The complementary role of root and leaf PIP1 and PIP2 aquaporins drives the anisohydric behavior in Helinathus annuus L.

This study aimed at determining the role of aquaporins (AQPs) in the anisohydric behavior of sunflower. Deactivation of AQPs by mercury revealed that they could play vital role in the stomatal regulation and this role was not time-dependent under the control conditions. Sunflower seedlings were exposed to drought or to osmotic or ionic factor of NaCl and KCl. Under control and optimal conditions, sunflower leaves behaved in an anisohydric manner (maintaining high stomatal conductance (g_s) at 13:00 compared to 9:00). At 13:00: 1) the upregulation of PIP1;1 could improve the mesophyll conductance to CO₂, leading to enhancement of the photosynthetic rate (A) and consequently increasing the g_s and 2) PIP2s were downregulated in the leaves, perhaps to decrease the hydraulic conductance and control excess water loss via transpiration but at the same time these genes were upregulated in the roots to improve root to leaf hydraulic conductance and maintain the positive water balance of the whole plant. Under drought, the decrease in g_s and A at 13:00 could imply a shift towards near-isohydric behavior. The downregulation of PIP1s and further downregulation of PIP2s could indicate their involvement in this shift by decreasing CO₂ permeability and avoiding the possible loss of water from the plant roots to soil, respectively. The anisohydric behavior of leaves treated with KCl, but not NaCl, (25 or 150 mM) could be due to the positive impact of K⁺ on stomata and the regulation of PIP1s and PIP2s. The anisohydric behavior could be achieved by a synchronized regulation of leaf and root AQPs.