Light and turgor affect the water permeability (aquaporins) of parenchyma cells in the midrib of leaves of Zea mays

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Abstract
In response to light, water relation parameters (turgor, half-time of water exchange, T1/2, and hydraulic conductivity, Lp; T1/2∝ 1/Lp) of individual cells of parenchyma sitting in the midrib of leaves of intact corn (Zea mays L.) plants were investigated using a cell pressure probe. Parenchyma cells were used as model cells for the leaf mesophyll, because they are close to photosynthetically active cells at the abaxial surface, and there are stomata at both the adaxial and abaxial sides. Turgor ranged from 0.2 to 1.0 MPa under laboratory light condition (40 μmol m−2 s−1 at the tissue level), and individual cells could be measured for up to 6 h avoiding the variability between cells. In accordance with earlier findings, there was a big variability in T1/2s measured ranging from 0.5 s to 100 s, but the action of light on T1/2s could nevertheless be worked out for cells having T1/2s greater than 2 s. Increasing light intensity ranging from 100 μmol m−2 s−1 to 650 μmol m−2 s−1 decreased T1/2 by a factor up to five within 10 min and increased Lp (and aquaporin activity) by the same factor. In the presence of light, turgor decreased due to an increase in transpiration, and this tended to compensate or even overcompensated for the effect of light on T1/2. For example, during prolonged illumination, cell turgor dropped from 0.2 to 1.0 MPa to –0.03 to 0.4 MPa, and this drop caused an increase of T1/2 and a reduction of cell Lp, i.e. there was an effect of turgor on cell Lp besides that of light. To separate the two effects, cell turgor (water potential) was kept constant while changing light intensity by applying gas pressure to the roots using a pressure chamber. At a light intensity of 160 μmol m−2 s−1, there was a reduction of T1/2 by a factor of 2.5 after 10–30 min, when turgor was constant within ±0.05 MPa. Overall, the effects of light on T1/2 (Lp) were overriding those of turgor only when decreases in turgor were less than about 0.2 MPa. Otherwise, turgor became the dominant factor. The results indicate that the hydraulic conductivity increased with increasing light intensity tending to improve the water status of the shoot. However, when transpiration induced by light tends to cause a low turgidity of the tissue, cell Lp was reduced. It is concluded that, when measuring the overall hydraulic conductivity of leaves, both the effects of light and turgor should be considered. Although the mechanism(s) of how light and turgor influence the cell Lp is still missing, it most likely involves the gating of aquaporins by both parameters.