Sensitivity of root system to low temperature appears to be associated with the root hydraulic properties through aquaporin activity

The sensitivity of cucumber and figleaf gourd root system to low temperature was evaluated in terms of water transport capacity. Plants were grown hydroponically and low solution temperature was imposed for different periods. Low temperature severely reduced xylem sap transport in cucumber but not in figleaf gourd to the same extent. Root pressure generated in figleaf gourd during the night under low temperature was able to transport xylem sap but there was little transport in cucumber when measured with heat-balance sap-flow gauge. The different capability in generation of root pressure in two species upon exposure to low temperature was confirmed by measuring the root pressure with excised root system that was related to the activity of plasma membrane H+-ATPase. Measurement of half-times of water exchange and hydraulic conductivity of cortical cells with cell pressure probe showed the insensitivity of figleaf gourd cells to low temperature. External application of HgCl2, which is known to inhibit the bulk water transport through aquaporins, also showed an insensitivity of individual figleaf gourd cortical cells as well as entire root system while marked reduction in hydraulic conductivity was observed in cucumber plants. Therefore, it is concluded that the activity (open/closed state) of aquaporin may be associated with the rate of water uptake and the sensitivity of root systems to low temperature.