Plants grown in elevated CO2 environments may exhibit photosynthetic acclimation or down regulation, which is characterised by reduced rates of photosynthesis. In most cases of CO2-induced photosynthetic acclimation, the reduced rates of photosynthesis were still higher than those detected in plants growing at ambient CO2 concentrations. In this work we present a study on the behaviour of seedlings of Hymenaea courbaril, a late secondary/climax species that is one of the most important trees in mature tropical forests of the Americas. After germination, the seedling of H. courbaril increases its rate of growth due to the mobilisation of massive amounts of a storage cell wall polysaccharide (xyloglucan) from its cotyledons. In our experiments, germinated seeds were incubated in open top chambers with increased concentration of atmospheric CO2 (720 ppm) (control at 360 ppm). To test the effects of the presence of the storage compound on the responses of growing seedlings, cotyledons were detached before the start of polysaccharide mobilisation and parameters such as dry mass, leaf area, CO2 assimilation rates and chlorophyll a fluorescence were measured during 98 days. A comparison between 360 and 720ppm growing seedlings showed a significant increase in leaf area only in metaphylls of seedlings growing under higher CO2. However, a marked and persistent increase (2 fold) in photosynthesis (CO2 assimilation) was observed in all cases (with or without cotyledons). Changes in the levels of sucrose have been suggested to act as a signalling mechanism that switches on/off the storage or development mode in plant tissues. Thus, the explanation for our general observation that the differential response in terms of growth of seedlings ceases to exist when storage mobilisation is functioning, might be related to the fact that higher levels of sucrose are produced as a result of carbon storage compounds degradation. By the results obtained, it appears that plants grown under enriched CO2 did not acclimate and therefore under the climatic conditions forecasted on the basis of the present carbon dioxide emissions, Hymenaea courbaril should establish faster in its natural environment and might also serve as an efficient mechanism of carbon sequestration within the forest.
Photosynthesis; CO2 enrichment; Hymenaea courbaril; storage mobilisation; root:shoot ratio; seedling growth; cotyledons; open top chamber; xyloglucan; biodiversity