ABA in Leaves of Field-Grown Soybean Under Water Stress
Abscisic acid (ABA) and leaf water potential were measured in leaves from field-grown soybean (Glycine max (L.) Merrill) during the latter part of the 1978 growing season in Varna, NY. Two cultivars were compared, ‘Chippewa 64’ which was fairly drought hardy and ‘Wilkin’ which was drought susceptible. Water was withheld from stressed plots from July 20 to August 25. Topmost fully expanded trifoliate leaves were sampled from unstressed and stressed plants of both cultivars at 0700 hr, 1600 hr, and 2200 hr on August 11 and 16. Three replications were made for each treatment. Leaf water potential measurements were made at sampling with a pressure chamber, samples were immediately placed on dry ice, subsequently freezer- dried and analyzed for ABA. Continuous in field measurements of C02-exchange rate (CER) were made with a differential infrared gas analyzer on comparable leaflets attached to plants of the two cultivars. At the time of sampling, soil water potential at 10 cm depth was -0.05 MPa and -0.46 MPa for unstressed and stressed plots, respectively. While plants were more stressed on August 16, on both days a diurnal cycle in leaf water potential was observed in unstressed and stressed plants in each cultivar. ABA fluctuated diurnally in leaves from stressed plants but in unstressed plants there was little diurnal change. On August 16 there was a linear correlation between ABA and leaf water potential. The relationship was different in stressed leaves of the two cultivars. ‘Wilkin’ did not accumulate as much ABA as ‘Chippewa 64’ yet the leaves from ‘Wilkin’ had more negative water potentials. CER in leaflets from stressed plants was reduced almost equally in both cultivars when compared to leaflets from unstressed plants.