Diurnal depression in leaf hydraulic conductance at ambient and elevated [CO2] and reveals anisohydric water management in field-grown soybean

Authors: LOCKE, ANNA - University Of Illinois; Ort, Donald

Submitted to: Environmental and Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2015
Publication Date: 8/1/2015
Citation: Locke, A.M., Ort, D.R. 2015. Diurnal depression in leaf hydraulic conductance at ambient and elevated [CO2] and 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 ('leaf), even as high VPD drives rapid evaporation of water from the intercellular air spaces. Two water management strategies have been described in response to high mid-day VPD: isohydric, in which stomatal conductance declines to maintain constant leaf, or anisohydric, in which stomata remain open at the cost of a drop in 'leaf. Thus, the anisohydric strategy allows a more variable 'leaf in order to maintain open stomata open and higher photosynthetic rates for longer periods, even as leaf water potential declines. This strategy allows anisohydric plants to attain higher carbon gain than isohydric plants when water is abundant and even when moderately limiting. However, under conditions of intense drought, this risk taking behavior could lead to a persistent collapse in carbon gain that the more conservative behavior of isohydric plants would avoid. This study examined the fluctuation of soybean leaf water status and Kleaf at ambient and elevated [CO2] over the course of the day to test the hypothesis that Kleaf does not increase with increasing VPD and limits soybean photosynthesis on a daily basis.

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 ('leaf) if leaf hydraulic conductance (Kleaf) is insufficient to supply water to intercellular airspaces in pace with demand. Kleaf 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 Kleaf 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 Kleaf. Stomatal conductance typically remains high at mid-day for soybean, suggesting either a mid-day increase in Kleaf 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 Kleaf and 'leaf, showing a mid-day depression in Kleaf in a pattern closely reflecting that of 'leaf, indicating that Kleaf depression is the result of cavitation in leaf xylem. The diurnal depression of Kleaf was not prevented by growth at elevated [CO2]. 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 finds that mid-day Kleaf 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, Kleaf even at the depressed level was in excess of that needed to sustain photosynthesis in soybean.