Root water uptake in relation to plant transpiration

Typically, root water uptake (RWU or U_tot) is said to be driven by transpiration (Tr). It is however more accurate to state that transpiration causes a reduction in leaf water content that reduces the leaf water potential such that a water potential gradient builds up between leafs and soil water, such that water can be extracted from the soil. For herbaceous plants, the amount of water that is hereby lost is typically assumed to be negligible so the plant can be treated as a resistive system. In how far this is true is open to discussion as quantifying shoot water changes is not easily feasible, especially when the soil-root system is drying out. A balance cannot observe water moving between the soil and the shoot and shoots have empty spaces such that 3D cameras provide an incomplete picture. Shoot weight determination requires that the amount of soil water is independently assessed to discriminate between the two pools of water. This can be achieved when a balance is combined with a Soil Water Profiler (SWaP) on the same soil-plant system. The precision of the SWaP is comparable to that of an expensive balance (<10mg for a 6kg system).

Here we performed experiments with the SWaP – balance combination under modulated light with progressive soil dehydration for sunflower and faba bean (N=4). Our data shows that transpiration precedes U_tot by about 5 to 10 mins under wet conditions (pF~2) and U_tot can exceed Tr by up to 20%. Gradually, with decreasing soil water content we find that U_tot becomes smaller than Tr and at the same time the delay between Tr and U_tot increases. For pF>3.5 most of the transpired water stems from the shoot, not from root water uptake, indicating that Tr is a poor proxy for RWU for pot experiments where soil is drying at a rate of ~5% per day at well watered conditions. This is very important for calculations of root conductance during drying scenarios. We found significant differences between sunflower sensitivity to soil drying as compared to faba beans that are somewhat more sensitive. We also present data that shows that the delay between Tr and local water uptake is rather dependent on depth and not so much dependent on local pF, which is typically lower for shallow sections of the pot. This may potentially be explained by loss of root water when Tr increases with light, analogous to shoot water losses.

The combination of the SWaP and gravimetric methods opens up a new way of looking at root water uptake as driven by transpiration and shoot water loss dynamics as it provides hitherto inaccessible information about these processes.