Growth and reproductive potential with respect to season and photosynthetic gas exchange behavior under elevated (short-term) CO2 at varying temperature, relative humidity (RH), and irradiance levels were investigated in ragweed parthenium (also known as carrot grass and congress grass), a noxious weed in India. Lower values of biomass, relative growth rate, net assimilation rate, crop growth rate, leaf area duration, leaf area index, and numbers of flowers and seeds in winter compared with summer stands showed that ragweed parthenium is greatly suppressed by low temperatures during winter. This was due to constrained vegetative growth, seedling emergence, and seed to flower ratio. The species showed maximum photosynthetic response to temperature at 25 to 35 C, and the net photosynthetic rate was reduced considerably at a low temperature (7 C). These temperatures approximately corresponded to the normal temperatures experienced by summer (25–35 C) and winter (7 C) stands of ragweed parthenium. Elevated CO2 enhanced leaf net photosynthetic efficiency, maximum photosynthetic rate, and water use efficiency (WUE) but decreased the light compensation point for net photosynthesis, stomatal conductance, and transpiration rate. The interactive effects of elevated CO2 and temperature resulted in a decrease in light-limited and light-saturated net photosynthetic rates and WUE. The interactive effects also reduced an elevated CO2-induced decrease in light compensation point relative to elevated CO2 alone. Stomatal conductance was insensitive to photosynthetic photon flux but was greatly influenced by RH. Leaves of the species may show increasing rates of net photosynthesis with a rise in CO2 and temperature. However, excessive increase in transpiration with temperature, especially at 47 C (noon temperature during summer in the plains of northern India), appears to be disadvantageous for the leaves when conservation of water is of prime importance.

Predicting the effects of climate change on Antarctic terrestrial vegetation requires a better knowledge of the ecophysiology of common moss species. In this paper we provide a comprehensive matrix for photosynthesis and major environmental parameters for three dominant Antarctic moss species (Bryum subrotundifolium, B. pseudotriquetrum and Ceratodon purpureus). Using locations in southern Victoria Land, (Granite Harbour, 77°S) and northern Victoria Land (Cape Hallett, 72°S) we determined the responses of net photosynthesis and dark respiration to thallus water content, thallus temperature, photosynthetic photon flux densities and CO2 concentration over several summer seasons. The studies also included microclimate recordings at all sites where the research was carried out in field laboratories. Plant temperature was influenced predominantly by the water regime at the site with dry mosses being warmer. Optimal temperatures for net photosynthesis were 13.7°C, 12.0°C and 6.6°C for B. subrotundifolium, B. pseudotriquetrum and C. purpureus, respectively and fall within the known range for Antarctic mosses. Maximal net photosynthesis at 10°C ranked as B. subrotundifolium > B. pseudotriquetrum > C. purpureus. Net photosynthesis was strongly depressed at subzero temperatures but was substantial at 0°C. Net photosynthesis of the mosses was not saturated by light at optimal water content and thallus temperature. Response of net photosynthesis to increase in water content was as expected for mosses although B. subrotundifolium showed a large depression (60%) at the highest hydrations. Net photosynthesis of both B. subrotundifolium and B. pseudotriquetrum showed a large response to increase in CO2 concentration and this rose with increase in temperature; saturation was not reached for B. pseudotriquetrum at 20°C. There was a high level of variability for species at the same sites in different years and between different locations. This was substantial enough to make prediction of the effects of climate change very difficult at the moment.

The present study of structural and physiological changes during the development of the cushion moss, Grimmia pulvinata, quantifies the size-dependence of various parameters of water relations such as changes in surface: volume ratio (S/V) or water loss rates, and also measures net CO2 gas exchange in the light and the dark. Larger cushions had lower S/V values than smaller ones and featured lower rates of area-based evapotranspiration, owing to higher boundary-layer resistance, but did not differ in relative water storage capacity (expressed as a percentage of d. wt). In combination, this leads to considerably longer hydration periods in larger cushions. By contrast, CO2 gas-exchange parameters were negatively correlated with size : larger cushions showed significantly lower (mass-based) rates of net photosynthesis and dark respiration. Using these data, we estimated carbon budgets during a drying cycle as a function of cushion size. When including alternations of dark and light periods, the relationship proved to be rather complicated. Depending on the time of hydration, net carbon budgets not only varied quantitatively with size but sometimes took on both positive and negative values depending on cushion size. We conclude that neglecting plant size can lead to unrepeatable or even misleading results in comparative ecophysiological studies, and therefore urge for adequate attention to be paid to size in these studies.

Both sensitive and tolerant clones of aspen (Populus tremuloides) were exposed to ozone using four different exposure regimes under controlled environmental conditions. Based on data on ambient ozone from 10 cities in the USA, three treatments of 4-wk exposure to the same SUM06 (an accumulation of hourly O3 concentrations greater than 0.06 ml l−1) were constructed. The regimes allowed us to investigate: (a) the importance of long (3 wk, treatment 1) versus short (1 wk, treatment 2) duration of regimes with high peaks; (b) the effect of treatments with variable peak occurrence (treatments 1 and 2) versus uniform peak occurrence (treatment 3) during the exposure period. Nonfumigated control plants were maintained at ozone concentrations <10 nl l−1. Bifacial black necrosis, a typical symptom of ozone injury on aspen leaves, occurred on both clones after 2 wk exposure. Up to 60% of the leaves on the sensitive clone were injured, with an average of 6% of total leaf area injured. In the tolerant clone only 10% of the leaves were injured, with less than 1% of the total leaf area symptomatic. The severity of injury was consistently greatest in treatment 2, followed by treatments 1 and 3, respectively. The interval between peak exposures was less important than the occurrence of peaks versus a stable maximum concentration. Premature leaf abscission occurred in the sensitive clone. Measures of gas exchange demonstrated reduced photosynthesis under ozone fumigation, but exposure regime was not a significant factor. Concentrations of two antioxidants, ascorbic acid and glutathione, were almost always greater in the resistant than in the sensitive clone, but the differences were not statistically significant. The levels of these antioxidants in aspen leaves did not change with ozone fumigation or leaf age.

Information on ozone uptake by various tree species has been derived mainly from controlled-exposure studies of seedlings. Parallel measurements of foliar physiological processes in large trees are rare, as are measurements of local ozone concentrations within forest environments. Thus, our objectives were to characterize foliar ozone uptake in forest trees of different size and to identify a mechanistic linkage between small and large trees. Light-saturated net photosynthesis (Pn), stomatal conductance (gH2O), and ozone uptake (gO3) in seedling (<2 m), sapling (2–5 m) and mature (9–21 m) black cherry (Prunus serotina Ehrh.), northern red oak (Quercus rubra L.) and red maple (Acer rubrum L.) trees were measured monthly over two growing seasons at sites at high (1240 m) and low (600 m) elevation in the Great Smoky Mountains National Park. Seasonal ozone dose to northern red oak and red maple in the upper crown of mature trees was approx. double that of respective understorey seedlings and saplings. Seasonal ozone dose to black cherry was up to six times greater in mature trees than in seedlings and saplings. The lower ozone dose to understorey trees was mainly a function of lower gH2O, although canopy gradients in ozone concentration were indicated. Differences in Pn and gH2O between small and large deciduous forest trees were linked to variation in nitrogen concentration expressed on a leaf area basis.