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 4.6 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 micromoles.m-2.s-1 (400-700 nm). There were no significant differences in A at ambient temperatures between 16 and 34 degrees C. 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