Transpiration and water uptake of Senecio medley-woodii and Aloe jucunda under changing environmental conditions: measurements with a potometric water-budget-meter

Abstract

Transpiration, water uptake by the roots and CO 2 exchange of two leaf succulents, Senecio medley-woodii (Asteraceae) and Aloe jucunda (Asphodeliaceae), were monitored simultaneously and continuously with a gas exchange cuvette combined with an apparatus to quantify water uptake (= water-budget-meter). Measurements, which are primarily valid for plants with a sufficient water supply, were made with the same plant for up to 29 consecutive days. Ambient air temperature varied between 17 and 35 °C with a constant dewpoint temperature of 13°C of the ambient air and a 12h photoperiod at 400-500 μmol m -2 s -1 photon irradiance. The net water flux (J W (net)= water uptake -transpiration) and the water balance (J W (net) integrated for a timespan) were calculated. Various tests were made to determine the accuracy of the measurements made with this rather complex equipment. In most cases the errors for transpiration and uptake rates were much lower than 8% determined under the conditions of drastically (about 10 K per 30 min) increased or decreased ambient air temperatures. The experimental set-up proved to be a most valuable tool to determine and analyse interactions between transpiration and water uptake, changes in plant water status and the buffering of negative J W (net). Increasing the temperature of ambient air resulted, for both species investigated, in a quick and considerably enhanced transpiration, but there was only a minor impact on water uptake. Water loss exceeding uptake was buffered by internal water reserves which were refilled within about 1 d after the plant was relieved of heat and drought stress caused by a period of high ambient air temperatures and high water vapour saturation deficits of the air. Repeated simulation of such stress periods showed that the absolute values of transpiration and the water uptake for 24 h can vary, but the diurnal course of the values showed the same pattern if the environmental conditions were identical. Such standardized diurnal transpiration and water uptake curves could be very useful for the validation of mathematical models used to describe plant water relations.