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 wild-type and transgenic tobacco plants constitutively overexpressing PIP2;5 and PIP1;4 aquaporins (AQPs). Exposure of plants to high irradiance (∼350 μmol m−2 s−1 photosynthetic photon flux density, PPFD) reduced hydraulic conductivity of the bundle sheath cells (Lpc) 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 ∼10 μmol m−2 s−1 PPDF, Lpc in the wild-type plants was reduced by 3–4-fold with 50 and 100 μM HgCl2 treatments. A higher, 200 μM HgCl2 concentration was required to reduce Lpc in the transgenic plants. When the plants were exposed to ∼350 μmol m−2 s−1 PPFD, HgCl2 treatments did not affect Lpc implying that the mercury-sensitive water transport processes involving aquaporins had been inhibited by high irradiance. Exposure of leaves to ∼350 μmol 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 ∼10 μmol 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 Lpc. 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.