GROWTH-RATE, PLANT DEVELOPMENT AND WATER RELATIONS OF THE ABA-DEFICIENT TOMATO MUTANT SITIENS

Given the close relationship between a plant's growth rate and its pat tern of biomass allocation and the effects of abscisic acid (ABA) on b iomass allocation, we studied the influence of ABA on biomass allocati on and growth rate of wildtype tomato (Lycopersicon esculentum Mill. c v. Moneymaker) plants and their strongly ABA-deficient mutant sitiens. The relative growth rate of sitiens was 22% lower than that of the wi ldtype, as the result of a decreased specific leaf area. The net assim ilation rate and the leaf weight ratio were not affected. The mutant s howed a much higher transpiration rate and lower hydraulic conductance of the roots. These two factors resulted in sitiens having a signific antly lower leaf water potential and turgor, resulting in reduced leaf expansion and, consequently, a lower specific leaf area relative to t he wildtype. Addition of ABA to the sitiens roots resulted in phenotyp ic reversion to the wildtype. We conclude that the influence of ABA-de ficiency on biomass allocation and relative growth rate is the result of altered water relations in the plants, rather than of a direct effe ct on sink strength of different plant organs.