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

Abstract

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 (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 ∼10 μmol m −2 s −1 PPDF, Lp c in the wild-type plants was reduced by 3–4-fold with 50 and 100 μM HgCl 2 treatments. A higher, 200 μM HgCl 2 concentration was required to reduce Lp c in the transgenic plants. When the plants were exposed to ∼350 μmol m −2 s −1 PPFD, HgCl 2 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 ∼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 HgCl 2 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.