Stable isotope measurements are employed extensively in plant-water relations research to investigate physiological and hydrological processes from whole plant to ecosystem scales. Stable isotopes of hydrogen and oxygen are routinely measured to identify plant source water. This application relies on the assumption that no fractionation of oxygen and hydrogen isotopes in water occurs during uptake by roots. However, a large fraction of the water taken up through roots in halophytic and xerophytic plants transverses cell membranes in the endodermis before entering the root xylem. Passage of water through this symplastic pathway has been hypothesized to cause fractionation leading to a decrease in 2 H of root xylem water relative to that in the surrounding soil medium. We examined 16 woody halophytic and xerophytic plant species in controlled conditions for evidence of hydrogen isotope fractionation during uptake at the root-soil interface. Isotopic separation ( 2 H = 2 H soil water ¡ 2 H xylem water ) ranging from 3‰ to 9‰ was observed in 12 species. A signiWcant positive correlation between salinity tolerance and the magnitude of 2 H was observed. Water in whole stem segments, sapwood, and roots had signiWcantly lower 2 H values relative to soil water in Prosopis velutina Woot., the species expressing the greatest 2 H values among the 16 species examined. Pressurized water Xow through intact root systems of Artemisia tridentata Nutt. and Atriplex canescens (Pursh) Nutt. caused the 2 H values to decrease as Xow rate increased. This relationship was not observed in P. velutina. Destroying the plasma membranes of root cells by excessive heat from boiling did not signiWcantly alter the relationship between 2 H of expressed water and Xow rate. In light of these results, care should be taken when using the stable isotope method to examine source-water use in halophytic and xerophytic species.

Diffusion of CO from the leaf intercellular air space to the site of carboxylation (g ) is a potential trait for increasing net rates of CO assimilation (A ), photosynthetic efficiency, and crop productivity. Leaf anatomy plays a key role in this process; however, there are few investigations into how cell wall properties impact g and A . Online carbon isotope discrimination was used to determine g and A in Oryza sativa wild-type (WT) plants and mutants with disruptions in cell wall mixed-linkage glucan (MLG) production (CslF6 knockouts) under high- and low-light growth conditions. Cell wall thickness (T ), surface area of chloroplast exposed to intercellular air spaces (S ), leaf dry mass per area (LMA), effective porosity, and other leaf anatomical traits were also analyzed. The g of CslF6 mutants decreased by 83% relative to the WT, with c. 28% of the reduction in g explained by S . Although A /LMA and A /Chl partially explained differences in A between genotypes, the change in cell wall properties influenced the diffusivity and availability of CO . The data presented here indicate that the loss of MLG in CslF6 plants had an impact on g and demonstrate the importance of cell wall effective porosity and liquid path length on g .

In a seasonally dry plant community of central Florida, USA, that experiences water limitation in the dry season and high water availability in the wet season, we first tested whether evergreen woody species shift from shallow water in the wet season to deep water in the dry season. Second, we tested whether deciduous woody species restrict water uptake to the shallow soil during the wet season and cease water uptake during the dry season. To address these questions, we measured water source use of two deciduous and three evergreen species over 13 months using stable isotopes. As hypothesized in previous studies, we showed that leaflessness in deciduous plants is an important source of stem water isotopic fractionation. Therefore, we compared stable isotope ratios of stem water only when deciduous species had leaves and found that all species, except the evergreen Lyonia ferruginea, used proportionally the same water sources. Early dry season water use was based on water availability for all species except L. ferruginea, and deep soil (50–150 cm) was the most important water source. During the late dry and wet seasons, water uptake from each soil layer was based on its respective proportion of fine roots. Nevertheless, deep water remained an important water source throughout the year. This study clearly demonstrates the limitations of using stable isotopes of stem water when comparing deciduous versus evergreen species. Further, this study is the first to directly quantify depth of water uptake via isotope analysis and couple these findings with root distribution. Copyright © 2014 John Wiley & Sons, Ltd.

Improving crop productivity while simultaneously reducing agricultural water input is essential to ensure the security of our global food supply and protect our diminishing freshwater resources. The irrigation requirements needed to mitigate the productivity loss associated with drought stress makes agriculture the largest industrial consumer of fresh water (Boyer, 1982; Hamdy et al., 2003). Addressing these challenges will require an integrated approach that combines irrigation practices that minimize water loss and the deployment of crop plants with superior water use efficiency

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Drought is a major agricultural problem worldwide. Therefore, selection for increased water use efficiency (WUE) in food and biofuel crop species will be an important trait in plant breeding programs. The leaf carbon isotopic composition (δ(13)Cleaf) has been suggested to serve as a rapid and effective high throughput phenotyping method for WUE in both C3 and C4 species. This is because WUE, leaf carbon discrimination (Δ(13)Cleaf), and δ(13)Cleaf are correlated through their relationships with intercellular to ambient CO2 partial pressures (Ci/Ca). However, in C4 plants, changing environmental conditions may influence photosynthetic efficiency (bundle-sheath leakiness) and post-photosynthetic fractionation that will potentially alter the relationship between δ(13)Cleaf and Ci/Ca. Here we discuss how these factors influence the relationship between δ(13)Cleaf and WUE, and the potential of using δ(13)Cleaf as a meaningful proxy for WUE.

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