Osmoregulation in Cotton in Response to Water Stress' I
Total Page:16
File Type:pdf, Size:1020Kb
Plant Physiol. (1981) 67, 484 488 0032-0889/81/67/0484/05/$00.50/0 Osmoregulation in Cotton in Response to Water Stress' I. ALTERATIONS IN PHOTOSYNTHESIS, LEAF CONDUCTANCE, TRANSLOCATION, AND ULTRASTRUCTURE Received for publication June 13, 1980 and in revised form October 13, 1980 ROBERT C. ACKERSON AND RICHARD R. HEBERT Central Research and Development Department, Experimental Station, E. L du Pont de Nemours and Company, Wilmington, Delaware 19898 ABSTRACT paper in this series defines the role of specific cellular carbohy- drates in stress adaptation (2). Cotton plants subjected to a series of water deficits exhibited stress adaptation In the form of osmoregulation when plants were subjected to a subsequent drying cycle. After adaptation, the leaf water potential coincid- MATERIALS AND METHODS ing with zero turgor was considerably lower than in plants that had never Plant Material. Cotton (Gossypium hirsutum L. Tamcot SP37) experienced a water stress. The relationship between leaf turgor and leaf plants were grown from seed in controlled environment facilities water potential depnded O leaf age. as described (3). When the fifth leaf above the cotyledons was Nonstomatal factors severely limited photosynthesis in adapted plants about 75% expanded, one set of plants was subjected to repetitive at high leaf water potential. Nonetheless, adapted plants maintained pho- water stress cycles while another set was well watered. Each stress tosyPnthesis to a much lower leaf water potential than did control plants, in cycle consisted of allowing plants to dehydrate until midday leaf part because of increased stomatal conductance at low leaf water poten- water potentials reached approximately -20 bars. Dehydration tials. Furthermore, adapted plants continued to translocate recently derived required 24 to 48 h, depending on plant age. Five days ofrecovery photosynthate to lower leafwater potentials, compared with control plants. (plants well watered) were interspersed between successive stress Stress preconditioning modffied cellular ultrastructure. Chloroplasts of cycles. Plants were subjected to a total of five such cycles. Midday fully turgid adapted leaves contained extremely large starch granules, leaf water potentials of control plants ranged from -5 to -8 bars. seemed swollen, and had some breakdown of thylakoid membrane struc- Plants that had been subjected to the five stress cycles are referred ture. In addition, cells of adapted leaves appeared to have smaller vacuoles to as "adapted" plants. This should be construed as referring to and greater nonosmotic cell volume than did control plants. "phenotypic" adaptation, rather than genotypic. Control plants had never been stressed. Five days after the last stress cycle, all plants were subjected to stress. Data were obtained during this dehydration period from leaves at nodes 5 and 8. Leaves at node 5 were 75% expanded at the time of the first stress cycle, whereas the leaves at node 8 were just emerging. After completion of the last stress cycle, the areas Osmoregulation enables plants to withstand temporary or sus- of the leaves at node 5 were similar in adapted and nonadapted tained water deficits (19). Although cellular mechanisms that plants. In adapted plants, the leaves at node 8 were approximately induce or promote osmoregulation are unknown, solute accumu- one-half the size of equivalent leaves in control plants. lation within the cell seems to play a central role in the adaptive Water Potential, Leaf Conductance, and Photosynthesis. Leaf process (10-12, 19, 31, 32). In stress-adapted plants, lowering of water potentials and leaf conductances were obtained using iso- cellular osmotic potential through accumulation of osmotica per- piestic thermocouple psychometry and diffusion porometry as mits turgor maintenance at relatively low leaf water potentials (1, previously described (3). Osmotic potentials were determined 10, 11, 14, 16, 19-21, 31, 32). psychometrically on leaf discs (1.0 cm diameter), frozen in liquid Although many plants partially adapt to water deficits by N2, and thawed. Turgor pressure was calculated as the difference osmoregulation, thereby maintaining some growth during between total leaf water potential and osmotic potential. Osmotic drought, adaptation is normally associated with reduced growth potentials were not corrected for the dilution of cell sap with and productivity (9, 19). Two physiological processes necessary apoplastic water that occurs during freezing (22). Leaf resistances for growth and productivity are photosynthesis and translocation. were converted to conductances by taking the reciprocal of total Both are important in generation and distribution of osmotically leafresistance obtained by assuming individual surface resistances active solutes. In most plants, water stress reduces photosynthesis act in parallel. (6) and movement of assimilates out of the leaf (24, 28, 33, 34). Apparent photosynthetic rates were determined using the CO2 The purpose of the present study was to examine the process of pulse-labeling technique described by Naylor and Teare (26). osmoregulation in cotton focusing on photosynthesis, transloca- Translocation. Sections of intact plants consisting of the vege- tion, and leafcarbohydrate status and their responses to leafwater tative apex and four to five monopodial and two to three sympo- status. dial branches were excised under water and the stems were placed In this paper we describe photosynthesis, translocation, and in distilled H20. The plants were enclosed in a Plexiglas chamber cellular ultrastructure in relation to osmoregulation. A companion containing a fan to facilitate mixing, and air containing about 320 PI 1-' CO2 was passed through the chamber (open system). Dew I Contribution 2803 from the Central Research and Development De- point of the air was adjusted to between 24 and 26 C. Plants were partment, E. I. du Pont de Nemours and Co. allowed to photosynthesize for 1 h at 27 C with PAR of 850 ,IE 484 Plant Physiol. Vol. 67, 1981 WATER STRESS ADAPTATION IN COTTON 485 m-2 s-' provided by a Xenon arc lamp. This PAR was equivalent to that during the growth period in the controlled environment chamber (3). After 1 h, the chamber was sealed and 5 to 10 ,uCi "CO2 were injected into the chamber. The pulse period was 15 to 20 min, followed by 15 to 20 min chase with air (320,d 1-1). After the chase period, discs from leaves at node 6 or 7 were removed aa and their radioactivity was counted. The percentage ofradioactiv- Ca ity remaining in the leaves after selected time intervals was deter- j mined. In each experiment, translocation was determined in a c fully turgid plant (stems in and a in 2 H20) wilting plant (stems 0.7 ai M mannitol after the pulse and chase periods). Excised, rather - C than intact plants, were used to dehydrate the control and adapted L plants at about the same rate. LLJi Leaf and Cellular Ultrastructure. Leaf samples were fixed in I (,:, 10 phosphate-buffered 5% glutaraldehyde (pH 7.0) for 2 h. Samples UI o CONTROL r = 0.94* * ILA were rinsed with buffer and treated with phosphate-buffered r. 8 * ADAPTED r = 0.88** osmium tetroxide (2%) for 1.5 h. Tissue was dehydrated by a 6- . graded series of ethanol and embedded in Spurr. Thin sections NODE 5 were cut and stained with lead citrate and uranyl acetate. Sections -jJ were examined on a Zeiss EM 10 at 60 kv. 4 2 RESULTS AND DISCUSSION 0 A series of brief water deficits reduced growth of cotton (Fig. 1). After each successive stress cycle the lead conductances offully -2 _ .I control 0 -4 -8 -12 -16 -20 turgid and adapted plants were similar (Fig. 1). Following LEAF WATER the first three stress cycles, photosynthetic rates of adapted plants POTENTIAL, bars were lower than those of control plants. However, immediately FIG. 2. Relationship between leaf water potential and leaf pressure after the last two stress cycles, adapted leaves photosynthesized potential of cotton leaves at the eighth (upper) and fifth (lower) node more rapidly than control leaves. This increase in photosynthesis above the cotyledonary node. Each data point is the mean of two mea- appeared to be transient inasmuch as fully turgid adapted leaves surements and data from two independent experiments were pooled to consistently had lower rates of photosynthesis than control plants derive the relationships. (0), control plants; (0), adapted plants. Asterisks 5 days after the last stress cycle (Fig. 5). Even though stress indicate significance at the 0.01 level. inhibited growth, plants subjected to water stress preconditioning exhibited signs of adaptation when subjected to a subsequent water stress. Similarly, Cutler and Rains (9) demonstrated greater drought tolerance in cotton subjected to limited irrigation than in 1.4 frequently irrigated cotton, even though limited water availability reduced growth. The adaptation exhibited - 12 by stressed plants appeared to be due to osmoregulation, at least with respect to leaves at node 8. This E 0 8 was inferred from the leafturgor-leaf water potential relationships 0.6 in adapted and control leaves (Fig. 2). Adapted, young leaves X E60.4 o CONTROL (node 8) exhibited greater leafpressure potentials than did control leaves at all leaf water potentials, prior to complete loss of turgor 0.2 * ADAPTED A (Fig. 2). Accordingly, adapted, younger leaves must have accu- 01 mulated solutes during the preconditioning phase of the experi- 100 ° ment. Osmoregulation in these young leaves was similar to that CONTROL observed in slowly adapted sorghum (21). In older leaves (node .- 80 - * ADAPTED 5), approximately the same turgor was observed in control and W 60 adapted plants at high leaf water potentials (2-5 bars). During z 40 dehydration, adapted plants sustained greater pressure potentials than did control plants as leaf water potentials declined (Fig. 2). X- 20 8 Consequently, adapted leaves approached zero turgor at lower 0 leaf water potentials than did control leaves. The pressure poten- 08 tial-leafwater potential relationships in these fully expanded older Z70.6- leaves resembled those observed in fully expanded sorghum leaves that had undergone moderate stress in the field (5).