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Mauricio Tejera (Walker lab)

Date & Location: September 29, 2020, at 12p; Virtual talk

Subject: Drought effects on source-sink balance

Abstract: Source and sink activities are particularly susceptible to water-deficit conditions. As water-deficit conditions progress, reduced growth imposes sink limitations because photoassimilates are not consumed in growing organs, stomatal closure imposes source limitations as CO2 supply for photosynthesis is restricted, and the production of osmoprotectant solutes create new sinks as metabolic pathways are upregulated. The relative impact of these limitations in perennial grass CO2 assimilation and portioning is poorly understood. During 2020 growing season I studied the physiological response to drought of the perennial C4 grass Switchgrass (Panicum virgatum L.). We imposed drought treatments using 20m2 rainout shelters. While the shelters successfully excluded rainfall and limited soil water content to only 10% of rainfed treatments, the drought treatment did not have clear effects on Switchgrass leaf water potential (LWP), diurnal course of CO2 assimilation and stomatal conductance, and light potential curves. Over the five sampling dates throughout the season, switchgrass CO2 assimilation was ~15% higher (p < 0.05) in rainfed treatments only in the samplings of mid and late summer. Interestingly, these differences did not correspond with significant changes in LWP. Similarly, significant changes in LWP did not correspond with changes in CO2 assimilation. In addition, light potential curves showed similar patter across treatments. These results suggest that i) switchgrass successfully adjusts and maintains high levels of net CO2 assimilation even under low LWP, or ii) other factors have a stronger control over photosynthesis. To test the first hypothesis, we did a 13C-CO2 labeling event at the end of the growing season to compare carbon allocation to different metabolic pathways in drought treatment and rainfed plants. Sink limitations in perennial grasses become more important towards the end of the growing season when growth has ended, grains are filled, and reserves have been replenished. To test whether carbohydrates buildup was limiting CO2 assimilation toward the end of the growing season, sampled leaves and rhizomes are being analyzed for sucrose and starch content. Altogether, this ongoing project will help clarify the impact of in-season limitations caused by drought and late-season sink limitations caused by carbohydrate buildup in switchgrass CO2 assimilation and carbon partitioning.

Speaker Lab: Dr. Berkley Walker