Root-shoot interactions in the response of sugarcane to drought

Abstract

Studies using greenhouse-grown plants of sugarcane cultivars known to have varying levels of resistance to drought were conducted to identify distinctive physiological features that may confer drought resistance in sugarcane. When leaf water relations, stomatal conductance (g), and shoot growth rate (SGR) were measured concurrently during a soil drying cycle, it was found that both osmotic and elastic adjustment occurred in the leaves of all cultivars in response to soil drying and diurnal water deficit. These adjustments led to almost complete maintenance of symplast volume, but only partial maintenance of turgor. During the early stages of drought, reductions in g and SGR were not accompanied by significant reductions in bulk leaf water status, suggesting that signals originating within the roots may have regulated shoot behavior. When the hydraulic properties of entire root systems and isolated roots were characterized by the transpiration gradient and pressure-flux techniques, cultivar differences in both root- and leaf-specific root hydraulic conductance (Groot) were discerned. At high soil moisture, transpiration and Groot differed considerably among cultivars and were positively correlated, whereas leaf water potential (ψL) was similar among cultivars. Within a narrow range of soil water suction (0 to 0.1 MPa), over which Groot and g fell to nearly zero, ψL remained nearly constant because the vapor phase conductance of the leaves and the liquid phase conductance of the roots declined in parallel. These patterns reinforced the suggestion that control of g in sugarcane plants exposed to drying soil was exerted primarily at the root rather than at the leaf level. Cultivar variation in water relations characteristics, especially bulk tissue elasticity, was more distinct in the roots than in the leaves, suggesting that the previously reported cultivar differences in drought resistance were likely to be root-based. It was hypothesized that coordination of g with declining Groot during soil drying was accomplished by a chemical signal moving from the roots to the leaves via the transpiration stream. Decreased root osmotic potential may have stimulated export of this putative substance from the roots.