Physiological responses of cotton leaves and roots to water deficit induced by polyethylene glycol

The characterization of plant water relations is a prerequisite for subsequent selection and genetic manipulation for drought tolerance. To evaluate roots in addition to leaves, a method based on PEG was developed to obtain clean, stressed roots quickly while avoiding toxic effects associated with PEG treatment. A mild water deficit of - 0.3 MPa was induced with polyethylene glycol (PEG 6000) around the roots of four cotton (Gossypium hirsutum) genotypes with different water deficit tolerances. Treatment with PEG during diurnal dark period avoided toxic effects observed with treatment during the light period. The genotypes examined included: (1) Siokra L-23, a drought-tolerant cultivar; (2) T-1521, a wild-type with significant osmotic adjustment capability; (3) CS-50, a moderately susceptible cultivar; and (4) Stoneville 506, a susceptible cultivar. The water potentials (psi(w)) and osmotic potentials (psi(s)) of roots and leaves of stressed and nan-stressed plants were determined psychrometrically. In response to the water deficits Siokra L-23 and T-1521 showed 25 and 20% reductions in leaf psi(s),, respectively, compared with unstressed controls. At the same time, psi(w) of these two genotypes did not change significantly. Conversely, the decrease in the psi(s) of CS-50 and Stoneville 506 was not significant, while their psi(w) decreased significantly. By maintaining a higher psi(w) during stress, the tolerant genotypes were able to maintain photosynthesis, stomatal conductance and relative water content near unstressed control levels. The use of PEG 6000 to maintain a constant psi(w) in the root environment provides an efficient method for controlling psi(w) while allowing for rapid sampling of clean root tissue for analysis. The results demonstrated that the empirically determined differences in drought tolerance of these cultivars can be related to measurable physiological parameters. These results suggest that physiological monitoring can be an effective tool in germplasm selection and improvement. (C) 1998 Elsevier Science B.V. All rights reserved.