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 transpiratio n of whole Zygophylium dumosum Boiss. shrubs were examined with a fiel d IRGA system during a rainless summer. Daily photosynthesis and trans piration activities were not notably different on a unit phyllode area basis among shrubs at naturally differing soil water potentials. Irri gation of shrubs caused phyllodes to increase significantly in water c ontent and new leaflets to appear. Leaflets had three times as many st omata per unit area (23 000 stomata cm-2) as phyllodes (7100 stomata c m-2) but photosynthesis and transpiration rates were not measurably di fferent between irrigated and non-irrigated shrubs on a unit area basi s. This finding suggests that sufficient soil moisture will lead to in creased carbon uptake of the entire shrub simply because the total are a of photosynthesizing tissue increases. Gas exchange rates appear to be controlled solely by atmospheric conditions under the stresses of s ummer.