Submitted to: Environmental and Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2015
Publication Date: 8/1/2015
Citation: Locke, A.M., Ort, D.R. 2015. Diurnal depression in leaf hydraulic conductance at ambient and elevated [CO2] reveals anisohydric water management in field-grown soybean. Environmental and Experimental Botany. 116:39-46. Interpretive Summary: Leaves must contend with dramatic environmental changes over the course of even a single day. Light and air temperature both tend to peak around the middle of the day, and vapor pressure deficit (VPD) typically peaks with leaf temperature, coinciding with maximum light and air temperature. For plants in temperate climates during the peak growing season, this means that transpiration demand is very high while the potential for maximum light-driven carbon acquisition requires fully open stomata. Maintenance of open stomata is only possible if the leaf interior can remain sufficiently hydrated, maintaining leaf water potential, even as high VPD drives rapid evaporation of water from the intercellular air spaces. This study examined the fluctuation of soybean leaf water status and leaf hydraulic conductance at ambient and elevated [CO2] over the course of the day showing that soybean photosynthesis is sustained even as leaf hydraulic conductance was depressed revealing that leaf hydraulic conductance for soybean is in excess of that needed to sustain a stomatal conductance sufficient to prevent depression of photosynthesis in soybean.
Technical Abstract: Diurnal cycles of photosynthesis and water use in field-grown soybean (Glycine max) are tied to light intensity and vapor pressure deficit (VPD). At high mid-day VPD, transpiration rates can lead to a decline in leaf water potential if leaf hydraulic conductance is insufficient to supply water to intercellular airspaces in pace with demand. Leaf hydraulic conductance is determined by leaf xylem conductivity to water, as well as extra-xylem pathways that are likely mediated by aquaporin water transport proteins. When transpiration demand exceeds the maximum capacity of leaf hydraulic conductance to supply water, high tension in the water column can cause cavitation in xylem, and these emboli-blocked xylem vessels reduce water transport and thus lower leaf hydraulic conductance. Stomatal conductance typically remains high at mid-day for soybean, suggesting either a mid-day increase in leaf hydraulic conductance or that photosynthesis may be maintained at the cost of leaf water status, indicative of an anisohydric water management strategy in soybean. This study examined diurnal fluctuations in leaf hydraulic conductance and 'leaf, showing a mid-day depression in leaf hydraulic conductance in a pattern closely reflecting that of leaf water potential, indicating that leaf hydraulic conductancedepression is the result of cavitation in leaf xylem. The diurnal depression of leaf hydraulic conductance was not prevented by growth at elevated [CO2], which lowered stomatal conductance. Diurnal transcription patterns of aquaporin genes were measured showing that a total of 34 genes belonging to 4 aquaporin families were expressed in soybean leaves, of which 22 were differentially expressed between at least two time points. Overall, this study indicates that mid-day leaf hydraulic conductance depression was driven primarily by cavitation at increasing xylem water tensions. It is further concluded that because soybean photosynthesis is typically sustained at mid-day, leaf hydraulic conductance even at the depressed level was in excess of that needed to sustain a stomatal conconductance sufficient to prevent depression of photosynthesis in soybean.