Variation in hydraulic architecture and gas-exchange in two desert sub-shrubs, Hymenoclea salsola (T. & G.) and Ambrosia dumosa (Payne)

Adjustment of hydraulic architecture in response to environmental condition s was studied in two warm-desert sub-shrubs, Hymenoclea salsola and Ambrosi a dumosa, both at the level of genetic adaptation along a climatic gradient and plastic response to immediate growth conditions: Individuals of both s pecies originating from southern populations developed higher leaf-specific hydraulic conductance in the common greenhouse than individuals from north ern populations. Hy draulic conductance was higher in plants grown at high temperature, but did not vary as a function of growth relative humidity. Hy draulic conductance was not correlated within species with individual varia tion in vessel diameter, cavitation vulnerability, or root:shoot ratio, but was strongly, negatively correlated with the fraction of total plant bioma ss allocated to leaves. For both species, stomatal conductance (g(s)) at hi gh leaf-to-air vapor pressure difference (nu) was tightly correlated with v ariability in hydraulic conductance, as was the sensitivity of stomatal clo sure to increasing nu. Experimentally increasing shoot water potential by s oil pressurization, under conditions where high nu had already caused stoma tal closure, led to substantial stomatal reopening in both species, but rec overy was significantly higher in H. salsola. Hydraulic conductance was hig her in H, salsola than A. dumosa. H. salsola also differed from A. dumosa b y being a representative of a highly specialised group of desert shrubs whi ch use the twigs as a major photosynthetic organ. The southern population o f H. salsola produced far fewer leaves and relied much more heavily on twig photosynthesis than the northern population. At the whole-plant level, inc reased reliance on twig photosynthesis was associated with higher leaf-spec ific hydraulic conductance, but equivalent whole-plant photosynthesis on ei ther a dry weight (mu mol CO2 g(-1)) or nitrogen basis (mu mol CO2 g(-1))). This suggests that twig photosynthesis might be one way of increasing hydr aulic conductance per unit photosynthetic canopy by increasing allocation t o an organ which simultaneously performs photosynthetic, support, and trans port functions.