Effect of Water Stress on Enzyme Activities in Wheat and Related Wild Species: Carboxylase Activity, Electron Transport and Photophosphorylation in Isolated Chloroplasts

Abstract

Drought sensitivity of Triticum aestivum (cv. Sion) and two wild related species, T. longissimum and T. kotschyi was assessed by measuring the activity of ribulose-1,5-bisphosphate carboxylase (RuP₂ carboxylase) and phosphoenolpyruvate carboxylase (PEP carboxylase), determination of total leaf soluble protein and chlorophyll content and by measuring the rates of electron transport and photophosphorylation in isolated chloroplasts. T. aestivum and T. longissimum showed relatively high drought sensitivity. The activity of the two carboxylating enzymes as well as chlorophyll and protein contents decreased with decreasing leaf water potential (Ψ). Rates of electron transport and photophosphorylation in isolated chloroplasts declined as soon as the Ψ value was reduced from - 0.8 MPa to - 1.0 MPa. In T. kotschyi, all the above activities remained stable down to a ¨ value of about - 2 MPa. These results point to a clear correlation between the ability of T. kotschyi to grow in arid areas and its better performance in all the tests carried out under controlled water stress. In T. kotschyi an initial increase in PEP carboxylase activity was detected between Ψ values of -0.8 MPa and -2.1 MPa, followed by a sharp decline. No uncoupling of the chloroplasts by water stress was observed in any of the three species.