Aquaporin involvement in controlling leaf hydraulic capacitance and resistance.

Hydraulic capacitance (Ch) and resistance (Rh) were assessed on detached leaves sampled from grapevines overexpressing the aquaporin gene VvPIP2;4N by following recovery dynamics after leaf dehydration. The dehydration was imposed by submitting the leaves at 0.5 to 1 MPa overpressure through a Scholander bomb. The recovery after dehydration was checked in dark, in light non-transpirative condition, and in light-transpirative condition. In leaves processed in dark conditions, Rh decreased and Ch increased compared to wild type plants, suggesting that in transgenic leaves, trans-cellular pathways activated during dehydration superimposed on apoplastic ones, saving water inside cells, as in/out cell-to-apoplasm water movements were aquaporin-controlled. Upon light, either when leaves transpired or not (either by depressing vapor pressure deficit or by feeding leaves with abscisic acid, causing stomatal closure) the described effects disappeared. We conclude that light activates leaf aquaporins masking the transgene effect on controlling leaf Ch and Rh, and confirming that the in-vivo leaf hydraulics is not affected by transgenic aquaporin VvPIP2;4N overexpression.