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 commerci al fruit batches. More than 50% of freshly isolated, cleaned (sarcotesta re moved) but undried seeds germinated at 26 degrees C. However, desiccation t o approx. 20% seed RH reduced germination at this temperature to < 10%. A s ubstantial increase in germination at alternating temperatures (33/19 degre es C) indicated that desiccation induced seed dormancy rather than viabilit y loss. Dormancy could be removed in a large proportion of the population b y the application of a single heat shock to rehydrated seeds for 4 h at 36 degrees C, with subsequent return to 26 degrees C for germination. Longer ( days) and shorter (minutes) periods of heat shock were less effective for r eleasing dormancy. Heat shock was generally applied 5 or 14 d after rehydra tion had started, but the treatment was equally effective after imbibition for only 1 d. Light was always applied during both imbibition and the post- heat shock treatment, but was not essential during the actual heat shock tr eatment. Rehydration and post-heat shock temperature treatments in the rang e of 16 degrees C to 36 degrees C revealed the same optima of 26 degrees C. Dormancy was re-imposed in heat-shocked seeds when they were subsequently dried to seed relative humidities of 25 to 75% (5 to 11% moisture content [ fresh weight basis]), but this state could be removed by a further heat sho ck. The longer heat-shocked seeds were held on agar-water at 26 degrees C p rior to re-drying, the greater the level of desiccation intolerance.