Temperature-Dependent Water and Ion Transport Properties of Barley and Sorghum Roots

Abstract

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 250 and 350C, respectively. Hydraulic conductance (Lp) of both intact plants and detached exuding roots of barley increased with increasing root temperature to a high value at 250C, remaining high with further warming. In sorghum, the Lp of intact plants and of detached roots peaked at 350C. In both species, root temperature did not affect water potentials of the expanded leaf blade or the growing region despite marked changes in Lp. Extreme temperatures greatly decreased ion flux, particularly K+ and N03-, 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.Root zone temperature fluctuates both diurnally and seasonally, exerting diverse effects on plants. In many species, shoot growth responds strongly to changes in the temperature of the root environment (4-6, 18, 19, 27). In addition, root temperature significantly influences stomatal behavior (3,18,22), leaf water status (3,22), and the expression of symptoms due to nutrient deficiencies and other environmental stresses (4,22,26). Despite these important consequences, the mechanism(s) coupling root temperature to shoot responses are poorly characterized. Root temperature is reported to affect both water and ion transport (10,13) several reports have implicated nonhydraulic messengers (perhaps hormonal) from the root system in the control of shoot responses (18,22,27,30). The existence of such messengers is inferred, in part, from the failure to find changes in shoot water status associated with changes in shoot behavior. Although negative evidence may allow inferences about the existence of messengers, it does not per se establish their nature. Further deductions about the involvement of hormones require more detailed knowledge of the possible alternative messengers functioning within a system.Here we characterize the short-term responses of barley and sorghum to root temperatures from 15 to 40°C. These two species are acclimated to widely different temperature ranges during the growing season and were expected to be affected differently by root temperature. Root water transport properties (J2v and Lp) were studied using both intact plants and excised roots. Fluxes of K+, NO3-, and P043-were studied with detached roots. Because ion influx into the root and release to the xylem may be differentially regulated (12,21), an attempt was also made to separate these two processes and assess t...

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