Suan Chin Wong

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

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 18O 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.

Environmental Effects on Oxygen Isotope Enrichment of Leaf Water in Cotton Leaves

The oxygen isotope enrichment of bulk leaf water (Δb) was measured in cotton (Gossypium hirsutum) leaves to test the Craig-Gordon and Farquhar-Gan models under different environmental conditions. Δb increased with increasing leaf-to-air vapor pressure difference (VPd) as an overall result of the responses to the ratio of ambient to intercellular vapor pressures (ea/ei) and to stomatal conductance (gs). The oxygen isotope enrichment of lamina water relative to source water \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(({\bar{{\Delta}}}_{1}),\) \end{document} which increased with increasing VPd, was estimated by mass balance between less enriched water in primary veins and enriched water in the leaf. The Craig-Gordon model overestimated Δb (and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\bar{{\Delta}}}_{1}),\) \end{document} as expected. Such discrepancies increased with increase in transpiration rate (E), supporting the Farquhar-Gan model, which gave reasonable predictions of Δb and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\bar{{\Delta}}}_{1}\) \end{document} with an L of 7.9 mm, much less than the total radial effective length Lr of 43 mm. The fitted values of L for \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\bar{{\Delta}}}_{1}\) \end{document} of individual leaves showed little dependence on VPd and temperature, supporting the assumption that the Farquhar-Gan formulation is relevant and useful in describing leaf water isotopic enrichment.

Seasonal change in water use and carbon assimilation of irrigated wheat

Abstract Measurements of evaporation and net assimilation rate of irrigated wheat at Griffith, New South Wales, were made during 73 days from full cover to physiological maturity. Micrometeorological determinations of flux densities of water vapour and CO 2 were based on the energy balance or Bowen ratio technique. Complementary measurements involving weighing lysimetry, soil water content and plant dry matter indicated that crop water use and growth could be explained in terms of a series of hourly values of these fluxes. Daily rates of evaporation increased ∼3-fold to 10 mm day −1 during the study period. Estimates of potential evaporation consistently underestimated measured evaporation until the final 20 days, when declining soil water and crop senescence reduced evaporation rates. Net daytime assimilation rates of CO 2 were nearly constant with time prior to anthesis, with day-to-day variability being an order of magnitude less than that for evaporation rate. Both daily and peak rates of net assimilation were generally greater than those reported for dryland wheat in southern Australia, but were comparable with those for irrigated sunflower. During the post-anthesis phase, a progressive decline in rate was observed. The initial decline was attributed to crop lodging, but later reductions are thought to be due to the combined effect of soil water depletion and crop sensecence. A loss in water use efficiency of 24% from the pre- to post-anthesis stages was determined from harvests of standing crop material near anthesis and at physiological maturity. The ratio of hourly evaporation to net assimulation was positively correlated with vapour pressure deficit to explain, in part, the observed reduction as a response to decreasing relative humidity with progression into the reproductive stage. Lodging and senescence of the crop were identified as additional factors which depressed water-use efficiency during this stage. An improved understanding of the dynamics of water-use efficiency requires further development in measurement to partition fluxes of gaseous exchange between plant and soil.

Promotion of shoot development and tuberisation in potato by expression of a chimaeric cytokinin synthesis gene at normal and elevated CO2 levels

The bacterial cytokinin biosynthesis gene ipt under control of a chalcone synthase promoter (PCHS) was introduced into potato (Solanum tuberosum L.). Two transgenic lines were selected for detailed study, because in these, root development was reduced only moderately, thus, enabling the plants to be grown in pots. Expression of the PCHS-ipt gene elevated the level of zeatin cytokinins markedly in the apical bud, subapical stems and leaves. The transgenic (IPT) plants exhibited a lower and denser leaf canopy relative to wild-type (WT) plants owing to reduction in main stem length, increase in node number per stem and promotion of lateral shoot development. Main stem diameter was increased markedly due to promotion of cell division associated with activation of cyclin-dependent kinase in the subapical stem. Expression of the PCHS-ipt gene induced aerial stolons, promoted growth of underground stolons and increased tuber number but reduced tuber weight and nitrogen content. The gene expression also increased pinnae and pinnule number per leaf, increased thickness of pinnae and promoted transpiration, photosynthesis and stomatal conductance – effects monitored by gas exchange and 18O and 13C analysis. The elevation of [CO2] to 900 μmol mol–1 promoted growth of both WT and IPT plants, ameliorated the negative effect of high cytokinin on tuber weight and interacted additively with ipt gene expression to promote stem growth.

Effects of root restriction on growth and associated cytokinin levels in cotton (Gossypium hirsutum)

Root restriction experiments on cotton plants were conducted under conditions where water and nitrogen supplies were not limiting. Xylem sap and leaf tissues were obtained before significant biomass reduction as a result of root restriction. Xylem-derived cytokinins (CTKs) entering a leaf and the CTK levels in the same excised leaf were measured, thus, enabling xylem sap and foliar CTKs to be compared concurrently under root-restrictive and control conditions. Abscisic acid (ABA) was also measured simultaneously in the same xylem sap samples. Root restriction reduced photosynthesis and δ18O of leaf matter showed that leaves of root-restricted plants generally had lower stomatal conductances. Root-restriction significantly decreased (37%) the delivery rate per unit leaf area of CTK bases (dihydrozeatin (DZ) and N6-(2-isopentenyl)adenine (iP)) to the leaf but did not alter the export of CTK O-glucosides and ABA from the roots to leaf 4. Root-restriction also did not alter significantly the levels of total CTK bases in the leaves but increased (54–57% higher) the CTK O-glucosides (especially zeatin O-glucosides). Root restriction also slowed down the apparent CTK turnover rates in the leaves. The slow down in CTK turnover rate can be interpreted as a reduction in leaf CTK metabolism during root restriction.

Rhizobium-induced elevation in xylem cytokinin delivery in pigeonpea induces changes in shoot development and leaf physiology

Inoculation with Rhizobium strain IC3342 induces in pigeonpea (Cajanus cajan (L) Millsp.) a leaf curl syndrome and elevated cytokinin levels in the xylem sap. High nitrogen (N) nutrition was found to inhibit onset of the syndrome which could then be induced by N-free nutrient after development of seven trifoliate leaves. This provided a new system to study the role of xylem cytokinin in shoot development and yielded plants suitable for determining the rate of delivery of xylem cytokinin to the shoot which for IC3342-inoculated plants was found to be three times that of control plants. Relative to leaves of control plants, the non-curled leaves of these IC3342 plants exhibited higher nitrogen and chlorophyll content and greater photosynthetic rate and stomatal conductance. Induction of the syndrome increased leaf thickness in developing leaves but not in expanded leaves already formed. Diameter of stems and number of laterals were also increased markedly by IC3342 inoculation which in addition induced leaf hyponasty. Exogenous cytokinins when applied directly to control leaves induced leaf curl and increased leaf thickness. The present studies are discussed in relation to the role of xylem cytokinins in plant development and especially the release of lateral buds from apical dominance.

Unsaturation of vapour pressure inside leaves of two conifer species

Stomatal conductance (gs) impacts both photosynthesis and transpiration, and is therefore fundamental to the global carbon and water cycles, food production, and ecosystem services. Mathematical models provide the primary means of analysing this important leaf gas exchange parameter. A nearly universal assumption in such models is that the vapour pressure inside leaves (ei) remains saturated under all conditions. The validity of this assumption has not been well tested, because so far ei cannot be measured directly. Here, we test this assumption using a novel technique, based on coupled measurements of leaf gas exchange and the stable isotope compositions of CO2 and water vapour passing over the leaf. We applied this technique to mature individuals of two semiarid conifer species. In both species, ei routinely dropped below saturation when leaves were exposed to moderate to high air vapour pressure deficits. Typical values of relative humidity in the intercellular air spaces were as low 0.9 in Juniperus monosperma and 0.8 in Pinus edulis. These departures of ei from saturation caused significant biases in calculations of gs and the intercellular CO2 concentration. Our results refute the longstanding assumption of saturated vapour pressure in plant leaves under all conditions.