Photosynthetic acclimation to elevated CO2 in poplar grown in glasshouse cabinets or in open top chambers depends on duration of exposure

The effects of elevated CO2 were studied on the photosynthetic gas exchange behaviour and leaf physiology of two contrasting poplar (Populus) hybrids grown and treated in open top chambers (OTCs in Antwerp, Belgium) and in closed glasshouse cabinets (GHCs in Sussex, UK). The CO2 concentrations used in the OTCs were ambient and ambient +350 µmol mol–1 while in the GHCs they were c. 360 µmol mol–1 versus 719 µmol mol–1. Measurements of photosynthetic gas exchange were made for euramerican and interamerican poplar hybrids in combination with measurements of dark respiration rate and Rubisco activity. Significant differences in the leaf anatomy and structure (leaf mass per area and chlorophyll content) were observed between the leaves grown in the OTCs and those grown in the GHCs. Elevated CO2 stimulated net photosynthesis in the poplar hybrids after 1 month in the GHCs and after 4 months in the OTCs, and there was no evidence of downward acclimation (or down-regulation) of photosynthesis when the plants in the two treatments were measured in their growth CO2 concentration. There was also no evidence of down-regulation of Rubisco activity and there were even examples of increases in Rubisco activity. Rubisco exerted a strong control over the light-saturated rate of photosynthesis, which was demonstrated by the close agreement between observed net photosynthetic rates and those that were predicted from Rubisco activities and Michaelis-Menten kinetics. After 17 months in elevated CO2 in the OTCs there was a significant loss of Rubisco activity for one of the hybrid clones, i.e. Beaupré, but not for clone Robusta. The effect of the CO2 measurement concentration (i.e. the short-term treatment effect) on net photosynthesis was always larger than the effect of the growth concentration in both the OTCs or GHCs (i.e. the longterm growth CO2 effect), with one exception. For the interamerican hybrid Beaupré dark respiration rates in the OTCs were not significantly affected by the elevated CO2 concentrations. The results suggest that for rapidly growing tree species, such as poplars, there is little evidence for downward acclimation of photosynthesis when plants are exposed to elevated CO2 for up to 4 months; longer term exposure reveals loss of Rubisco activity

Mechanisms of polyamine action during senescence responses induced by osmotic stress

The peeled Avena sativa L. leaf-system has been used as a simple and rapid model, which allows the study of the mechanisms of polyamine action during senescence responses induced by osmotic stress. The use of guazatine, an inhibitor of polyamine oxidase activity, has demonstrated the existence of a positive correlation between high levels of spermidine and spermine and delay of senescence in oat leaves and mesophyll protoplasts. The availability of antibodies specific for key polypeptides of thylakoid membranes in combination with ultrastructural studies have led to the discovery that spermine stabilizes the molecular composition and preserves the integrity of thylakoid membranes. The analyses of malondialdehyde and lipoxygenase levels has revealed that polyamines and guazatine treatments may prevent membrane destabilization, at least in part, by inhibiting lipid peroxidation. Finally, the availability of a cDNA coding for oat arginine decarboxylase (ADC) and the generation of ‘site-directed’ antibodies specific for ADC have led to the establishment of the basis for a model of regulation of ADC gene expression in oat leaves

Asymmetric responses of adaxial and abaxial stomata to elevated CO2: impacts on the control of gas exchange by leaves.

The response of adaxial and abaxial stomatal conductance in Rumex obtusifolius to growth at elevated atmospheric concentrations of CO2 (250 μmol mol−1 above ambient) was investigated over two growing seasons. The conductance of both the adaxial and abaxial leaf surfaces was found to be reduced by elevated concentrations of CO2. Elevated CO2 caused a much greater reduction in conductance for the adaxial surface than for the abaxial surface. The absence of effects upon stomatal density indicated that the reductions were probably the result of changes in stomatal aperture. Partitioning of gas exchange between the leaf surfaces revealed that increased concentrations of CO2 caused increased rates of photosynthesis only via the abaxial surface. Additionally, leaf thickness was found to increase during growth at elevated concentrations of CO2. The tendency for these amphistomatous leaves to develop a distribution of conductance approaching that of hypostomatous leaves clearly reduced their maximum photosynthetic potential. This conclusion was supported by measurements of stomatal limitation, which showed greater values for the adaxial surfaces, and greater values at elevated CO2. This reduction in photosynthesis may in part be caused by higher diffusive limitations imposed because of increased leaf thickness. In an uncoupled canopy, asymmetrical stomatal responses of the kind identified here may appreciably reduce transpiration. Species which show symmetrical responses are less likely to show reduced transpirational rates, and a redistribution of water loss between species may occur. The implications of asymmetrical stomatal responses for photosynthesis and canopy transpiration are discussed