INVESTIGATION OF THE LIMITATIONS TO PHOTOSYNTHESIS INDUCED BY LEAF WATER DEFICIT IN FIELD-GROWN SUNFLOWER (HELIANTHUS-ANNUUS L)

The diurnal cycling of leaf water potential (PSI-leaf) in field-grown sunflower (Helianthus annuus) was used to investigate the cause of water deficit-induced limitation of net photosynthesis. Daily mid-afternoon decreases in PSI-leaf of up to 1.5 MPa and in net photosynthesis of up to 50% were typical for irrigated sunflower during seed filling. These mid-afternoon 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 PSI-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.