Leaf hydraulic conductance declines in coordination with photosynthesis, transpiration and leaf water status as soybean leaves age regardless of soil moisture

Photosynthesis requires sufficient water transport through leaves for stomata to remain open as water transpires from the leaf, allowing CO2 to diffuse into the leaf. The leaf water needs of soybean change over time because of large microenvironment changes over their lifespan, as leaves mature in full sun at the top of the canopy and then become progressively shaded by younger leaves developing above. Leaf hydraulic conductance (K(leaf)), a measure of the leaf's water transport capacity, can often be linked to changes in microenvironment and transpiration demand. In this study, we tested the hypothesis that K(leaf) would decline in coordination with transpiration demand as soybean leaves matured and aged. Photosynthesis (A), stomatal conductance (g(s)) and leaf water potential (Ψ(leaf)) were also measured at various leaf ages with both field- and chamber-grown soybeans to assess transpiration demand. K(leaf) was found to decrease as soybean leaves aged from maturity to shading to senescence, and this decrease was strongly correlated with midday A. Decreases in K(leaf) were further correlated with decreases in g(s), although the relationship was not as strong as that with A. Separate experiments investigating the response of K(leaf) to drought demonstrated no acclimation of K(leaf) to drought conditions to protect against cavitation or loss of g(s) during drought and confirmed the effect of leaf age in K(leaf) observed in the field. These results suggest that the decline of leaf hydraulic conductance as leaves age keeps hydraulic supply in balance with demand without K(leaf)becoming limiting to transpiration water flux.