Investigation of the limitations to photosynthesis induced by leaf water deficit in field‐grown sunflower (Helianthus annuus L.)

Abstract. The diurnal cycling of leaf water potential (Ψ_leaf) in field‐grown sunflower (Helianthus annuus) was used to investigate the cause of water deficitinduced limitation of net photosynthesis. Daily midafternoon decreases in Ψ_leaf of up to 1.5 MPa and in net photosynthesis of up to 50% were typical for irrigated sunflower during seed filling. These midafternoon values were lowered an additional 0.6 to 0.8 MPa by prolonged drought treatment. There was a nearly linear relationship between the decline in net photosynthesis and reductions in leaf conductance over the course of the day. Thus, it was unexpected to find that the low, midafternoon rates of photosynthesis were associated with the highest intercellular CO₂ concentrations. These and other observations suggest that the daily decline in photosynthesis represents a ‘down regulation’ of the biochemical demand for CO₂ that is coordinated with the diurnally developing need to conserve water, thus establishing a balanced limitation of photosynthesis involving both stomatal and non‐stomatal factors. There were no indications that either short term (i.e. diurnal declines in Ψ_leaf) or long term (i.e. drought treatment) water deficits caused any damage or malfunctioning of photosynthesis. Rather, both the daily declines in photosynthesis and the nearly 25% decrease in leaf area induced by prolonged drought appeared to be well‐controlled adaptive responses by field‐grown sunflower plants to limited water availability.