Limitations to photosynthesis were explored in leaves from four canopy positions of field-grown, unshaded coffee (Coffea arabica L.), a tropical tree species classified as shade-obligatory. Overall, compared to shade (lower) leaves, sun (upper) leaves had higher net carbon assimilation rate (A) (4.5 against 2.0 micromol m(-2)s(-1) at most) associated with higher electron transport rate (due to a greater irradiance availability) but unrelated to stomatal and mesophyll conductances, which were similar regardless of leaf position. Neither physiological variable directly involved with photosynthetic carbon gain nor those involved with light capture were able to adjust themselves to match the capacity of the photosynthetic machinery to the light supply. We concluded that: (i) there was no major difference in photosynthetic capacity between sun and shade leaves; (ii) the intrinsic low A in coffee was greatly associated with remarkable low diffusive limitations rather than with biochemical or photochemical constraints; and (iii) morphological (e.g., variations in specific leaf area and leaf inclination) or anatomical plasticity should be of greater acclimative value than physiological plasticity as a mean of coffee leaves to respond to changing irradiance.

a b s t r a c tHeavy bearing is a typical phenomenon on unshaded coffee (Coffea arabica L.) trees and limits both the production and retention of leaves, leading to branch dieback, and, thus, results in a strong biennial bearing pattern. The major goals of this study were to investigate the physiological mechanisms that may be associated with the leaf-to-fruit ratio (LFR), branch dieback, biennial production and the relationships between carbohydrate and mineral fluctuations and branch dieback in coffee plants. The trees were grown in north-south-oriented hedgerows under conditions of full sunlight. Leaves and plagiotropic branches from the upper and lower strata of the east-and west-facing sides of the hedgerow were examined. A strong biennial pattern of coffee production was observed over three harvests. Overall, the east face of the hedgerow produced a more sellable crop than the west face, and this was associated with more light availability for the east-facing branches. The branch growth rate was higher with an increasing LFR during 2006-2007, regardless of the canopy position, and no compensatory increase in the photosynthetic rate was found in response to a decreasing LFR. Due to a relatively low fruit yield in 2007-2008, there was no branch dieback. The extent of branch dieback increased dramatically with decreasing LFR and was probably not closely related to changes in the concentrations of carbohydrates, amino acids and minerals. The extent of branch dieback was apparently unrelated to the differences in the photosynthetic rates per unit leaf area, carbon isotope composition, or oxidative stress, as was assessed by the electrolyte leakage from the leaf tissues. We discuss these responses in terms of the relative lack of branch autonomy in coffee trees.

Stomata are crucial in land plant productivity and survival. In general, with lower irradiance, stomatal and epidermal cell frequency per unit leaf area decreases, whereas guard-cell length or width increases. Nevertheless, the stomatal index is accepted as remaining constant. The aim of this paper to study the influence of ordinary epidermal cells and stomata on leaf plasticity and the influence of these characteristics on stomata density, index, and sizes, in the total number of stomata, as well as the detailed distribution of stomata on a leaf blade. As a result, a highly significant positive correlation (R 2 a = 0.767 p ≤ 0.001) between stomatal index and stomatal density, and with ordinary epidermal cell density (R 2 a = 0.500 p ≤ 0.05), and a highly negative correlation between stomatal index and ordinary epidermal cell area (R 2 a = -0.571 p ≤ 0.001), were obtained. However in no instance was the correlation between stomatal index or stomatal density and stomatal dimensions taken into consideration. The study also indicated that in coffee, the stomatal index was 19.09% in shaded leaves and 20.08% in full-sun leaves. In this sense, variations in the stomatal index by irradiance, its causes and the consequences on plant physiology were discussed. Keywords: stomatal index, stomatal density, sun and shade plants, Coffea arabica L.Qual a influência das células epidérmicas ordinárias e dos estômatos na plasticidade foliar de cafeeiros desenvolvidos a pleno sol e sob condições de sombra? ResumoOs estômatos são fundamentais na fisiologia vegetal e na sobrevivência das plantas. Em geral, com a diminuição da irradiância, a frequência estomática, ou mesmo epidérmica, por unidade de área foliar diminui, enquanto as dimensões dos estômatos aumentam. Por outro lado, acredita-se que o índice estomático, sendo uma característica intrínseca da planta, permanece constante. O objetivo deste trabalho foi estudar a influência das células epidérmicas ordinárias e das células estomáticas na plasticidade da folha e as influências destas características sobre a densidade e no índice estomático, bem como nas dimensões e no número de estômatos e descrever, em detalhes, os principais fatores que os alteram. Verificou-se, neste trabalho, uma correlação fortemente significativa (R 2 a = 0,767 p ≤ 0,001) entre o índice estomático e a densidade estomática, ou mesmo, entre este e a densidade de células epidérmicas (R 2 a = 0,500 p ≤ 0,05). Uma forte correlação negativa entre o índice estomático e a área das células epidérmicas (R 2 a = -0,571 p ≤ 0,001) também foi verificada, entretanto, nenhuma correlação foi verificada entre o índice estomático e as dimensões dos estômatos, assim como sugere a literatura. De qualquer modo, este estudo demonstrou que o índice estomático do cafeeiro é de 19,09% nas folhas sombreadas e de 20,08% nas folhas a pleno sol. Neste sentido, as variações no índice estomático causadas pela irradiância, suas causas e consequências para a fisiologia vegetal são discutidas em detalhes.Palavras-chave: índice e...

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