Responses of transpiration and hydraulic conductance to root temperature in nitrogen- and phosphorus-deficient cotton seedlings.

Suboptimal N or P availability and cool temperatures all decrease apparent hydraulic conductance (L) of cotton (Gossypium hirsutum L.) roots. The interaction between nutrient status and root temperature was tested in seedlings grown in nutrient solutions. The depression of L (calculated as the ratio of transpiration rate to absolute value of leaf water potential [Psi(w)]) by nutrient stress depended strongly on root temperature, and was minimized at high temperatures. In fully nourished plants, L was high at all temperatures >/=20 degrees C, but it decreased greatly as root temperature approached the chilling threshold of 15 degrees C. Decreasing temperature lowered Psi(w) first, followed by transpiration rate. In N- or P-deficient plants, L approached the value for fully nourished plants at root temperatures >/=30 degrees C, but it decreased almost linearly with temperature as roots were cooled. Nutrient effects on L were mediated only by differences in transpiration, and Psi(w) was unaffected. The responses of Psi(w) and transpiration to root cooling and nutrient stress imply that if a messenger is transmitted from cooled roots to stomata, the messenger is effective only in nutrient-stressed plants.