Efeitos do stress hidrico no metabolismo fotossintetico de Lupinus albus L

The effects of imposed soil dehydration on gas exchange and photosynthetic carbon metabolism was studied in potted plants of Lupinus albus. Photosynthesis in air and leaf conductance were strongly reduced in stressed (non-watered) plants. Photosynthetic capacity (in saturating CO_2 and irradiance) and chlorophyll a fluorescence parameters, together with the recovery capacity on rehydration, revealed that stomatal components were the most important limiting factor of photosynthesis in lupin plants submitted to moderate or rapidly induced water deficits. Photosynthetic capacity decreased only significantly when leaf RWC<65% or after prolonger water stress (over two weeks). Measurements carried out in Eucalyptus globulus and Vitis vinifera showed that the inhibition of photosynthesis under soil drought conditions was due in E. globulus mainly to closure of stomata; in V. vinifera modifications of the photo and biochemical functioning of the mesophyll were also important. In lupins water stress effects on photosynthesis were most severe in old leaves. Although young leaves showed a longer delay in stomatal reopening after rehydration, old leaves did not fully recover their photosynthetic capacity. RuBisCO initial activity decrease in all leaves. RuBisCO total activity and amount were only significantly reduced in old leaves. After rehydration both young and old leaves showed a further decrease in the amount of RuBisCO in parallel with soluble protein. The acceleration of the acropetal pattern of senescence induced by water deficits and subsequent rehydration is discussed in a whole plant approach. At the partition level of the recent assimilated carbon, water stress seemed to induce alterations both on sink (Chenopodium rubrum suspension cell cultures) and source (Lupinus albus leaves) tissues. Starch was depleted and cells acumulated or maintained high levels of soluble sugars. The resulting higher ratios, solubles/insolubles, are discussed in terms of osmotic regulation and in terms of down regulation of the photosynthetic activity mediated by mechanism involving genetic expression..Available from Fundacao para a Ciencia e a Tecnologia, Servico de Informacao e Documentacao, Av. D. Carlos I, 126, 1200 Lisboa / FCT - Fundação para o Ciência e a TecnologiaSIGLEPTPortuga