GAS-EXCHANGE OF A DESERT SHRUB (ZYGOPHYLLUM-DUMOSUM BOISS) UNDER DIFFERENT SOIL-MOISTURE REGIMES DURING SUMMER DROUGHT

The effects of soil water potential on photosynthesis and transpiration of whole Zygophylium dumosum Boiss. shrubs were examined with a field IRGA system during a rainless summer. Daily photosynthesis and transpiration activities were not notably different on a unit phyllode area basis among shrubs at naturally differing soil water potentials. Irrigation of shrubs caused phyllodes to increase significantly in water content and new leaflets to appear. Leaflets had three times as many stomata per unit area (23 000 stomata cm-2) as phyllodes (7100 stomata cm-2) but photosynthesis and transpiration rates were not measurably different between irrigated and non-irrigated shrubs on a unit area basis. This finding suggests that sufficient soil moisture will lead to increased carbon uptake of the entire shrub simply because the total area of photosynthesizing tissue increases. Gas exchange rates appear to be controlled solely by atmospheric conditions under the stresses of summer.