Effects of irradiance on cell water relations in leaf bundle sheath cells of wild-type and transgenic tobacco (Nicotiana tabacum) plants overexpressing aquaporins

Effects of low and high irradiance on cell water transport properties were studied in the leaves of wildtype and transgenic tobacco plants constitutively overexpressing PIP2;5 and PIP1;4 aquaporins (AQPs). Exposure of plants to high irradiance (similar to 350 mu mol m(-2) s(-1) photosynthetic photon flux density, PPFD) reduced hydraulic conductivity of the bundle sheath cells (Lp(c)) in the midrib, secondary, tertiary and quaternary veins of tobacco (Nicotiana tabacum) leaves. However. this decrease was not reflected in the decrease of the leaf hydraulic conductance and both wild-type and transgenic plants responded to high irradiance with an increase in transpiration rates. When the plants were exposed to similar to 10 mu mol m(-2) s(-1) PPDF, Lp(c) in the wild-type plants was reduced by 3-4-fold with 50 and 100 mu M HgCl2 treatments. A higher, 200 mu M HgCl2 concentration was required to reduce Lp(c) in the transgenic plants. When the plants were exposed to similar to 350 mu mol m(-2) s(-1) PPFD, HgCl2 treatments did not affect Lp(c) implying that the mercury-sensitive water transport processes involving aquaporins had been inhibited by high irradiance. Exposure of leaves to similar to 350 mu mol m(-2) s(-1) PPFD did not affect the modulus of elasticity in bundle sheath cells, but decreased turgor pressure (P) and this decrease was reversible by exposing the leaves to similar to 10 mu mol m(-2) s(-1) PPDF. The reduction in P was not affected by the HgCl2 treatments. However, 10 mM tetraethylammonium, a K+ channel and AQP inhibitor, prevented the high irradiance-induced decrease in P and decreased Lp(c). Exposure of leaves to high irradiance also increased the ratio of cell wall to protoplast K+ in the bundle sheath cells. Our results suggest that the effects of irradiance on cell water relations in tobacco leaves involve an inhibition of AQP-mediated water transport and changes in K fluxes leading to the decline in cell P. (C) 2008 Elsevier Ireland Ltd. All rights reserved.