Long-term effects of drought on wild and cultivated plants in the Negev desert. II. Diurnal patterns of net photosynthesis and daily carbon gain

The seasonal change in diurnal patterns of net photosynthesis and daily carbon gain is studied in relation to the plant water status of the irrigated and non-irrigated naturally growing desert species Hammada scoparia, Zygophyllum dumosum, Artemisia herba-alba and Reaumuria negevensis. Comparison is made to cultivated Prunus armeniaca. Under non-irrigated natural conditions Hammada scoparia, a C₄ plant, showed one-peaked flat diurnal courses of CO₂ uptake which changed into a pattern of a high morning peak of CO₂ uptake or slightly two-peaked curves in the late dry season. In contrast, the C₃ species Zygophyllum dumosum, Artemisia herba-alba and Prunus armeniaca changed from one-peaked to distinct two-peaked patterns. At the end of the dry season, non-irrigated plants showed respiration only. Reaumuria negevensis had one-peaked curves with a low level of CO₂ uptake. There is no general relation between day-time CO₂ gain and pre-dawn water potential for the investigated species. In order to characterize the effect of soil drought, the CO₂ gain during day-time of non-irrigated plants is expressed as a percentage of the CO₂ gain of the irrigated counterparts. After an initial period of minimal drought effect, the relative day-time CO₂ gain decreases almost linearly with cumulative water stress as determined by the daily addition of pre-dawn water potentials for the non-irrigated plants since the last rainfall. The slope of decrease differs from species to species. The relation of daily CO₂ gain to maximal net photosynthesis is discussed. Initially, at a good plant water status, the daily CO₂ gain does not decrease in proportion to the maximal photosynthetic rates as a result of stomatal control at high photosynthetic activity. At increasing water stress the daily CO₂ gain decreases more than proportionally to the decrease of the maximal rates.