Changes in photosynthesis,rlbulose bisphosphate carboxylase (RuBPCase), and proteolytic activity were followed in the leaves of field-grown soybeans [Giycine max (L.) Merr. cv. Kent] from flowering through senescence. These parameters were followed in relation to changes in leaf resistance, chlorophyll, protein, starch, total N levels, and seed development. In addition, changes in leaf ultrastructure were observed. The initial symptoms of senescence (evident 3 to 4 weeks after flowering) were a decline in photosynthesis, chlorophyll, and total leaf N and an increase in proteolytic activity. Preceding these changes there was a swelling of the chloroplasts and a disorientation of the chloroplast lamellae, possibly resulting from the apparent increase in starch deposition. Also, large numbers of osmiophilic granules appeared within the chloroplasts.These changes were evident prior to the time the seed entered its most rapid period of growth which was 4 to 7 weeks after flowering, The initial decline in photosynthesis did not appear to be due to an increase in leaf resistance or a decline in RuBPCase activity or level. The decline in protein levels began between 5 and 6 weeks after flowering and was paralleled by the decline in carboxylase activity and level. Associated with these changes were an increase in the size of the osmiophilic granules within the chloroplasts, a decrease in the number of chloroplasts with a corresponding increase in the apparent cellular breakdown products, and a dissolution of the vacuoles. No large increase in leaf resistance or change in specific activity of carboxylase was observed until late in senescence.

The threshold leaf water potential required to initiate stomatal closure in cotton (Stoneville 213) became progressively more negative when plants were subjected to a series of water stress cycles. The shift in the threshold water potential required for induction of stomatal closure was dependent on the number of previous stress cycles and leaf age. The basal level of endogenous abscisic acid (ABA) in fully turgid leaves increased in response to the stress treatments, whereas the amount accumulated in response to a subsequent stress did not differ greatly among plants that had experienced different degrees of stress conditioning.Stomatal sensitivity to (±)-ABA fed through the transpiration stream was enhanced in detached leaves of plants which had experienced repetitive water stresses. The increased sensitivity was apparently the result of ABA synthesized during the stress periods since foliar applications of ABA sensitized stomata in an analogous manner. Furthermore, the amount of (+)-ABA required to initiate stomatal closure in leaves from the various stress treatments was not related to the amounts accumulated during wilting.

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The effects of varying degrees of water stress on stomatal activity, photosynthesis, and nitrate reductase activity were examined in field grown cotton (Gossyplum hirsutum L., cv ‘Dunn 56C’). A relationship between plant water status and activity of each measured physiological parameter was established.Slight increases in leaf diffusive resistance were observed as leaf water potentials decreased although complete stomatal closure due to water stress was not generally observed. In many cases, visibly wilted leaves with zero turgor potentials exhibited minimal diffusive resistances. Morning and afternoon values of leaf diffusive resistance were distinctly different even though no correlation between leaf water potential and diffusive resistance was evident.Water stress substantially reduced photosynthesis in both vegetative and reproductive leaves of cotton. Photosynthetic rates of each leaf type responded differently to declining leaf water potentials. The data suggest that the photosynthetic reduction could not be attributed to stomatal closure.The activity of nitrate reductase was adversely affected by declining leaf water potentials. The nocturnal activity of nitrate reductase (respiratory linked) was also reduced by severe water stress. However, the reduction from maximum daily activity to minimum night time activity was similar in both stressed and non‐stressed plants. These data suggest that inhibition of nitrate reductase activity could be due to long term water stress effects rather than temporal changes in plant water status.The data presented indicate that stomata of field grown cotton are relatively insensitive to water stress, at least within the range of leaf water potentials observed in this study. Measurement of stomatal activity may not be a good criterion for assessing plant water status of cotton. The measurement of one or more physiological processes may prove a better index of plant water status as well as providing sensitive selection criteria for breeding more drought tolerant varieties.

Diurnal changes in tissue water potential components, photosynthesis, and specific leaf carbohydrates were examined in water stress-adapted and nonadapted cotton plants. Adapted plants exhibited lower daily minimum leaf water potentials and maintained turgor to lower leaf water potentials than nonadapted plants. Because Osmotic adjustment in response to water stress enables many plants to withstand moderate water deficits or grow in areas of limited water availability (11). Although osmotic adjustment occurs in several species (1, 4-6, 8-10, 12, 13, 21, 22), the mechanisms involved in this adaptive process remain obscure. Accumulation of solutes facilitates osmotic adjustment, although the nature of the osmotica has not been fully investigated. In cotton, sugars, and organic acids seem to accumulate in stress-hardened plants (6,7). Proline or glycine-betaine are the predominate organic solutes present in halophytes forced to osmoregulate in saline environments (14).Physical changes in cell size or elasticity can promote stress adaptation (6,7,9,12,13 turgor to lower leaf water potentials than larger cells (7). Changes in the elastic properties of cell walls may be important for concentrating solutes when plants undergo dehydration (9, 13). However, physical changes alone do not necessarily promote osmotic adjustment, and it seems likely that a combination of solute accumulation and changes in cell volume is involved in adaptation.A companion paper presented some general aspects of stress adaptation and osmoregulation in cotton conditioned to water stress (3). The present paper investigates the role of specific photosynthetically derived solutes and their relationship to osmotic adjustment in cotton.MATERIALS AND METHODS Plant materials were grown and treated as described (3). Leaf water potential and its components, photosynthesis, and leaf conductance were determined as previously described (2, 3).Samples of leaf material were obtained from leaves at selected times during dehydration, quick-frozen on Dry Ice and stored in a freezer for subsequent analysis. Tissue was extracted in 20 ml 80% ethanol three times. The fractions were pooled, evaporated to dryness under vacuum, and extracted in ether twice, and the H20-soluble materials were resuspended in 1 to 2 ml H20. This fraction was transferred to a coupled ion-exchange column containing a layer each of Dowex 50, Sephadex G-25, and AG Dowex 1. Sugars were eluted with 15 ml H20, evaporated to dryness, and resuspended in 1.0 to 1.5 ml H20. An aliquot of the resuspended material was used for determination of glucose and sucrose. Glucose was determined by incubating an aliquot of the neutral fraction in H20 at 35 to 40 C for 3 h and then determining glucose levels by reaction with Statzyme (Worthington Biochemicals). Sucrose in the same neutral fraction was determined by incubating an aliquot of the fraction with invertase (Sigma). After incubation for 3 h, glucose levels were determined by reaction with Statzyme. Sucrose levels were calculated by determi...

ABSTRACrCotton (Gossypium hirsutum) (L.) was grown in a sand and nutrient solution system at two levels of phosphorus (0.5 and 5.0 millimolar). Within each phosphorus treatment, plants were either watered daily or acclimated to water stress by subjection to several water stress cycles.Stress acclimation increased leaf starch at the low phosphorus level, but not at the high phosphorus level. High phosphorus increased leaf sucrose and glucose concentration in both acclimated and nonacclimated plants, but had little effect on osmotic adjustment or the relationship between turgor and water potential.In nonacclimated plants, high phosphorus increased both leaf conductance and photosynthesis at high water potentials. In acclimated plants, high phosphorus increased photosynthesis but decreased conductance, thus increasing water use efficiency at the single leaf level.Recent evidence suggests that the partitioning of photosynthetically fixed carbon between sucrose and starch may be regulated by cellular Pi levels (6, 8, 9, 1 1). Cotton leaves accumulate starch as a consequence of water stress acclimation (2, 3). Moreover, water stress significantly depresses phosphorus uptake (5, 7, 16) and low cellular Pi levels lead to starch accumulation in isolated chloroplasts and leaves (6, 8, 9, 11). Consequently, phosphorus fertility may play a role in altering the response of plants to water stress by changing the ratio of starch to soluble sugars in leaf cells.The influence of phosphorus fertility on internal water relations, leaf conductance, photosynthesis, and cellular carbohydrates in cotton is reported in this study. MATERIALS AND METHODSPlant Culture. Cotton (Gossypium hirsutum L. Tamcot SP37) was grown in 2 1-cm diameter plastic pots containing sand. Plants were thinned to two per pot after emergence. Conditions in the controlled environment chamber used for plant growth were as previously described (2, Five days after the last stress cycle, all plants were subjected to dehydration. During this dehydration period, data were obtained from leaves at nodes 6 and 7.Water Potential Leaf Conductance, Photosynthesis, and Leaf Carbohydrates. Water potentials and osmotic potentials were determined with isopiestic thermocouple psychrometers (4) as previously described (1-3). Procedures for diffusion porometry and for measurement of photosynthesis, and determination of glucose, sucrose, and starch levels in the leaves were described earlier (1-3).The data reported represent the combined results of three experiments. Based on plant height measurements, the two phosphorus fertility regimes did not differentially affect growth. Acclimated plants were shorter than the appropriate controls following cessation of the stress cycles as previously noted (3). RESULTS Phosphorus (P) fertility level did not significantly alter the relationship between leaf turgor and leaf water potential (Fig. 1). At any given water potential, plants grown on 5.0 mM P maintained leaf pressure potentials 0.5 to 1.0 bars higher than plants grown on 0.5 mM P. A...