Assimilação de carbono e fluorescência da clorofila do cafeeiro (Coffea arabica L.) sob condições contrastantes de irradiância, temperatura e disponibilidade de CO2
Ronquim, Júlio Cesar
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Leaf gas exchange and potential photochemical efficiency of photosystem II of Coffea arabica L. cultivars Catuaí Vermelho, Icatu Amarelo and Obatã were evaluated under field conditions on clear and cloudy days in the wet season. Additionally, were compared seasonal photosynthetic responses in wet and dry periods of year to irradiance, temperature and water stress of C. arabica cv. Obatã non-grafted or grafted on to C. canephora Pierre. Finally, in laboratory conditions were evaluated CO2 assimilation and electron flow through photosystems II and I, in young plants of C. arabica cvs. Catuaí Vermelho, Obatã and Ouro Verde exposed to different thermal treatments during 14 h, under regular and saturated carbon concentration. The results provided evidence that in clear day occur the midday depression of leaf gas exchange and potential photochemical efficiency of photosystem II, due to the greater atmospheric vapor pressure deficit, causing a strong decrease (about 70%) in daily carbon gain. On a cloudy day, the daily carbon gain was barely limited by irradiance below light saturation point. During dry period, the grafted plants showed accentuated higher net photosynthesis when compared with the non-grafted plants, what demonstrated lower susceptibility to water stress and the favorable effects of grafting on yearly carbon gain. The results suggest that maintaining greater stomatal conductance and transpiration rates during dry period is more important for carbon balance than the efficient use of water. The optimal temperature for maximum net photosynthesis measured at 355 ppm CO2 was from 17-23 °C. The difference in optimal temperature range of maximum net photosynthesis when compared to maximum electron transport rate through photosystem II (26-29 °C) showing an imbalance between photochemistry and biochemistry phases of photosynthesis, resulting in electron excess. Elevated CO2 concentration did not affect the response of net carbon assimilation to temperature, despite the variation in optimal range between 23-29 °C for Catuaí Vermelho. At temperature of above 29 °C were measured the higher mesophyll limitations to photosynthesis, which was greater than stomatal limitations. Photosynthetic process of C. arabica is strongly inhibited at leaf temperature about 32 °C.