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Drought is a major constraint to canola production around the world. There is potential for improving crop performance in dry environments by selecting for transpiration efficiency (TE). In this work we investigated TE by studying its genetic association with carbon isotope discrimination (Δ) and other traits, e.g. specific leaf weight (SLW) and leaf chlorophyll content (SPAD). Among the 106 canola genotypes - including open-pollinated, hybrid, inbred types and cytoplasmic variants - tested in the field and glasshouse there was significant genotypic variation for TE, Δ, plant total dry weight, SLW and SPAD. Strong negative correlations were observed between TE and Δ (-0.52 to -0.76). Negative correlations between Δ and SLW or SPAD (-0.43 to -0.78) and smaller but significant positive correlations between TE and SLW or SPAD (0.23 to 0.30) suggested that photosynthetic capacity was, in part, underpinning the variation in TE. A cytoplasmic contribution to genetic variation in TE or Δ in canola was also observed with Triazine tolerant types having low TE and high Δ. This study showed that Δ has great potential for selecting canola germplasm with improved TE.

Photosynthetic manipulation is seen as a promising avenue for advancing field crop productivity. However, progress is constrained by the lack of connection between leaf-level photosynthetic manipulation and crop performance. Here we report on the development of a model of diurnal canopy photosynthesis for well watered conditions by using biochemical models of C3 and C4 photosynthesis upscaled to the canopy level using the simple and robust sun-shade leaves representation of the canopy. The canopy model was integrated over the time course of the day for diurnal canopy photosynthesis simulation. Rationality analysis of the model showed that it simulated the expected responses in diurnal canopy photosynthesis and daily biomass accumulation to key environmental factors (i.e. radiation, temperature and CO2), canopy attributes (e.g. leaf area index and leaf angle) and canopy nitrogen status (i.e. specific leaf nitrogen and its profile through the canopy). This Diurnal Canopy Photosynthesis Simulator (DCaPS) was developed into a web-based application to enhance usability of the model. Applications of the DCaPS package for assessing likely canopy-level consequences of changes in photosynthetic properties and its implications for connecting photosynthesis with crop growth and development modelling are discussed.

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.

Tropical soils are often characterised by low phosphorus availability and tropical forest trees typically exhibit lower area-based rates of photosynthesis (Aa) for a given area-based leaf nitrogen concentration ([N]a) compared with plants growing in higher-latitude, N-limited ecosystems. Nevertheless, to date, very few studies have assessed the effects of P deprivation per se on Aa⟷[N]a relationships in tropical trees. Our study investigated the effect of reduced soil P availability on light-saturated Aa and related leaf traits of seven Australian tropical tree species. We addressed the following questions: (1) Do contrasting species exhibit inherent differences in nutrient partitioning and morphology? (2) Does P deprivation lead to a change in the nature of the Aa⟷[N]a relationship? (3) Does P deprivation lead to an alteration in leaf nitrogen levels or N allocation within the leaf? Applying a mixed effects model, we found that for these Australian tropical tree species, removal of P from the nutrient solution decreased area-based photosynthetic capacity (Amax,a) by 18% and reduced the slope of the Amax,a⟷[N]a relationship and differences among species accounted for around 30% of response variation. Despite greater N allocation to chlorophyll, photosynthetic N use efficiency was significantly reduced in low-P plants. Collectively, our results support the view that low soil P availability can alter photosynthesis-nitrogen relationships in tropical trees.

CO2 diffusion from substomatal intercellular cavities to sites of carboxylation in chloroplasts (mesophyll conductance; gm) limits photosynthetic rate and influences leaf intrinsic water-use efficiency (A/gsw). We investigated genotypic variability of gm and effects of gm on A/gsw among eleven wheat (Triticum aestivum L.) genotypes under light-saturated conditions and at either 2 or 21% O2. Significant variation in gm and A/gsw was found between genotypes at both O2 concentrations, but there was no significant effect of O2 concentration on gm. Further, gm was correlated with photosynthetic rate among the 11 genotypes, but was unrelated to stomatal conductance. The effect of leaf age differed between genotypes, with gm being lower in older leaves for one genotype but not another. This study demonstrates a high level of variation in gm between wheat genotypes; 0.5 to 1.0μmolm-2s-1 bar-1. Further, leaf age effects indicate that great care must be taken to choose suitable leaves in studies of genotypic variation in gm and water-use efficiency.

Stomata are the site of CO2 exchange for water in a leaf. Variation in stomatal control offers promise in genetic improvement of transpiration and photosynthetic rates to improve wheat performance. However, techniques for estimating stomatal conductance (SC) are slow, limiting potential for efficient measurement and genetic modification of this trait. Genotypic variation in canopy temperature (CT) and leaf porosity (LP), as surrogates for SC, were assessed in three wheat mapping populations grown under well-watered conditions. The range and resulting genetic variance were large but not always repeatable across days and years for CT and LP alike. Leaf-to-leaf variation was large for LP, reducing heritability to near zero on a single-leaf basis. Replication across dates and years increased line-mean heritability to ~75% for both CT and LP. Across sampling dates and populations, CT showed a large, additive genetic correlation with LP (rg=-0.67 to -0.83) as expected. Genetic increases in pre-flowering CT were associated with reduced final plant height and both increased harvest index and grain yield but were uncorrelated with aerial biomass. In contrast, post-flowering, cooler canopies were associated with greater aerial biomass and increased grain number and yield. A multi-environment QTL analysis identified up to 16 and 15 genomic regions for CT and LP, respectively, across all three populations. Several of the LP and CT QTL co-located with known QTL for plant height and phenological development and intervals for many of the CT and LP quantitative trait loci (QTL) overlapped, supporting a common genetic basis for the two traits. Notably, both Rht-B1b and Rht-D1b dwarfing alleles were paradoxically positive for LP and CT (i.e. semi-dwarfs had higher stomatal conductance but warmer canopies) highlighting the issue of translation from leaf to canopy in screening for greater transpiration. The strong requirement for repeated assessment of SC suggests the more rapid CT assessment may be of greater value for indirect screening of high or low SC among large numbers of early-generation breeding lines. However, account must be taken of variation in development and canopy architecture when interpreting performance and selecting breeding lines on the basis of CT.

Non-photosynthetic, or heterotrophic, tissues in C 3 plants tend to be enriched in 13 C compared with the leaves that supply them with photosynthate. This isotopic pattern has been observed for woody stems, roots, seeds and fruits, emerging leaves, and parasitic plants incapable of net CO 2 fixation. Unlike in C 3 plants, roots of herbaceous C 4 plants are generally not 13 C-enriched compared with leaves. We review six hypotheses aimed at explaining this isotopic pattern in C 3 plants: (1) variation in biochemical composition of heterotrophic tissues compared with leaves; (2) seasonal separation of growth of leaves and heterotrophic tissues, with corresponding variation in photosynthetic discrimination against 13 C; (3) differential use of day v. night sucrose between leaves and sink tissues, with day sucrose being relatively 13 C-depleted and night sucrose 13 C-enriched; (4) isotopic fractionation during dark respiration; (5) carbon fixation by PEP carboxylase; and (6) developmental variation in photosynthetic discrimination against 13 C during leaf expansion. Although hypotheses (1) and (2) may contribute to the general pattern, they cannot explain all observations. Some evidence exists in support of hypotheses (3) through to (6), although for hypothesis (6) it is largely circumstantial. Hypothesis (3) provides a promising avenue for future research. Direct tests of these hypotheses should be carried out to provide insight into the mechanisms causing within-plant variation in carbon isotope composition.

Internal conductance to carbon dioxide is a key aspect of leaf photosynthesis although is still not well understood. It is thought that it comprises two components, namely, a gas phase component (diffusion from intercellular spaces to cell walls) and a liquid phase component (dissolution, diffusion in water, hydration equilibrium). Here we use heavy water (D 2 O), which is known to slow down CO 2 hydration by a factor of nearly three. Using 12 C/ 13 C stable isotope techniques and Xanthium strumarium L. leaves, we show that the on-line carbon isotope discrimination (Δ 13 C, or Δ obs ) associated with photosynthesis is not significantly decreased by heavy water, and that the internal conductance, estimated with relationships involving the deviation of Δ 13 C, decreased by 8-40% in 21% O 2 . It is concluded that in typical conditions, the CO 2 -hydration equilibrium does not exert an effect on CO 2 assimilation larger than 9%. The carbon isotope discrimination associated with CO 2 addition to ribulose-1,5,bisphosphate by Rubisco is slightly decreased by heavy water. This effect is proposed to originate from the use of solvent-derived proton/deuteron during the last step of the catalytic cycle of the enzyme (hydration/cleavage).

While photosynthetically evolved O 2 has been repeatedly shown to have nearly the same oxygen isotope composition as source water so that there is no corresponding 16 O/ 18 O isotope effect, some recent 18 O-enrichment studies suggest that a large isotope effect may occur, thus feeding a debate in the literature. Here, the classical theory of isotope effects was applied to show that a very small isotope effect is indeed expected during O 2 production. Explanations of the conflicting results are briefly discussed.

Post-photosynthetic carbon isotope fractionation might alter the isotopic signal imprinted on organic matter (OM) during primary carbon fixation by Rubisco. To characterise the influence of post-photosynthetic processes, we investigated the effect of starch storage and remobilisation on the stable carbon isotope signature (δ 13 C) of different carbon pools in the Eucalyptus delegatensis R. T. Baker leaf and the potential carbon isotope fractionation associated with phloem transport and respiration. Twig phloem exudate and leaf water-soluble OM showed diel variations in δ 13 C of up to 2.5 and 2‰, respectively, with 13 C enrichment during the night and depletion during the day. Damped diel variation was also evident in bulk lipids of the leaf and in the leaf wax fraction. δ 13 C of nocturnal phloem exudate OM corresponded with the δ 13 C of carbon released from starch. There was no change in δ 13 C of phloem carbon along the trunk. CO 2 emitted from trunks and roots was 13 C enriched compared with the potential organic substrate, and depleted compared with soil-emitted CO 2 . The results are consistent with transitory starch accumulation and remobilisation governing the diel rhythm of δ 13 C in phloem-transported OM and fragmentation fractionation occurring during respiration. When using δ 13 C of OM or CO 2 for assessing ecosystem processes or plant reactions towards environmental constraints, post-photosynthetic discrimination should be considered.

Slow stomatal oscillations (70-95 min), associated with feedback within the plant hydraulic systems, were studied in cotton (Gossypium hirsutum L.). Oscillations were only evident when the whole plant was exposed to light, and were not influenced by reductions in intercellular CO 2 concentrations (C i ) in intact, attached leaves. Oscillations were synchronised among different leaves of the same plant, even when the leaf-to-air vapour pressure difference (VPD) was reduced in a cuvette enclosing one of the leaves. In the trough phase of stomatal oscillations the apparent C i was higher than expected from the combination of the observed assimilation rate and the A(C i ) relationship measured in the absence of oscillations. Using chlorophyll fluorescence imaging we found evidence of stomatal heterogeneity in this phase. Finally, we found that stomatal oscillations appeared to be correlated with xylem embolism, with more vessels filled with gas at the peak than at the troughs of stomatal oscillations.

Methods for determining carbon isotope discrimination, Δ, or kinetic isotope effects, α, for CO 2 -consuming enzymes have traditionally been cumbersome and time-consuming, requiring careful isolation of substrates and products and conversion of these to CO 2 for measurement of isotope ratio by mass spectrometry (MS). An equation originally derived by Rayleigh in 1896 has been used more recently to good effect as it only requires measurement of substrate concentrations and isotope ratios. For carboxylation reactions such as those catalysed by d-ribulose-1,5-bisphosphate carboxylase / oxygenase (RuBisCO, EC 4.1.1.39) and PEP carboxylase (PEPC, EC 4.1.1.31), this has still required sampling of reactions at various states of completion and conversion of all inorganic carbon to CO 2 , as well as determining the amount of substrate consumed. We introduce a new method of membrane inlet MS which can be used to continuously monitor individual CO 2 isotope concentrations, rather than isotope ratio. This enables the use of a simplified, new formula for calculating kinetic isotope effects, based on the assumptions underlying the original Rayleigh fractionation equation and given by.

Plant nitrate reductase, the enzyme that reduces nitrate (NO 3 - ) to nitrite (NO 2 - ), is known to fractionate N isotopes, depleting nitrite in 15 N compared with substrate nitrate. Nearly 20 years ago, the nitrogen isotope effect associated with this reaction was found to be around 1.015. However, the relationships between the isotope effect and the mechanism of the reaction have not yet been examined in the light of recent advances regarding the catalytic cycle and enzyme structure. We thus give here the mathematical bases of the 14 N /  15 N and also the 16 O /  18 O isotope effects as a function of reaction rates. Enzymatic nitrate reduction involves steps other than NO 3 - reduction itself, in which the oxidation number of N changes from +V (nitrate) to +III (nitrite). Using some approximations, we give numerical estimates of the intrinsic N and O isotope effects and this leads us to challenge the assumptions of nitrate reduction itself as being a rate-limiting step within the nitrate reductase reaction, and of the formation of a bridging oxygen as a reaction intermediate.

A recent study by Keppler et al. (2006; Nature 439, 187-191) demonstrated CH 4 emission from living and dead plant tissues under aerobic conditions. This work included some calculations to extrapolate the findings from the laboratory to the global scale and led various commentators to question the value of planting trees as a greenhouse mitigation option. The experimental work of Keppler et al. (2006) appears to be largely sound, although some concerns remain about the quantification of emission rates. However, whilst accepting their basic findings, we are critical of the method used for extrapolating results to a global scale. Using the same basic information, we present alternative calculations to estimate global aerobic plant CH 4 emissions as 10-60 Mt CH 4 year -1 . This estimate is much smaller than the 62-236 Mt CH 4 year -1 reported in the original study and can be more readily reconciled within the uncertainties in the established sources and sinks in the global CH 4 budget. We also assessed their findings in terms of their possible relevance for planting trees as a greenhouse mitigation option. We conclude that consideration of aerobic CH 4 emissions from plants would reduce the benefit of planting trees by between 0 and 4.4%. Hence, any offset from CH 4 emission is small in comparison to the significant benefit from carbon sequestration. However, much critical information is still lacking about aerobic CH 4 emission from plants. For example, we do not yet know the underlying mechanism for aerobic CH 4 emission, how CH 4 emissions change with light, temperature and the physiological state of leaves, whether emissions change over time under constant conditions, whether they are related to photosynthesis and how they relate to the chemical composition of biomass. Therefore, the present calculations must be seen as a preliminary attempt to assess the global significance from a basis of limited information and are likely to be revised as further information becomes available.

Variation in the oxygen isotope composition of within-canopy CO 2 has potential to allow partitioning of the ecosystem respiratory flux into above- and below-ground components. Recent theoretical work has highlighted the sensitivity of the oxygen isotope composition of leaf-respired CO 2 (δ Rl ) to nocturnal stomatal conductance. When the one-way flux model was tested on Ricinus communis L. large enrichments in δ Rl were observed. However, most species for which the isotope flux partitioning technique has been or would be applied (i.e. temperate tree species) are much more conservative users of water than R. communis. So, high stomatal conductance and very high enrichment of δ Rl observed may not be typical for temperate tree species. Using existing gas-exchange measurements on six temperate tree species, we demonstrate significant water loss through stomata for all species (i.e. statistically significantly greater than cuticular loss alone) at some time for some leaves during the night. δ Rl values predicted by the one-way flux model revealed that δ Rl might be very much more enriched than when the net flux alone is considered, particularly close to sunrise and sunset. Incorporation of the one-way flux model into ecosystem respiration partitioning studies will affect model outputs and interpretation of variation in the oxygen isotope composition of atmospheric CO 2 .

An expression is derived for the isotopic composition of water in leaves under conditions where the composition of water entering the leaf is not necessarily the same as that of water being transpired. The treatment is simplified and considers the average composition of the lamina and of the sites of evaporation. The concept of 'isostorage' is introduced as the product of leaf water content and the isotopic enrichment of leaf water above source water. It is shown that the rate of increase of isostorage is minus the 'isoflux' through the stomata, with the latter expressed as the product of the transpiration flux and the enrichment of the transpired water beyond source water. The approach of the isostorage to the steady state depends on the deviation of the isotopic enrichment of water at the evaporating sites from the steady value, and on the gross (one way) diffusive flux out of the leaf. To achieve model closure, it is assumed that the relationship between leaf water enrichment and that at the sites of evaporation depends on the radial Péclet number in the same manner as in the steady state. The equations have an analytical solution, and we also show how to calculate the results simply using a commonly available computer tool. The form of the equations emphasises that the one-way fluxes of water into and out of the stomata must sometimes be considered separately, rather than as a net outward flux. In this narrow sense we come to the interesting conclusion that more water usually enters the leaf from the air than from the roots.

Carbon isotope effects of enzymes involved in primary carbon metabolism are key parameters in our understanding of plant metabolism. Nevertheless, some of them are poorly known because of the lack of in vitro experimental data on purified enzymes. Some studies have focused on theoretical predictions of isotope effects. Here we show how quantum chemical calculations can be adapted for calculation of isotope effects for the Rubisco-catalysed carboxylation and oxygenation reactions and the citrate synthase reaction. The intrinsic isotope effect of the carboxylation by Rubisco appears to be much smaller than previously thought, being close to the overall isotope effect of the reaction that is, between 25 and 30 per mil. The same applies to the enzyme citrate synthase, that catalyses the first step of the Krebs cycle, with an isotope effect of around 23 per mil. Combined with the isotope effects of equilibrium reactions calculated with β-factors, the Krebs cycle then has an overall isotope effect that depletes organic acids in 13 C.

The temperature response of J max , the irradiance-saturated potential rate of photosynthetic electron transport in the absence of Rubisco limitation, has usually been modelled by a complicated, modified Arrhenius type of equation. Light saturation can be difficult to achieve and reduces the precision of fluorescence measurements. Consequently, we calculated the rate of electron transport at 1200 μmol photosynthetically active radiation (PAR) quanta m -2 s -1 from chlorophyll fluorescence measurements on intact soybean leaves [Glycine max (L.) Merr] as temperature increased from 15 to 43°C with 1250 μmol mol -1 ambient [CO 2 ]. Electron transport rate was maximal around 37°C and the decline in rate following further increases in leaf temperature to 43°C was found to be completely reversible immediately upon return to lower temperatures. We report a convenient, new equation for the temperature dependence of the rate of electron transport under high irradiance:...

We measured the oxygen isotope composition of both the water and dry matter components of phloem sap exported from photosynthesising Ricinus communis L. leaves. The 18 O / 16 O composition of exported dry matter matched almost exactly that expected for equilibrium with average lamina leaf water (leaf water exclusive of water associated with primary veins) with an isotope effect of α o =1.027, where α o =R o / R w , and R o and R w are 18 O / 16 O of organic molecules and water, respectively. Average lamina leaf water was enriched by 14-22‰ compared with source water under our experimental conditions, and depleted by 4-7‰, compared with evaporative site water. This showed that it is the average lamina leaf water 18 O / 16 O signal that is exported from photosynthesising leaves rather than a signal more closely related to that of evaporative site water or source water. Additionally, we found that water exported in phloem sap from photosynthesising leaves was enriched compared with source water; the mean phloem water enrichment observed for leaf petioles was 4.0 ± 1.5‰ (mean ± 1 s.d., n = 27). Phloem water collected from stem bases was also enriched compared with source water. However, the enrichment was approximately 0.8 times that observed for leaf petioles, suggesting some mixing between enriched phloem water and unenriched xylem water occurred during translocation. Results validated the assumption that organic molecules exported from photosynthesising leaves are enriched by 27‰ compared with average lamina leaf water. Furthermore, results suggest that the potential influence of enriched phloem water should be considered when interpreting the 18 O / 16 O signatures of plant organic material and plant cellulose.