Physiological aspects of diapause and cold tolerance during overwintering in Pieris brassicae

Abstract

Abstract The relationship between diapause‐associated metabolic suppression and carbohydrate metabolism linked with cold tolerance was investigated in pupae of Pieris brassicae L. Cold tolerance was assessed by measuring the crystallization temperature (Tc) and by estimates of pre‐freeze mortality. Metabolic suppression was measured using 31P nmr and carbohydrates by GLC.Sorbitol (a possible cryoprotectant) accumulated from the onset of diapause in October until December reaching a concentration of c. 40 mMolal in both years of the study, but then declined from January until adult eclosion in May. The Tc remained between ‐23 and ‐25oC throughout the winter except for a slight rise before eclosion in May. The absence of a significant Tc suppression is as predicted from the low concentration of sorbitol accumulated. The pre‐freeze mortality experiments indicate that pupae are most cold tolerant in the period December‐February when sorbitol concentration is high, suggesting an alternative cryoprotective role for sorbitol. Glycogen declined at the beginning of diapause until February after which there was some recovery suggesting that it may be the source of carbon for sorbitol synthesis.Diapause‐associated metabolic suppression is evident in the low 31P nmr resonances of ATP during November‐February compared with non‐diapause pupae and diapause pupae soon after pupation. The suppression of metabolism at this time may have a direct role in cryoprotection and by itself (rather than sorbitol) account for the increased pre‐freeze cold tolerance. ATP appears to increase slowly from February until a sharp increase occurs shortly before eclosion. Arginine phosphate remains high during diapause until late February‐March when it begins a decline which continues until eclosion. A period of change in energy and carbohydrate metabolism is apparent at the same time which may indicate diapause termination and related changes in cold tolerance mechanisms.It is argued that in P.brassicae sorbitol accumulates as a result of metabolic suppression and may have no cryoprotective role. However, for species living in, or colonizing, low temperature environments it is a short evolutionary step to exploit this pathway and accumulate high concentrations of polyols as a specialized cold tolerance strategy.