Abstract

MELHORADO, F.  and  MOREIRA, T.. Seasonal pattern of the stomatal regime of irrigated holm-oak (Quercus rotundifolia Lam.). Rev. de Ciências Agrárias [online]. 2007, vol.30, n.1, pp.212-222. ISSN 0871-018X.

This project aims to study the possible variation in stomata regime between irrigated oak trees Quercus rotundifolia Lam. and the same trees under natural environmental conditions at the different seasons of the year. The study was carried in a montado situated in Mitra, southeast of Portugal, at 13 km from Évora. For this project two isolated trees were selected, one irrigated to keep the plant regularly water supplied and the other subjected to the typical dryness of our summer (we will call this the standard tree). The stomatal resistance was measured with a porometer (?AP4) at three daily periods (morning, noon and sunset) in the hydrologic year of 2001-02. During the summer the stomata resistances of the irrigated tree were significantly different from the standard (99%; alfa = 0,01), presenting always the double of the stomatic aperture of the standard tree. This relation lasted through all daily periods of the season. In the summer the average of stomata resistances in both trees follows an exponential curve (r² = 0,99), with a maximum of resistance at the sunset. In winter, the differences in stomata resistance between the two trees are significative (at 99%; alfa = 0,01) over all daily period. And we have observed a change in the daily variation pattern of the resistances, becoming more visible a midday-closure, especially in the standard tree. The resistance curves follow a bimodal pattern (r² = 1) in both trees with the maximum at noon. In the irrigated tree, however, the maximum resistance occurs at noon as in the standard tree, but with a much lower value, and not very different from the values that occur in the morning and at the sunset. In the analysis taken to the variation of seasonal pattern in stomatal regime, we have saw the existence of significant differences between irrigated and standard Quercus rotundifolia Lam. in stomatal aperture, but the daily behavior over the seasons (Summer-Winter) is identical in both trees, following the same type of curves. We have concluded that the difference between summer and winter in the behavior of the species, that is, the passage of an exponential type of stomata response in summer to the bimodal in winter, is due, probably, to the inferior evaporative demand at sunset during wintertime. We have confirmed the existence of stomata midday-closure in this species, showing that adaptation of the specie isn’t rigid and depends on the water availability in the soil.

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