Plant water relations of Tamarix ramosissima in response to the imposition and alleviation of soil moisture stress

The effect of an extended drydown on the water relations of Tamarix ramosissima (saltcedar), a desert phreatophyte, was investigated along the floodplain of the Virgin River (southern Nevada) during a hot dry summer period. Seedlings of Tamarix were grown in lysimeters (120 cm depth, 51 cm radius) positioned along a gradient from the desert's edge to the interior of the stand and monitored for growth and evapo-transpiration over a 2-year period prior to this study. Water tables were maintained at approximately 60 cm during the evapo-transpiration study by applying weekly irrigation directly to the water table via piezometers. At the end of the 2-year period, three lysimeters were selected for a drydown experiment (desert's edge, river's edge, open stand). On 7 July 1995 all irrigation was terminated for a 29-day period. After the drydown period was over, irrigation was applied for an 18-day period to re-establish water table depths. Sapflow (transpiration) was monitored during the entire 47-day period (drydown, wetup) with stem how gauges, soil moisture with time domain reflectometry, and plant water relations with a steady state porometer and pressure chamber. Results indicated that sapflow decreased significantly as water tables and stored soil water declined. Daily sapflow totals on a leaf area basis were higher for the plant growing along the river's edge, with midday hourly values significantly higher when a water table was present. On day 26, sapflow reached zero in the open stand lysimeter. As sapflow approached zero, a significant decrease in the time in which daily sapflow stopped occurred, rather than a delayed sapflow start time. Upon application of water on day 29, all three plants growing in the lysimeters responded by increasing sapflow within a 24-h period. Relative stomatal conductance (p = 0.001), leaf xylem water potential (p = 0.001) and sapflow (p = 0.001) measured during both the drydown and wetup stages were air linearly correlated with relative soil water in storage, indicating that relative soil water in storage could decline by as much as 90% and that upon alleviation of the soil moisture stress no hysteresis would be observed within these parameters. Such results indicate that Tamarix can be subjected to significant soil water deficits and still respond rapidly to the presence of water. We conclude that significantly longer drydown periods without plant contact with a water table would be required to cause catastrophic cavitation in Tamarix. (C) 1997 Academic Press Limited.