Respostas de quatro populações de azinheira ao stress hídrico | Responses of four populations of Holm oak to water stress

Mestrado em Engenharia Florestal e dos Recursos Naturais - Instituto Superior de Agronomia

Holm oak is a tree with a relatively wide distribution due to its resistance to high temperature conditions (associated to very dry summers) and low temperatures. However, Holm oak’s sustainability depends heavily on the water factor. Thus, the study of Holm oak responses to water stress becomes relevant, within a scenario that predicts climate changes in the Mediterranean region (associated to soil degradation and increasing use of local water resources), since it may provide additional information that can be integrated in future management strategies of structures like the montados. The content of this study focuses on four Holm oak populations, from different provenances: one Croatian (Biograd na Moru) and three Portuguese (Bragança, Mora and Castro Marim). We considered that the native provenances (Quercus rotundifolia Lam) and the Croatian one (Quercus ilex L.) as two distinct species, based on morphological differences exhibited by these plants. Half of the plants were submited to total watering suspension and the main goal of this work consisted on the assessment of physiologic and morphological responses of the plants to water stress and its comparison between different provenances. The main response to water stress took place at a vegetative growth level, especially in plants from Mora e Castro Marim provenances. In these plants (that exhibited greater number of leaves and height) transpiration rate increased, and as such, there was a greater demand in water supply. The reduction of the aerial part was due to growth inhibition, lower production of new leaves and leave senescence (the latter more intense, after the recovery period). The analysis of dry root biomass showed that it was not affected with the characteristic intensity of the aerial part. We even verified a tendency for root growth investment on stressed plants, in Mora’s provenance. Water stress only had a negative effect on the root growth in plants from Castro Marim’s provenance. The values of stomata conductance decreased significantly with the reduction of leaf water content, and therefore, stomata closure was one of the first strategies observed, with the purpose for reduction of water loss by transpiration, in detriment of CO2 assimilation. With the increase of water stress, the most influent limitations became metabolic (non-stomata limitations). After the re-hydration period, only Castro Marim’s plants did not show full recovery of this parameter, suggesting that the damage caused in these plants might be irreversible or a necessity for a longer recovery period. V From the evaluation of chlorophyll a fluorescence we verified that it decreased in plants submitted to water stress, however the recovery of this parameter occurred in plants of all provenances, except for Castro Marim’ plants. The assessment of relative and total chlorophyll content revealed, in most cases, reductions of these parameters as a result from water stress effects. Nevertheless, the expeditious relative chlorophyll content method showed that in re-hydrated plants from Castro Marim provenance there was no recovery of this parameter, while the chlorophyll extractive method did not detect this difference. The soluble protein content suffered notorious changes only in the third week of watering suspension, period during which, we observed significantly higher values in stressed plants from Biograd na Moru e Bragança. The observed increases were possibly a result of an induced stimulus by water stress in protein synthesis, with probable functions of cellular protection.