Anthocyanins under drought and drought-related stresses in bilberry (<em>Vaccinium myrtillus</em> L.)

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>The aim was to study the effects of drought and indirect drought-related stresses on anthocyanin content in bilberry (<em>Vaccinium myrtillus</em> L.). Anthocyanin content was also studied in relation to developmental stage (juvenile vs. mature leaves, previous vs. current-year stems). It was hypothesised that drought-stressed plants accumulate anthocyanins, but their direct role in osmotic regulation was questioned. </p><p>Direct drought was created by preventing water supply. Freezing-induced dehydration was accomplished by removal of snow. Effects of summertime chilling (+2 °C during active growth, +5/0 °C during frost hardening) on water status were tested. The trace metal Nickel (Ni) was applied to the soil, as Ni may interfere with plant water relations at the rhizospheric level. Salt (Na⁺) was added to the soil to observe salt-induced disturbance in osmotic balance and ion homeostasis. </p><p>Tissue water content (TWC) decreased and anthocyanin level increased under direct drought, especially in the mature leaves. The freezing-stressed plants contained the same TWC and anthocyanin levels in mid-winter compared to plants that overwintered below snow. The freezing-stressed plants had lower TWC and a similar anthocyanin level in early spring, and lower TWC and anthocyanin level in late spring than plants that overwintered below snow. In the summer and autumn following snow removal, current-year stems of freezing-stressed plants had the same TWC, but higher levels of anthocyanins than current-year stems of plants that overwintered below snow. New growth was thus affected by the freezing stress experienced by previous-year stems. Chilling had no effect on anthocyanins. Although TWC decreased and anthocyanin level increased from active growth to the beginning of frost hardening, no increase was observed during frost hardening. Ni did not cause drought stress in the aboveground shoots, but anthocyanin level decreased in the aboveground shoots along with Ni accumulation in the belowground stems. Na⁺ increased TWC in the belowground stems, but decreased TWC and anthocyanin level in the aboveground stems. </p><p>It is proposed that anthocyanins do not have direct role in osmotic regulation, or in the development of freezing tolerance. It is suggested that the increase in anthocyanin level under direct drought stress is mainly due to the photoprotection of chlorophylls by anthocyanins. This is supported by two facts: (1) At increased anthocyanin level in the juvenile leaves, chlorophyll <em>a</em> was stabilized despite continuing drought stress, and (2) after the initial peak in the mature leaves, the accumulation of anthocyanins ceased although the drought became more severe. As chlorophyll <em>a</em> decreased in the mature leaves due to senescence, there was less demand for such high levels of anthocyanins. </p></div>