Limitações da fotossíntese e metabolismo do carbono em folhas de diferentes posições da copa do cafeeiro (Coffea arabica L.)

• Autor(a) principal: Araujo, Wagner Luiz
• Data de Publicação: 2006
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/4302
• topic: Fotossíntese; Estresse oxidativo; Coffea arabica; Photosynthesis; Oxidative stress; Coffea arabica; CNPQ::CIENCIAS BIOLOGICAS::BOTANICA::FISIOLOGIA VEGETAL

This study aimed to explore the physiological and biochemical strategies associated to the acclimation of the photosynthetic apparatus due to irradiance attenuation downwards the canopy of coffee plants, as well as the causes of their low photosynthetic rates. Sampling and measurements were made using outer leaves from upper and lower strata from east- and westfaced sides of a coffee hedgerow (north-south orientation). Photosynthetic active radiation (PAR) incident on the canopy varied from 500 to 1850 mmol (photons) m-2 s-1 but PAR effectively intercepted by leaves was smaller, ranging from 20 to 800 mmol (photons) m-2 s-1 for lower leaves, and from 50 to 1400 mmol (photons) m-2 s-1 for the upper ones. In general, net carbon assimilation rate (A) was larger in upper leaves (135%), while internal to ambient CO2 concentration ratio (Ci/Ca) was always larger, and carbon isotope composition smaller, in lower leaves. By contrast, both stomatal (gs) and mesophyll (gm) conductances were similar for both upper and lower leaves. The compensation and saturation irradiances as well as the apparent quantum yield were also similar regardless leaf position. Light-saturated A was relatively low even in upper leaves, suggesting that limitations other than light could be largely associated with the low photosynthetic rates of coffee plants. Initial and total activities of Rubisco, as well as its activation state, varied lightly along the treatments. These results, combined with those from curves A/Ci, suggest that: (i) spatial variation of photosynthetic rates in recently expanded leaves were not a result of biochemical or diffusional limitations, but mainly of photochemical limitations associated with low-light availability; (ii) low photosynthetic rates per se should have resulted mainly from diffusional limitations, as could be deduced from the low values of gs and gm irrespective of leaf position, but not necessarily due to a low mesophyll capacity for CO2 fixation. Even at Ci [>or=] 1000 mmol mol-1 (Ca [~] 1600 mmol mol-1) A was not saturated. In fact, small variations in carbohydrate concentrations as well as in the activities of several enzymes associated with carbon metabolism suggest that the coffee tree presents a low plasticity to adjust its biochemical apparatus for CO2 fixation in response to decreasing light availability. The greatest activities of sucrose-phosphate synthase and fructose-1,6-bisphosphatase in upper leaves appeared to be strongly associated with the greatest photosynthetic rates observed in these leaves in order to guarantee their abilities to maintain sucrose synthesis and export. Photoinhibition of photosynthesis was not observed even in the most exposed leaves. The quantum yield of electron transport was almost always smaller, while the electron transport rate and leaf angles were always larger, in the upper leaves than in the lower leaves. The differences observed in A were not related to a differential allocation of N for the production of photosynthetic pigments, which did not vary along the canopy. Taken together, despite the relatively low values of A, the photosynthetic apparatus of the coffee plants appears to exhibit a relatively low plasticity in response to varying irradiance.