Seasonal changes in photoprotective mechanisms of leaves from shaded and unshaded field-grown coffee (Coffea arabica L.) trees

Coffee is native to shady environments, but often grows and yields better without shade. Thus, it may be reasoned that coffee leaves should display enough plasticity to acclimate themselves to contrasting light environments. However, little is known about mechanisms associated with such plasticity in coffee. This work aimed, therefore, to explore differences in leaf photoprotective mechanisms. Plants were grown in the field and received either 48 or 100% natural light. Evaluations were made using outer leaves from the sun-facing sides of the coffee hedgerow in Vicosa (Brazil) in August and October, when growth and photosynthetic rates are expected to be minimal and maximal, respectively, and in December, when temporary depressions in those variables are common. Regardless of light treatments, coffee leaves showed: (1) very low photosynthetic rates (generally below 2.5 mu mol m(-2) s(-1)), (2) chronic photoinhibition in August (dry, cool season) that was accompanied by strong loss of pigment concentration, and (3) discrete, dynamic photoinhibition in October and December (rainy, warm season). Compared with shaded leaves, sunlit leaves generally exhibited lower pigment concentration, lower quantum yield of electron transport, steeper inclinations and similar electron transport rate. Total ascorbate pool tended to be larger in sunlit than in shaded leaves (but with similar redox state), whereas activities of key antioxidant enzymes, as well as malondialdehyde accumulation and electrolyte leakage, were similar between those leaf types. As a whole, the photosynthetic apparatus of the coffee tree showed a low phenotypic plasticity to varying irradiance.