Temperature-dependent water and ion transport properties of barley and sorghum roots : I. Relationship to leaf growth.

Root temperature strongly affects shoot growth, possibly via "nonhydraulic messengers" from root to shoot. In short-term studies with barley (Hordeum vulgare L.) and sorghum (Sorghum bicolor L.) seedlings, the optimum root temperatures for leaf expansion were 25 degrees and 35 degrees C, respectively. Hydraulic conductance (L(p)) of both intact plants and detached exuding roots of barley increased with increasing root temperature to a high value at 25 degrees C, remaining high with further warming. In sorghum, the L(p) of intact plants and of detached roots peaked at 35 degrees C. In both species, root temperature did not affect water potentials of the expanded leaf blade or the growing region despite marked changes in L(p). Extreme temperatures greatly decreased ion flux, particularly K(+) and NO(3) (-), to the xylem of detached roots of both species. Removing external K(+) did not alter short-term K(+) flux to the xylem in sorghum but strongly inhibited flux at high temperature in barley, indicating differences in the sites of temperature effects. Leaf growth responses to root temperature, although apparently "uncoupled" from water transport properties, were correlated with ion fluxes. Studies of putative root messengers must take into account the possible role of ions.