Utilizable water in leaves of 8 arid species as derived from pressure-volume curves and chlorophyll fluorescence

The water-storage properties of leaves from 8 co-occurring arid species, ranging from 4 to 36 in turgid weight/dry weight ratio, were studied. Total water content at saturation varied from 75 to 97%, with 65-93% of this utilizable when bound water (B) was subtracted, and 72-93% when water content at a critical level of optimal fluorescence (F-c) was subtracted. F-c represented the turning point from slight to substantial impairment of chlorophyll fluorescence, and either coincided with B (two species) or was slightly below B (6 species). An incipient level of fluorescence was also recognized, corresponding to the lowest water content before any effect on maximum variable fluorescence/maximum fluorescence could be detected. This lay closer to B than to water content at turgor loss point (TLP), rendering TLP of no significance in dictating the fraction of stored water utilizable (UWC). Under laboratory conditions, turgid branchlets of the 8 species were estimated to take from 1.5 days to 15 weeks to reach F-c. The rate of water loss was almost completely explained by variations in leaf thickness. Under field conditions in mid-spring, UWC at F-c on a harvest weight basis ranged from 51 to 87% predawn and from 45 to 86% by early afternoon (EA). The UWC of plants severed from their root systems 6 weeks earlier ranged from 0 to 63% during the EA. Overall, two groups of species could be recognized: thick-leaved species whose UWC is high and varies little during the day and which use their stored water conservatively and have limited reliance on their root system for replenishment after winter; and thinner leaved species whose UWC is moderate and fluctuates daily, and whose stored water falls rapidly unless replenished continuously from the soil.