WATER DEFICITS AND ENVIRONMENTAL-FACTORS AFFECT PHOTOSYNTHESIS IN LEAVES OF CUCUMBER (CUCUMIS-SATIVUS)

Cucumber plants were cultured in a greenhouse and subjected to either well-watered or water deficit conditions that reduced leaf water potential to -0.6 MPa. Leaf gas exchange measurements were conducted using an open gas exchange system. Carbon dioxide assimilation (A) attained saturation at a photon flux density (PFD) of 1000 mumol.m2.s-1 (400-700 nm). There were no significant differences in A at ambient temperatures between 16 and 34C. Water use efficiency decreased rapidly with increasing vapor-pressure deficits to 2.5 kPa. Water stressed plants had lower stomatal conductances and CO2 assimilation rates. The decrease in A was only partially due to stomatal closure. The A vs. intercellular CO2 (C(i)) relationship for stressed leaves revealed a change in the CO2 compensation point, and that nonstomatal factors were contributing to the decrease in A in stressed plants. Thus, feedback inhibition of A may have occurred through photoassimilate accumulation. The concentrations of sucrose and raffinose were higher, and the concentration of stachyose was lower in leaves of stressed than of well-watered plants.