Are carbon and oxygen isotope compositions of bulk leaf material reliable predictors of water use efficiency in slow-growing droughtadapted species?

摘要

Regulation of water use efficiency (WUE) is particularly important in Mediterranean ecosystems, where plants are periodically exposed to severe water stress. An important technique to measure WUE is carbon isotope discrimination (DELTA~ (13)C) in leaf material, which represents an integrative measure over the leaf lifetime. However, in bulk leaf material of slow-growing evergreen species the influence of seasonal environmental constraints may be buffered by leaf longevity. Furthermore, under conditions of limited resources, interpretation of DELTA~ (13)C may not be straightforward, since regulation may occur through a coherent change in stomatal restriction of CO_2 diffusion and down-regulation of photosynthetic capacity, which may result in similar WUE. Recently it has been suggested that the oxygen isotope ratio (delta ~(18)O) of bulk leaf material may reflect evaporative conditions and may be used to determine whether variations in DELTA ~(13)C result from differences in photosynthetic capacity or in stomatal conductance (Farquhar et al. 1998). While ~(13)C is depleted during photosynthesis, leaf water is enriched in ~(18)O during transpiration, since diffusion and enzymatic incorporation generally favours the lighter isotope. This enrichmentin ~(18)O of leaf water is passed on to organic molecules due to an isotopic exchange of the water oxygen and the carbonyl group oxygen during biosynthesis. The oxygen isotope composition of organic material is further influenced by the isotopic composition of the source water. The degree of leaf water enrichment depends on the ratio of the vapour pressure differences in the atmosphere and the intercellular spaces (ea/ei), whereby low relative humidity causes an increase in leaf water enrichment. In general, bulk leaf water is somewhat less enriched than water at the evaporating surface, due to a gradient within the leaf as a result of the shifting balance between the convective evaporation flux of unfractionated water through the leaf, and the back diffusion of isotopically enriched water away from the evaporating sites (Yakir & Sternberg 2000). This phenomenon, termed the Peclet effect, has been modelled by Farquhar and Lloyd (1993) predicting an inverse relationship between delta ~(18)O of bulk leaf water and the rate of transpiration.