LEAF EXPANSION IN FIELD-GROWN SUNFLOWER IN RESPONSE TO SOIL AND LEAF WATER STATUS

Leaf responses to soil water deficits in controlled environments may be mediated by nonhydraulic root signals (RS). The aim of this study was to evaluate in the field the effects of RS and leaf water potential (psi(l)) on leaf expansion rate (LER) of sunflower (Helianthus annuus L.). Two experiments were performed on a deep sandy-loam soil (Typic Xerofluvent) in a Mediterranean environment under spring (Exp. 1) and summer conditions (Exp. 2). WET and DRY treatments were established at the 16th-leaf stage in both experiments. WET plots were irrigated daily with an amount of water equal to the reference evapotranspiration of the previous day. DRY plots received no water during the treatment period which was maintained until LER declined to about 30% of the WET controls. A split-root (SR) treatment with the root zone divided in dry and wet sections was included in Exp. 2. Soil water content (theta(a)), LER and psi(l) were measured. Responses of LER to soil drying were better described by psi(l) than by plant available water (PAW); i.e., the response of LER to PAW depended on evaporative demand, whereas a single regression of psi(l) on LER fitted all the data (P < 0.001). Leaf expansion rate and psi(l) of DRY plants began to decline with respect to that Of WET plants at similar PAW thresholds. The soil water content in the dry soil zone of the SR plots WaS Similar to that in the DRY plots during the first week of treatment whereas theta(a) in the wet zone allowed SR plants to maintain a water status similar to that Of WET controls. Split-rooted plants did not behave as DRY controls, as expected in the case of a significant root signal effect, but maintained LER similar to the WET plants. It is concluded that, under the conditions of the present experiments, hydraulic effects were probably of more importance than nonhydraulic root signals in the regulation of sunflower leaf expansion.