Seedlings of Fremont cottonwood (Populus fremontii), a California native riparian foundation species, are vulnerable to water stress from rapid water-table declines and the interactions of the effect water stress with excess light and supra-optimal temperatures that are common on barren point-bars where it establishes. Reduced Fremont cottonwood seedling establishment can impair biological filter functions and biodiversity benefits of riparian forests for terrestrial and aquatic species. The purpose of this study was to determine 1) photosynthetic responses of Fremont cottonwood to water, light and heat stresses, 2) stomatal and biochemical limitations of photosynthesis under stress, 3) damage thresholds for photoinhibition and photosynthetic carbon assimilation, and 4) the effect of leaf orientation and water stress on leaf temperature. Experiments were performed during two growing seasons on fully irrigated (0 days without water) and water-stressed (2, 3, 4 days without water) >90 day-old seedlings to compare the effects of controlled changes in [CO2], light intensity, temperature, and leaf angle. Leaf angle manipulations affected leaf energy balance, light absorption and leaf temperature.
After three days of water stress under ambient field conditions, midday leaf water potential was reduced 23%. Stomatal closure reduced transpiration by 35%, but limited CO2 supply for photosynthesis by 15% resulting in a large increase in water use efficiency. Three days without water (3-day treatment) had no effect on photosynthetic function under moderate temperature and light conditions, but, when combined with heat and light stress, photosynthesis declined and photoinhibition occurred. Zero-day (control) seedlings had photosynthetic rates 1.6-fold greater than 3-day seedlings (24.9±1.7 and 15.8±1.3 μmolCO 2 m-2s-1). Control seedlings maintained photosynthesis up to 40°C; whereas, photosynthesis in 3-day water-stressed seedlings decreased with temperatures >30°C. Thresholds for photochemical damage from heat stress differed by only 3°C (0 day=41°C; 3 day=38°C). Leaf orientation and water stress had a strong effect on leaf temperature, such that horizontally restrained leaves, mimicking young seedling leaf orientation, were significantly warmer with a greater effect occurring in water-stressed leaves due to reduced transpirational cooling. Overall these thresholds for damage by heat, light and water stress, when compared to naturally occurring microclimate extremes, suggest that photoinhibition and reduced photosynthesis are common for Fremont cottonwood seedlings on exposed point bars. Such damage is likely to greatly affect growth and establishment of this species especially when the water table recedes too quickly for seedling root growth to keep up. The natural shift in leaf orientation that occurs in Fremont cottonwood seedlings about 50-60 days after germination appears to be an effective mechanism to mitigate these effects. This study contrasts with previous conclusions that water stress alone causes mortality of cottonwood seedlings. The cause is more likely due to water stress predisposing leaves to damage by excess light and high temperatures, which are common on the open point bars where these plants establish.
|Advisor:||Richards, James H.|
|Commitee:||DeJong, Theodore M., McElrone, Andrew J.|
|School:||University of California, Davis|
|Department:||Soils and Biogeochemistry (formerly Soil Science)|
|School Location:||United States -- California|
|Source:||MAI 50/03M, Masters Abstracts International|
|Subjects:||Ecology, Plant sciences, Environmental science|
|Keywords:||Fluorescence, Microclimate, Photoinhibition, Photosynthesis, Populus fremontii, Water stress|
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