Effect of low root temperature on hydraulic conductivity of rice plants and the possible role of aquaporins.

The role of root temperature T(R) in regulating the water-uptake capability of rice roots and the possible relationship with aquaporins were investigated. The root hydraulic conductivity Lp(r) decreased with decreasing T(R) in a measured temperature range between 10 degrees C and 35 degrees C. A single break point (T(RC) = 15 degrees C) was detected in the Arrhenius plot for steady-state Lp(r). The temperature dependency of Lp(r) represented by activation energy was low (28 kJ mol(-1)) above T(RC), but the value is slightly higher than that for the water viscosity. Addition of an aquaporin inhibitor, HgCl(2), into root medium reduced osmotic exudation by 97% at 25 degrees C, signifying that aquaporins play a major role in regulating water uptake. Below T(RC), Lp(r) declined precipitously with decreasing T(R) (E(a) = 204 kJ mol(-1)). When T(R) is higher than T(RC), the transient time for reaching the steady-state of Lp(r) after the immediate change in T(R) (from 25 degrees C) was estimated as 10 min, while it was prolonged up to 2-3 h when T(R) < T(RC). The Lp(r) was completely recovered to the initial levels when T(R) was returned back to 25 degrees C. Immunoblot analysis using specific antibodies for the major aquaporin members of PIPs and TIPs in rice roots revealed that there were no significant changes in the abundance of aquaporins during 5 h of low temperature treatment. Considering this result and the significant inhibition of water-uptake by the aquaporin inhibitor, we hypothesize that the decrease in Lp(r) when T(R) < T(RC) was regulated by the activity of aquaporins rather than their abundance.