Response of osmotic adjustment and ascorbate-glutathione cycle to heat stress in a heat-sensitive and a heat-tolerant genotype of wucai (Brassica campestris L.)

High temperature stress (>30 °C) from late summer to early fall is one of the main limiting factors in wucai (Brassica campestris L.) growth in southern China. To assess the osmotic and antioxidative reponses of two wucai genotypes (WS-6 and WS-1) differing in heat tolerance, three-week-old seedlings were subjected to three heat-stress treatments for 5 days. Heat treatments above at 30 °C resulted in significant decreases in leaf area and leaf relative water content (RWC) in both genotypes, but being more in heat-sensitive WS-6. This was due to greater accumulations of osmolytes including soluble sugar, soluble protein and proline (Pro), as well as higher levels of reduced ascorbic acid (AsA) and glutathione (GSH) in heat-tolerant WS-1 under heat stress as compared to those in heat-sensitive WS-6. It is worthy noting that mild hyperthermia (30 °C) caused significant increases of AsA and GSH in both genotypes, which might be resulted from their enhanced activities of monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR), helping recycle AsA and GSH. In contrast, the AsA and GSH levels of both genotypes were decreased markedly at 40 °C, especially in heat-sensitive WS-6, despite that the enzymes (DHAR and GR) activities involved in their regeneration remained high levels as compared to their respective controls. In addition, larger decrease in total antioxidant capacity (T-AOC) by heat stress was observed in WS-6 than in WS-1. Our results indicated that the heat-sensitive and the heat-tolerant genotype of wucai might possess the same pathway but different capacities of antioxidant defences to cope with heat stress.