Differential drying rates of recalcitrant Trichilia dregeana embryonic axes: a study of survival and oxidative stress metabolism

Abstract

Studies to elucidate the biochemical basis of survival of excised embryonic axes (EAs) of recalcitrant seeds of Trichilia dregeana at different drying rates revealed significant differences between slow and rapid drying. Rapid drying allowed these EAs to survive dehydration to much lower water contents (WCs; ca. 0.31 g g⁻¹ dry mass basis with 73% germination) compared with slow drying, where 90% of the EAs lost viability at a WC of ca. 0.79 g g⁻¹. In EAs slowly dried within seeds, the levels of hydroxyl radical (three- to fivefold at WCs > 0.5 g g⁻¹) and lipid peroxidation (50% at similar WC) were significantly higher compared with those dried rapidly to comparable WCs. When EAs were dried slowly, enzymic antioxidant levels were not sustained and declined significantly with prolonged storage. In contrast, sustained activity of enzymic antioxidants was detected in rapidly dried EAs even at relatively low WCs. Furthermore, the greater decline in glutathione (GSH)/GSH disulphide ratio in EAs slowly dried within seeds compared with rapidly dried EAs and a shift in GSH redox potential to relatively more positive values in the EAs slowly dried within seeds was correlated with considerable viability loss. It is apparent from this study that greater retention of viability to lower WCs in rapidly dried EAs from recalcitrant seeds may at least be partly explained by the retention of functional antioxidant status. It is also suggested that the reduction of viability in rapidly dried EAs at very low WCs appears to be a non-oxidative process.

Evidence for the absence of enzymatic reactions in the glassy state. A case study of xanthophyll cycle pigments in the desiccation-tolerant moss Syntrichia ruralis

“…With duration of time spent in the desiccated state, viability is gradually lost, and this coincides with the failure of the antioxidant system (Kranner et al , 2002; Varghese et al , 2011). Accordingly, following long-term storage in the desiccated state, the ability of S. ruralis upon rehydration to recover its maximal photochemical efficiency of PSII progressively decreased (Fig.…”

The use of plant stress biomarkers in assessing the effects of desiccation in zygotic embryos from recalcitrant seeds: challenges and considerations

“…This can lead to damaging levels of reactive oxygen species (ROS) production and affect the functioning of antioxidant systems (Varghese & Naithani ), which can subsequently cause lethal damage even at relatively high WCs (Varghese et al . ). However, even when this dehydration‐induced damage does not necessarily compromise viability after drying, it can be exacerbated during subsequent cooling and rewarming, leading to viability loss (Sershen et al .…”

“…The influence of explant drying rate has therefore become a particularly important consideration in recent times (Varghese et al . ; Ballesteros et al . ) since desiccation damage in recalcitrant seeds appears to be a function of two interrelated parameters: the degree and the duration of dehydration (Pammenter et al .…”

“…); superoxide production, lipid peroxidation and antioxidant activity, all of which give an indication of oxidative stress in seed tissues (Varghese et al . ); and cellular vacuolation, the degree of which often increases in partially dried zygotic embryos/embryonic axes from recalcitrant seeds (Berjak et al . ; Sershen et al .…”

Implications of the lack of desiccation tolerance in recalcitrant seeds

“…The major features which become apparent in embryo cells, the underlying mechanisms and processes involved and their interactions in developing seeds upon acquisition of desiccation tolerance, have been sequentially extensively reviewed (Vertucci and Farrant, 1995; Pammenter and Berjak, 1999; Kermode and Finch-Savage, 2002; Berjak et al, 2007; Berjak and Pammenter, 2008). Consequently, to provide the context for comparison of the desiccation-sensitive seed situation, these are briefly described, but not detailed here.…”