam, 1970; Banks and Whitecross, 1971). Heavier wax blooms have been observed more often in individuals of a given species growing in arid regions than on individuals in more mesic 39 40 areas (Daly, 1964). It has been shown that the removal and/ or disorganization of this structured wax results in higher rates of water loss from plant organs (Possingham, et al., 1967; Hall and Jones, 1961; Denna, 1970). Theoretical calculations (Jeffree, Johnson and Jarvis, 1971) would seem to support the idea that epicuti- cular waxes are an adaptive advantage in dry regions. Rook, et al. (1971) found that the imposition of drought stress on Pinus radiata did not alter the appear- ance of needle surface waxes. In this study, the epicuticular waxes of six North American species of spruce were examined after they had been growing under alternating dry and wet conditions. Materials and Methods Seed from each of the twelve previously described seedlots (see Table l) were sown in plastic pots in a sandy loam soil enriched with shredded peat to 20% organic matter on January 9 in a glasshouse. The plants received natural daylight as well as continuous artificial illumi- nation from fluorescent bulbs from the time of seedling emergence through the end of the experiment. Through April 1 the pots were kept well watered. Watering was done so as to avoid wetting the foliage. The height of the trees was measured at that time. The trees were then left unwatered until May 2, at which time the height of each tree was Ililll'llll' [II]! III] 41 again measured. The plants were watered on May 2 and as needed thereafter to avoid further drought stress through- out the remainder of the experiment. Six to eight weeks following the end of the drought period, observations were made of the epicuticular waxes of the needles from repre- sentative trees. Needles growing on those parts of the main stem that were formed before, during and after the drought period were examined. Cotyledons and the needles immedi- ately above them were not used, nor were very young succu- lent needles. The magnitude of the drought stress was sufficient to kill more than 50% of the trees in two seedlots. Needles were picked fresh from the trees and pre- pared for observation under the scanning electron microscope within one hour. Needles were gold coated prior to exami- nation in an Applied Science Laboratory EMX-SM instrument. Photographs were taken by a Polaroid camera at lOOOX magnification except as noted. Care was taken to photo- graph a "typical" section of each needle. Needles were examined during six different sessions over a two week period. Results In most cases, the surfaces of all needles, regard- less of species or position on the stem looked basically [{(IIH[I'H[[[[[tflll‘lllllll‘H‘H {all 42 alike. Common features include the presence of a large amount of wax in the epistomatal chamber which resembled filaments joined together in a three dimensional fishnet type of configuration (Figure 3a). Clusters of wax fila- ments are spread across the needle surface. Between these are areas of varying size which contain little or no structured wax (Figure 3b,c). Some exceptions to this general uniformity of the material were noted. Occasionally, a needle was observed in which the epistomatal chamber was free or nearly free from filamentous wax (Figure 3d,e). A lack of clusters of filamentous wax on the surface of the needle was invariably associated with this condition. Instead, the surface exhibited an undulating appearance which was in sharp contrast to the relatively smooth surface normally observed between filamentous wax clusters (Figure 3f). This condi- tion was very consistent in seedlot Eng—Col on needles formed prior to the drought and was also observed on some of the needles in the Sit-Ore seedlot that were formed prior to the drought (Figure 4a). Needles of seedlot Wh-Alas formed prior to the drought appeared to have either fewer but considerably thickened wax filaments in the epistomatal chamber (Figure 4b), or the chamber appeared to be completely occluded with what appeared to be molten wax (Figure 4c,e,f). In these cases, the clusters of fibrous surface wax were less 43 Figure 3.-—Surface waxes of spruce foliage, Part I (a, pre- drought Blu—SS 3125X; b, post-drought Eng-Col 625x; c, pre-drought Red-WVa 625x; d, post-draught Eng- Col 625x; e, pre-drought Sit-Ore 625x; f, pre-drought Eng—Col 625X). 44 , firmer wax. I... . I wwwpbc‘u . t... . A. a. was. \fidnfik, ND“? '9‘ Q 13 ...~....v.. \ Figure 3 45 Figure 4.--Surface waxes of spruce foliage, Part II (a, pre- drought Sit-Ore 625x; b,c, pre—drought Wh—Alas 625x; d, pre-drought Red—Que 1250X; e,f, pre-drought Wh- Alas 625X). 46 C Figure 4 47 fibrous and more amorphous. This condition was also observed on some needles formed prior to the drought in seedlot Red-Que (Figure 4d). Thus, the only distinctive and relatively consistent differences observed in this study occurred in needles prior to the drought period in seedlots Eng-Col and Wh-Alas. In the Eng-Col seedlot, the needles appeared to have little or no fibrous structured surface wax and the epistomatal chambers appeared to be free of wax. In seedlot Wh-Alas, the needles formed prior to the drought had stomates which for the most part appeared to be completely occluded with wax 0 Discussion The epicuticular waxes on needles formed prior to the drought differed considerably from the waxes on needles formed after the drought stress in seedlots Wh-Alas and Eng-Col. This would indicate that the extent of drought stress at the time of needle elongation is not, in itself, a sufficient explanation for surface wax differences. The striking differences observed in seedlots Eng- C01 and Wh-Alas could be explained in several ways. The waxes were either formed in the observed configurations or their configurations were altered after formation. If they were formed originally in the observed configurations, two explanations are offered: First, the differences could be due to developmental changes as the plants grew older. 48 This would necessitate the coincidence of a developmental change with the imposition of a drought stress, which is unlikely. A second possibility is that the imposition of a drought stress triggered a change in the subsequent formation of surface waxes that was irreversible, at least for the duration of the study. If the presence of wax is indeed a useful adaptation to drought, then it might be that drought would trigger increased wax formation on new needles. This explanation is supported by the response of seedlot Eng—Col. However, seedlot Wh-Alas seems to respond opposite to this pattern; pre—drought formed needles had occluded stomates and the needles formed during and after the drought had less completely occluded stomates. Post-formation alteration of the surface waxes is another possible explanation of the observations. This might be the result of mere "weathering" (Reicosky, 1973) or it could be an active plant process. It is possible that the undulations of the surface of the Eng-Col needles are a result of the breakdown of pre-existing fibrous wax clusters. In the case of the pre- drought formed needles in seedlot Wh—Alas, it is not diffi- cult to imagine a progression from fine fibrous filaments in the epistomatal chambers to thickened filaments, to the "molten wax" stage. Presuming post-formation alteration does occur, one advantage to the trees in seedlot Wh-Alas can readily 49 be seen. In times of severe drought, the sealing of the stomates would tend to prevent water loss which would be an adaptive advantage. There are two reasons why the pre- drought needles might be sealed whereas those needles formed during the drought would not be. Clark (1961) has shown that the photosynthetic rate of older needles is less than that of fully formed new needles. Thus, stomatal occlusion of the older needles by wax would be less disadvantageous to photosynthesis than if the younger needles were sealed off. Secondly, as the environment is known to affect plant anatomy, as in the case of sun and shade leaves, it is possible that the needles formed during the drought are anatomicly adapted to the dry conditions while the needles formed during the moist period are so poorly adjusted to drought that adjustments are made by wax occlusion of the epistomatal chamber. It should be noted that seedlot Wh-Alas had, in other studies, an unusual tendency to set buds and halt height growth in response to'drought. This would certainly be compatible with the sealing of stomatal openings, with the attendent lowering of the photosynthetic rate of the seedlings. In summation, there appeared to be little difference between the surface waxes of any of the species examined when they were grown under similar conditions. It appeared 50 that conditions of drought stress influenced the needle surface waxes in only two of the twelve seedlots examined. These differences were of an Opposite nature in the two seedlots, but both were of possible importance in improving the water status of the plant during droughty periods. CHAPTER IV CONCLUSIONS AND RECOMMENDATIONS The responses of the six species and seedlots to drought, as indicated in Figure l are highly variable and not in the expected pattern based on the habitat from which the seed was collected. From the statistical analysis it must be concluded that a northern Michigan black spruce seedlot behaved in a manner similar to blue spruce. This is a somewhat surprising result in View of the fact that the black spruce in question were growing in a swamp, an area of apparently adequate moisture, while blue spruce grow in regions of apparent aridity. The finding that drought stress can cause changes in the relative size of the organs of seedlings is without precedent, and may be an adaptive feature allowing for growth under a variety of conditions. However, the rather large number of studies that have shown that drought stress is not able to change the relative size of plant organs would seem to indicate that this adaptive mechanism is used only under extreme drought conditions. The results of the photosynthesis study fit well with the expected pattern. Species adapted to dry areas 51 52 are less affected by drought than species adapted to wet areas. In addition, this study indicates that more than one aspect of the response of a plant to photosynthesis must be investigated to obtain an adequate picture of plant adaptation. The differences between seedlots for both rate of transpiration under conditions of no soil water stress and rate of change of the transpiration rate with changes of soil water potential do not consistently form an inter-' pretable pattern. Further experiments appear necessary before conclusions about the adaptive role of transpiration in the genus Picea can be made. The differences between surface waxes of the spruces are slight or non-existant. The conditions of this experi- ment may not have been the best for observing differences between species, but it is evident that drought alone is not a sufficient stimulus to the immediate change of sur- face wax characteristics. Further research in certain areas touched by this study that might prove fruitful are: 1. An investigation of the root development of young seedlings. It is quite probable that early root development is an important determiner of drought resistance. Casual observations made during this study indicate that large differences exist between species. 53 2. An investigation of the rates of growth of these species using higher moisture levels. Several seed- lots were obviously stunted at all water levels used in this study. What might occur at wetter treatment levels is hard to say, but of great interest. 3. An investigation of the water used per gram of dry matter produced (water efficiency) between water levels and seedlots might be interesting. Data was col- lected for this purpose in this experiment, but the variation within treatments was too high to allow interpretation. 4. The effect of drought stress on surface wax development should be followed over a long period of time. Perhaps a drought stress occuring during bud growth in the fall would result in waxier needles during the following growing season. 5. A study of the anatomy of the needles, stems and roots of the species might be interesting in that structural differences might suggest functional changes within the plant with regards to water relations. 6. A study designed to determine if some of the differences found here are in fact of adaptive significance. This might well be carried out in the natural ranges of several of the Species. BIBLIOGRAPHY 54 BIBLIOGRAPHY Babaloa, O., L. Boersma and C. T. Youngberg. 1968. Photo- synthesis and transpiration of Monterey pine seedlings as a function of soil water suction and soil temperature. Plant Physiol. 43: 515-521. Banks, J. C. G. and M. I. Whitecross. 1971. Ecotypic variation in Eucalyptus viminalis Labill. I. Leaf surface waxes, a temperature X origin interaction. Aust. J. Bot. 19: 327-34. Bates, Carlos G. 1924. 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