Non-Stomatal Inhibition of Photosynthesis Associated with Partitioning of the Recent Assimilates into Starch and Sucrose in Sunflower Leaves under Water Stress.

Abstract

The effects of water stress on the photosynthesis (Pn) of potted sunflower Helianthus annuus L. plants were examined by monitoring the photosynthetic O2 evolution rates under saturated CO2 (10 kPa). These conditions eliminate stomatal limitation and partitioning of the recent assimilates into sucrose and starch in leaves was measured with decreasing leaf relative water content (RWC). Water-stressed leaves at RWC lower than 60% required at least 8 min pre-illumination at a saturated irradiance to reach the maximum O2 evolution rate (Pmax), compared to 2 min for non-stressed leaves, suggesting slower light-activation of enzymes involved in CO2 fixation in the Calvin cycle. Light-saturated photosynthesis and apparent quantum yield decreased with decreasing leaf RWC, but the former was inhibited more than the latter by the water stress. Pmax of both non-stressed and ABA-fed leaves required a very high CO2 pressure of 10 kPa to overcome stomatal closure but Pn never completely recovered in dehydrated leaves at RWC lower than 65%. We concluded that a non-stomatal inhibition played an important role in reducing photosynthesis of severely dehydrated leaves. Water stress increased sucrose content and decreased starch content in dehydrated leaves, leading to an increase in sucrose/starch ratio with decreasing RWC and Pmax. These results suggest that photosynthesis of water-stressed sunflower leaves was depressed on account of stomatal and non-stomatal limitations, which might be a feedback regulation of sucrose and starch syntheses influenced by water stress.