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

Young coffee (Coffea arabica L.) plants grown under low irradiance (PPFD up to ca. 150 μmol m-2 s-1) and high (2N), medium (1 N) and low (0N) nitrogen availability conditions were exposed to natural sunlight (noon PPFD up to ca. 1,700 μmol m-2 s-1) for 14-15 days, during which parameters related to photosynthesis and nitrogen assimilation were monitored. In the plants with lower N availability, a donor and an acceptor side photoinhibition may have affected PSII functioning. During the high irradiance stress the plants with higher N availability maintained the highest contents of cytochromes (cyt) b(559HP), b(559LP), b563 and f, and in plastoquinone-9 (PQ-9). Those plants also presented significant increases in rubisco content and activity, and a preferential investment in rubisco rather than in light harvesting components by the end of the high irradiance stress. Despite the effects observed on PSII and rubisco, the PSI activity and cyt b6/f complex were much more affected in all N treatments. Leaf nitrate reductase activity decreased whereas nitrate and amino acid contents increased during the high irradiance in 2 N and 1 N plants. Our data shows that high irradiance affected both the photochemical and enzymatic reactions of photosynthesis (especially in 0 N and 1 N plants), that PSI was a preferential photoinhibitory target and that the higher N availability promoted the recovery of important physiological parameters, such as electron transport rates, PQ-9 and rubisco activity and content.