AN EVALUATION OF HYDRAULIC VS. NON-HYDRAULIC ROOT SIGNALS CONTROLLING SHOOT RESPONSE TO SOIL WATER DEFICITS IN CUCUMBER

Whole-root and split-root experiments were conducted to determine whether a non-hydraulic signal from roots in drying soil affects stomatal conductance (ks) and leaf expansion rate (LER) in cucumber. No evidence of leaf osmotic adjustment, which could have altered ks and LER stress responses, was observed in our studies. A linear decrease in ks and significant reduction in LER began 8 to 10 days after water was withheld from the entire root system, and this coincided with the initial decline in soil water potential (_soil) and leaf water potential (_L). This result suggests that leaf turgor-mediated mechanisms of response to soil water deficit were operative under these experimental conditions. In split-root experiments, LER significantly declined when _soil of the dry half of the root system fell to -0.36 MPa, even though _L and leaf turgor potential (_p) were maintained at control levels. The ks of plants with half their root system below -0.3 MPa also tended to be lower than the control, but the effect was not statistically significant. Fluctuating leaf-to-air vapor pressure gradient (vpg) during the experiments, and stomatal response to vpg, may have obscured evidence of a root signal effect on ks. These results suggest that there may be a non-hydraulic root signal affecting shoot response to drought. However, if this signal exists, its effects on both LER and stomatal behavior may be masked by turgor-mediated responses when leaf water deficits develop, and by stomatal response to vpg.