The present investigations were conducted on suniower (var. Morden) plants under pot culture condition. Moisture stress was applied at four developmental stages viz. seedling, vegetative, flowering and seed filling. Net photosynthesis (Pn), leaf stomatal conductance, chlorophyll content and rate of partial light reactions were measured in the upper fully expanded leaves at varying degrees of leaf water potential (ψI). Stomotal conductance and net photosynthetic rate declined sharply with increasing levels of moisture stress at pre-flowering stage. Vegetative stage was found more susceptible to moisture stress as regards to stomatal conductance and net carbon exchange rate. Chlorophyll content, at decreasing leaf water potential, declined gradually alongwith the age of the plant. Reduction in rate of electron transport via PS II and PS I was, however, comparatively more at flowering than other stages at all the levels of moisture stress.
