Tissue-specific changes in remobilisation of fructan in the xerophytic tussock species Festuca novae-zelandiae in response to a water deficit

Abstract

Cellular responses of the native New Zealand grass species Festuca novae-zelandiae (Hack.) Cockayne to a water deficit and re-hydration treatment were investigated. Leaf extension ceased after 28 d of withholding water, while mitotic activity within the intercalary meristem still continued. By 35 d, no evidence of mitotic activity could be detected. At the apical meristem, cell division commenced 24 h post re-watering after 49 d of dry-down and was accompanied by synthesis of storage oligosaccharides of the fructan type. Changes in water-soluble carbohydrates over the course of the water deficit were examined in consecutive leaf segments comprising the leaf base (meristem region), elongation zone, the enclosed and the exposed lamina, as well as basal sheath segments from the two next oldest leaves. In fully hydrated leaf tissue, the fructan pool was mainly composed of the low molecular weight fructans of the inulin and neokestose series with higher concentrations towards the leaf base. Fructan concentrations decreased over the course of the water deficit with the leaf base retaining significantly higher concentrations than any other tissue, until tissue water content fell below 45%. Sucrose content increased in each tissue during the course of the dry-down, and was highest at the leaf base, where a concentration of 200 μmol g DW^–1 was measured after 49 d of dry-down.

In 1.5 h after re-watering, levels of 1-kestotriose increased relative to the level of sucrose at the leaf base, indicating re-synthesis of fructans from accumulated sucrose. By 24 h post re-watering, the ratio of sucrose to 1-kestotriose declined from six in non-watered plants to two and higher molecular weight fructans became detectable. The negative correlation between fructan and sucrose content, which indicates an inter-conversion depending on tissue water content, suggests that, in this species, fructans serve as a carbohydrate pool, while sucrose stabilises the meristem during extreme water deficit.