Carbon Gain in Coffea Arabica During Clear and Cloudy Days in the Wet Season

Three cultivars of Coffea arabica , Catuaí Vermelho IAC 81, Icatu Amarelo IAC 2944 and Obatã IAC 1669–20, were evaluated in relation to leaf gas exchange and potential photochemical efficiency of photosystem II under field conditions on clear and cloudy days in the wet season in southeast Brazil. Independent of levels of irradiance, leaf water potential (υ leaf ) values were always higher than the minimum required to affect daily net photosynthesis (P N ). P N , stomatal conductance (g s ), leaf transpiration (E) and the index of photochemical efficiency (F v /F m ) declined on a clear day in all cultivars. The depression of leaf gas exchange and F v /F m (specially around midday) caused a strong decrease (about 70 %) in daily carbon gain on a clear day. Under cloudless conditions, g s and P N were correlated with the air vapour pressure deficit (VPD air ), but not with photosynthetic photon flux density (PPFD) values. On a cloudy day, the daily carbon gain was barely limited by PPFD below 800 μmol m −2 s −1 , the F v /F m values showed a slight decrease around midday, and g s and P N were positively correlated with PPFD but not with VPD air . By contrast, irrespective of the contrasting irradiance conditions during the day, P N and E were correlated with g s .