13C discrimination between atmosphere and bulk leaf matter (D 13 Clb) is frequently used as a proxy for transpiration efficiency (TE). Nevertheless, its relevance is challenged due to: (1) potential deviations from the theoretical discrimination model, and (2) complex time integration and upscaling from leaf to whole plant. Six hybrid genotypes of Populus deltoides ¥ nigra genotypes were grown in climate chambers and tested for whole-plant TE (i.e. accumulated biomass/water transpired). Net CO2 assimilation rates (A) and stomatal conductance (gs) were recorded in parallel to: (1) 13 C in leaf bulk material (d 13 Clb) and in soluble sugars (d 13 Css) and (2) 18 O in leaf water and bulk leaf material. Genotypic means of d 13 Clb and d 13 Css were tightly correlated. Discrimination between atmosphere and soluble sugars was correlated with daily intrinsic TE at leaf level (daily mean A/gs), and with whole-plant TE. Finally, gs was positively correlated to 18 O enrichment of bulk matter or water of leaves at individual level, but not at genotype level. We conclude that D 13 Clb captures efficiently the genetic variability of whole-plant TE in poplar. Nevertheless, scaling from leaf level to whole-plant TE requires to take into account water losses and respiration independent of photosynthesis, which remain poorly documented.
Poplar genotypes differ in transpiration efficiency (TE) at leaf and whole-plant level under similar conditions. We tested whether atmospheric vapour pressure deficit (VPD) affected TE to the same extent across genotypes. Six Populus nigra genotypes were grown under two VPD. We recorded (1) (13)C content in soluble sugars; (2) (18)O enrichment in leaf water; (3) leaf-level gas exchange; and (4) whole-plant biomass accumulation and water use. Whole-plant and intrinsic leaf TE and (13)C content in soluble sugars differed significantly among genotypes. Stomatal conductance contributed more to these differences than net CO2 assimilation rate. VPD increased water use and reduced whole-plant TE. It increased intrinsic leaf-level TE due to a decline in stomatal conductance. It also promoted higher (18)O enrichment in leaf water. VPD had no genotype-specific effect. We detected a deviation in the relationship between (13)C in leaf sugars and (13)C predicted from gas exchange and the standard discrimination model. This may be partly due to genotypic differences in mesophyll conductance, and to its lack of sensitivity to VPD. Leaf-level (13)C discrimination was a powerful predictor of the genetic variability of whole-plant TE irrespective of VPD during growth.
Genetic differences in δ¹³C (isotopic composition of dry matter carbon) have been evidenced among poplar genotypes at juvenile stages. To check whether such differences were maintained with age in trees growing in plantations, we investigated the time course of δ¹³C as recorded in annual tree rings from different genotypes growing at three sites in southwestern France and felled at ∼15-17 years. Wood cores were cut from tree discs to record the time course of annual basal area increment (BAI). The isotopic ratio δ¹³C was recorded in bulk wood and in extracted cellulose from the annual rings corresponding to the period 1996-2005. Discrimination against ¹³C between atmosphere and tissues (Δ¹³C) was computed by taking into account the inter-annual time course of δ¹³C in the atmosphere. Annual BAI increased steadily and stabilized at about 8 years. An offset in δ¹³C of ∼1‰ was recorded between extracted cellulose and bulk wood. It was relatively stable among genotypes within sites but varied among sites and increased slightly with age. Site effects as well as genotype differences were detected in Δ¹³C recorded from the cellulose fraction. Absolute values as well as the genotype ranking of Δ¹³C remained stable with age in the three sites. Genotype means of Δ¹³C were not correlated to annual BAI. We conclude that genotypic differences of Δ¹³C occur in older poplar trees in plantations, and that the differences as well as the genotype ranking remain stable while trees age until harvest.
A greenhouse experiment was conducted to investigate the effects of water deficit on growth and physiological parameters of Ficus benjamina and Conocarpus erectus. The results revealed that all growth parameters such as plant height, stem diameter, no. of leaves, no. of branches and chlorophyll contents significantly decreased under water deficit condition. Interestingly, although leaf, stem and total biomass production and allocation decreased significantly under water deficit, but root biomass production and allocation increased significantly. Similarly, stomatal conductance to water vapor decreased significantly and CO2 assimilation rate remained similar to control under water deficit condition. Resultantly, a significant increase in water use efficiency was evident in both species under water deficit condition. These results suggested that, in spite of a significant decrease in biomass production, young Conocarpus erectus and Ficus benjamina can tolerate water deficit which is due to sustained CO2 assimilation rate and increase in root biomass.
In the context of the predicted increasing frequency of summer droughts in the northeastern deciduous forest of North America due to climate change, we investigated the acclimation capacity of yellow birch, an economically important native tree species, to soil water deficit. We carried out an integrated examination of allocation of biomass, leaf physiology, branching pattern and in situ 3D crown display. Potted seedlings were subjected to moderate soil water deficit for four consecutive months during their second growing season. Individuals under soil water deficit showed a 40% decrease in biomass accumulation but no change in the relative allocation of biomass to the different plant compartments. Net CO assimilation rates at leaf level decreased under water deficit (~15%) but could not alone explain the total reduction in growth, excluding the carbon starvation hypothesis. The observed reduction in net CO assimilation rates was related to a decrease in stomatal conductance and chlorophyll content. STAR (in situ silhouette to total leaf area ratio; a proxy for light interception efficiency) decreased under soil water deficit due to shifts in biomass allocation within the branch compartment from long upper axes to short bottom axes. Despite the fact that the understanding of the processes involved in growth reduction and branching pattern alteration will need more attention in future research, we conclude that under water deficit yellow birch at young stages will: (i) experience a substantial loss of growth and biomass; and (ii) acclimate through architectural plasticity rather than through changes in the relative allocation of root biomass to enhance its water management.
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