Drought tolerance of juvenile and mature leaves of a deciduous dwarf shrub Vaccinium myrtillus L. in a boreal environment

The difference between drought tolerance of juvenile and mature leaves of the winter-deciduous dwarf shrub bilberry (Vaccinium myrtillus L.) from a northern boreal environment was investigated. It was hypothesised that mature leaves are more drought sensitive than juvenile leaves. Bilberry plants were allowed to dry out by excluding irrigation when leaves were at juvenile and mature stages. Tissue water content decreased at both phenological stages, but the response was more pronounced in the mature leaves. Anthocyanin concentrations increased as the tissue water content decreased, and again this occurred to a greater extent in the mature leaves. Chlorophyll concentrations decreased only marginally at the juvenile stage, while the decrease was significant in the mature leaves. Chlorophyll degradation was enhanced by drought stress. Soluble proteins decreased and protein oxidation increased in the mature leaves, and degradation of oxidised proteins increased in the drought-stressed plants. The results suggest that leaves of bilberry are more sensitive to drought stress at the mature stage, and that drought stress accelerates senescence at the mature stage. The significance of the results is that dry periods during the juvenility of leaves are not as detrimental as they may be later in summer. In addition, the strategy of a winter-deciduous plant is obviously to protect its perennial parts from severe drought by accelerated leaf senescence at the mature stage. Therefore, the deciduous life form may provide an excellent adaptation against drought also in northern ecosystems. The role of anthocyanins in photoprotection under drought stress is also discussed.