18O Spatial Patterns of Vein Xylem Water, Leaf Water, and Dry Matter in Cotton Leaves

Abstract

Three leaf water models (two-pool model, Péclet effect, and string-of-lakes) were assessed for their robustness in predicting leaf water enrichment and its spatial heterogeneity. This was achieved by studying the 18 O spatial patterns of vein xylem water, leaf water, and dry matter in cotton (Gossypium hirsutum) leaves grown at different humidities using new experimental approaches. Vein xylem water was collected from intact transpiring cotton leaves by pressurizing the roots in a pressure chamber, whereas the isotopic content of leaf water was determined without extracting it from fresh leaves with the aid of a purpose-designed leaf punch. Our results indicate that veins have a significant degree of lateral exchange with highly enriched leaf water. Vein xylem water is thus slightly, but progressively enriched in the direction of water flow. Leaf water enrichment is dependent on the relative distances from major veins, with water from the marginal and intercostal regions more enriched and that next to veins and near the leaf base more depleted than the Craig-Gordon modeled enrichment of water at the sites of evaporation. The spatial pattern of leaf water enrichment varies with humidity, as expected from the string-of-lakes model. This pattern is also reflected in leaf dry matter. All three models are realistic, but none could fully account for all of the facets of leaf water enrichment. Our findings acknowledge the presence of capacitance in the ground tissues of vein ribs and highlight the essential need to incorporate Péclet effects into the string-of-lakes model when applying it to leaves.The isotopic composition of leaf water reflects local humidity, and its imprints on plant cellulose and other fossil materials have been widely explored for palaeoclimatic reconstruction. To date, isotopic values of wood cellulose (Epstein et al., 1977;Yapp and Epstein, 1982;Edwards et al., 1985;Edwards and Fritz, 1986;Roden et al., 2000), grassland phytoliths (Webb and Longstaffe, 2000), and deer bone (Cormie et al., 1994) have been shown to be related to leaf water isotopic composition. Leaf water isotopic signature is not only imprinted on plant organic matter but is also recorded in atmospheric CO 2 and O 2 . The CO 2 interacts and undergoes isotopic exchange with leaf water, and O 2 is released by the plant during photosynthesis. Changes in the oxygen isotope ratios of CO 2 and O 2 can thus be used to study variations in the net exchange of CO 2 in terrestrial ecosystems and in the balance of terrestrial and marine productivity (Bender et al., 1985;Bender et al., 1994). Because all of these applications critically depend on estimation of the leaf water oxygen isotopic ratio, a good understanding of the nature of leaf water enrichment is needed.The isotopic composition of leaf water is most commonly estimated from the model of a freely evaporating water surface (Craig and Gordon, 1965) where isotopic fractionation is driven by the lower vapor pressure and diffusivity of the heavier molecules. Although the Craig-Gordon m...

Progress and challenges in using stable isotopes to trace plant carbon and water relations across scales

“…Helliker and Ehleringer, 2000). Gan et al (2002) compared different leaf water evaporative enrichment models (i.e. the twopool model, the Péclet effect model and the string-of-lakes model), which assume different water isotopic gradients and different mixing of leaf water pools.…”

Contributions of evaporation, isotopic non‐steady state transpiration and atmospheric mixing on the δ18O of water vapour in Pacific Northwest coniferous forests

“…5 requires caution. Bulk leaf water is an ill-defined entity because it is a mixture of fractionated and non-fractionated water pools (Yakir 1992;Yakir et al 1994;Gan et al 2002). Predictions from current models specifically describe the enrichment at the sites of evaporation.…”

“…Gan et al (2002) used cotton leaves growing at different humidities to show that there were considerable spatial variations of leaf water enrichment within a leaf. They showed progressive δ…”

“…The Péclet correction considers the mixing of various water pools within leaves (Yakir, DeNiro & Gat 1990;Yakir 1992;Gan et al 2002;Farquhar & Gan 2003) that lowers the δ…”

Time for cotton to uptake water of a known isotopic signature as measured in leaf petioles

“…However, the use of stable isotopes has generally been limited to investigations of large changes in the water regime caused by rainy seasons or changes to the ground water table to determine the water source used by specific plants in the study area [7][8][9][10][11][12][13][14][15]. Investigations into shorter, more transient environmental changes, such as rain events of <25 mm have not been undertaken because of the relatively small changes in the isotope values and the need to take larger frequency samples to discriminate the effects of the rain on plant water uptake.…”