High irradiance impairments on photosynthetic electron transport, ribulose-1,5-bisphosphate carboxylase/oxygenase and N assimilation as a function ofn availability in Coffea arabica L. plants

Young coffee (Coffea arabica L.) plants grown under low irradiance (PPFD up to ca. 150 mu mol m(-2) s(-1)) and high (2 N), medium (1 N) and low (0 N) nitrogen availability conditions were exposed to natural sunlight (noon PPF D up to ca. 1,700 mu mol m(-2) s(-1)) for 14-15 days, during which paramete rs related to photosynthesis and nitrogen assimilation were monitored. In t he plants with lower N availability, a donor and an accept or side photoinh ibition may have affected PSII functioning. During the high irradiance stre ss the plants with higher N availability maintained the highest contents of cytochromes (cyt) b(559HP), b(559LP), b(563) and f, and in plastoquinone-3 (PQ-9). Those plants also presented significant increases in rubisco conte nt and activity, and a preferential investment in rubisco rather than in li ght harvesting components by the end of the high irradiance stress. Despite the effects observed on PSII and rubisco, the PSI activity and cyt b(6)/f complex were much more affected in all N treatments. Leaf nitrate reductase activity decreased whereas nitrate and amino acid contents increased durin g the high irradiance in 2 N and 1 N plants. Our data shows that high irrad iance affected both the photochemical and enzymatic reactions of photosynth esis (especially in 0 N and 1 N plants), that PSI was a preferential photoi nhibitory target and that the higher N availability promoted the recovery o f important physiological parameters, such as electron transport rates, PQ- 9 and rubisco activity and content.