Leaf mass per unit area (LMA) and internal leaf anatomy often affect net gas exchange because of their effects on internal CO2 conductance to the site of carboxylation, internal shading, competition for CO2 among carboxylation sites, nitrogen concentration and its partitioning. To evaluate effects of LMA and leaf anatomy on CO2 assimilation, water-use efficiency (WUE) and nitrogen-use efficiency (NUE), we measured LMA, leaf thickness, the thickness of mesophyll components, and gas exchange rates at ambient CO2 concentration in leaves of six woody deciduous and evergreen species with different leaf life spans. In two species, CO2 assimilation was also estimated at saturating CO2 concentrations. There were interspecific differences in all morphological variables studied. Long-lived leaves had higher LMA and were thicker than short-lived leaves. Species with high LMA had low assimilation rates and NUE, both in ambient and saturating CO2 concentrations. Thus, in species with high LMA, assimilation was reduced by non-stomatal limitations, possibly because of a lower allocation of N to the photosynthetic machinery than in species with low LMA. Within a species, thicker leaves tended to have a lower tissue density. In intraspecific comparisons under field conditions, increasing internal air volume had positive effects on WUE, probably because of enhanced internal CO2 conductance to the site of carboxylation. We conclude that, in interspecific comparisons, different patterns of N partitioning strongly influence NUE, whereas in intraspecific comparisons, internal leaf anatomy is a key factor regulating resource-use efficiency.

Biotic interactions play an important role in ecosystem function and structure in the face of global climate change. We tested how plant–plant interactions, namely competition and facilitation among grassland species, respond to extreme drought and heavy rainfall events. We also examined how the functional composition (grasses, forbs, legumes) of grassland communities influenced the competition intensity for grass species when facing extreme events. We exposed experimental grassland communities of different functional compositions to either an extreme single drought event or to a prolonged heavy rainfall event. Relative neighbour effect, relative crowding and interaction strength were calculated for five widespread European grassland species to quantify competition. Single climatic extremes caused species specific shifts in plant–plant interactions from facilitation to competition or vice versa but the nature of the shifts varied depending on the community composition. Facilitation by neighbouring plants was observed for Arrhenatherum elatius when subjected to drought. Contrarily, the facilitative effect of neighbours on Lotus corniculatus was transformed into competition. Heavy rainfall increased the competitive effect of neighbours on Holcus lanatus and Lotus corniculatus in communities composed of three functional groups. Competitive pressure on Geranium pratense and Plantago lanceolata was not affected by extreme weather events. Neither heavy rainfall nor extreme drought altered the overall productivity of the grassland communities. The complementary responses in competition intensity experienced by grassland species under drought suggest biotic interactions as one stabilizing mechanism for overall community performance. Understanding competitive dynamics under fluctuating resources is important for assessing plant community shifts and degree of stability of ecosystem functions.

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We have investigated the interactions between resource assimilation and storage in rosette leaves, and their impact on the growth and reproduction of the annual species Arabidopsis thaliana. The resource balance was experimentally perturbed by changing (i) the external nutrition, by varying the nitrogen supply; (ii) the assimilation and reallocation of resources from rosette leaves to reproductive organs, by cutting or covering rosette leaves at the time of early flower bud formation, and (iii) the internal carbon and nitrogen balance of the plants, by using isogenic mutants either lacking starch formation (PGM mutant) or with reduced nitrate uptake (NU mutant).When plants were grown on high nitrogen, they had higher concentrations of carbohydrates and nitrate in their leaves during the rosette phase than during flowering. However, these storage pools did not significantly contribute to the bulk flow of resources to seeds. The pool size of stored resources in rosette leaves at the onset of seed filling was very low compared to the total amount of carbon and nitrogen needed for seed formation. Instead, the rosette leaves had an important function in the continued assimilation of resources during seed ripening, as shown by the low seed yield of plants whose leaves were covered or cut off. When a key resource became limiting, such as nitrogen in the NU mutants and in plants grown on a low nitrogen supply, stored resources in the rosette leaves (e.g. nitrogen) were remobilized, and made a larger contribution to seed biomass. A change in nutrition resulted in a complete reversal of the plant response: plants shifted from high to low nutrition exhibited a seed yield similar to that of plants grown continuously on a low nitrogen supply, and vice versa. This demonstrates that resource assimilation during the reproductive phase determines seed production.The PGM mutant had a reduced growth rate and a smaller hiomass during the rosette phase as a result of changes in respiration caused by a high turnover of soluble sugars (Caspar et al. 1986; W. Schulze et al. 1991). During flowering, however, the vegetative growth rate in the PGM mutant increased, and exceeded that of the wildtype. By the end of the flowering stage, the biomass of the PGM mutant did not differ from that of the wild-type. However, in contrast to the wild-type, the PGM mutant maintained a high vegetative growth rate during seed formation, but had a low rate of seed production. These differences in allocation in the PGM mutant result in a significantly lower seed yield in the starchless mutants. This indicates that starch formation is not only an important factor during growth in the rosette phase, but is also important for whole plant allocation during seed formation. The NU mutant resembled the wild-type grown on a low nitrogen supply, except that it unexpectedly showed symptoms of carbohydrate shortage as well as nitrogen deficiency.In all genotypes and treatments, there was a striking correlation between the concentrations of nitrate and organic nitrogen...

Once per year, at the beginning of each growing season, the lysimeters were watered to field capacity, and consequently each tree experienced a seasonal drought in relation to the amount of available water depending on soil volume and tree age. Measurements of leaf conductance were performed on youngest fully expanded, sun-exposed leaves in climatized cuvettes. Leaf water potential was measured with a pressure chamber, which was also used for collecting xylem sap. Abscisic acid concentration of the xylem sap was determined with ELISA.There was no consistent diurnal relationship between abscisic acid concentration in xylem sap and leaf conductance. During a drying cycle, however, there was an inverse relationship between maximum leaf conductance and daily average abscisic acid concentration in xylem sap. With soil drying, the concentration of abscisic acid in xylem sap initially increased to a maximum in summer but decreased again towards autumn. Thus, during the first part of the drying cycle the predawn water potential of the leaf was significantly correlated with abscisic acid concentration in xylem sap. As predawn water potential becomes more negative during the first part of the drying cycle, the decline in maximum leaf conductance correlates with an increase in concentration of abscisic acid in xylem sap. With further decrease in predawn water potential towards the end of the drying cycle, the stomata remained closed even though there was a decreasing concentration of abscisic acid in the xylem sap.We conclude that the concentration of abscisic acid in xylem sap is not related to leaf conductance during the course of a day from end of May to September. However, during the course of a drying cycle the range of maximum leaf conductance seems to be dependent on the concentration of abscisic acid in xylem sap.