Desiccation-induced dormancy in papaya (Carica papaya L.)seeds is alleviated by heat shock

The effects of desiccation and temperature on the germination capability of Carica papaya L. were investigated for seeds extracted from three commercial fruit batches. More than 50 percent of freshly isolated, cleaned (sarcotesta removed)but undried seeds germinated at 26°C. However, desiccation to approx. 20 percent seed RH reduced germination at this temperature to < 10 percent. A substantial increase in germination at alternating temperatures (33/19°C)indicated that desiccation induced seed dormancy rather than viability loss. Dormancy could be removed in a large proportion of the population by the application of a single heat shock to rehydrated seeds for 4 h at 36°C, with subsequent return to 26°C for germination. Longer (days)and shorter (minutes)periods of heat shock were less effective for releasing dormancy. Heat shock was generally applied 5 or 14 d after rehydration had started, but the treatment was equally effective after imbibition for only 1 d. Light was always applied during both imbibition and the postheat shock treatment, but was not essential during the actual heat shock treatment. Rehydration and post-heat shock temperature treatments in the range of 16°C to 36°C revealed the same optima of 26°C. Dormancy was re-imposed in heat-shocked seeds when they were subsequently dried to seed relative humidities of 25 to 75percent (5 to 11 percent moisture content [fresh weight basis]), but this state could be removed by a further heat shock. The longer heat-shocked seeds were held on agar-water at 26°C prior to re-drying, the greater the level of desiccation intolerance.