Osmoregulation in Cotton in Response to Water Stress

Abstract

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...

Osmoregulation in Cotton in Response to Water Stress

“…1). The increased sugar levels could contribute only about 0.4 bar to the leaf osmotic potential, based on calculations of cellular volumes and the attendant sugar concentrations (2,3). This 'osmotic adjustment' due to high P is similar to the amount shown in Figure 1.…”

“…Plants were thinned to two per pot after emergence. Conditions in the controlled environment chamber used for plant growth were as previously described (2,3). Plants were watered twice daily; once with Hoagland complete nutrient solution and once with deionized H20.…”

“…Water potentials and osmotic potentials were determined with isopiestic thermocouple psychrometers (4) as previously described (1)(2)(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)(2)(3).…”

“…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).…”

Effects of Water Stress on Photosynthesis and Carbon Partitioning in Soybean (Glycine max [L.] Merr.) Plants Grown in the Field at Different CO2 Levels

“…The present study was conducted to obtain more information on the interactions between CO2 enrichment and plant water stress, both of which are known to affect carbohydrate concentrations in leaves (1,4,5,17,18). Specific objectives were to determine the long-term effects of CO2 enrichment on (a) CER and photosynthate partitioning and (b) SPS activity in leaves of nonstressed versus water-stressed plants.…”