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

Abstract

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% of freshly isolated, cleaned (sarcotesta removed) but undried seeds germinated at 26°C. However, desiccation to approx. 20% seed RH reduced germination at this temperature to < 10%. 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 75% (5 to 11% 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.

Conservation technologies for safeguarding and restoring threatened flora: case studies from Eastern Australia

“…Further work by Ashmore et al (unpublished) on S. luehmannii indicates that seeds are tolerant to desiccation to around 20% MC but also require dormancy-breaking treatments (gibberellic acid or heat shock) to achieve germination (68% and 82%, respectively). This is similar to reports in papaya seeds, where desiccation tolerance was observed down to 5% MC, but dormancy-breaking treatments were subsequently required for germination (Wood et al 2000a;Ashmore et al 2009). Shatnawi et al (2004) have also reported both in vitro propagation and shoot-tip cryopreservation via encapsulation-dehydration of Syzygium francisii (rose satinash) (Table 1).…”

Biomechanical, biochemical, and morphological mechanisms of heat shock-mediated germination inCarica papayaseed

“…Germination of ~70% of desiccated C. papaya seeds could be achieved by giving seeds a 2–6 h HS before returning them to ∼25 °C for relatively rapid and synchronous germination. In contrast, heat treatment had no effect on the dormancy behaviour of the desiccated C. papaya seed without prior imbibition, irrespective of variation in temperature and duration of treatment (Wood et al , 2000). Thus, hydration is an absolute prerequisite as germination subsequently occurred only in seeds previously imbibed for 1–5 d prior to HS treatment.…”

“…Carica papaya L. (papaya) is an opportunistic pioneer species native to tropical South America and is an important commercial crop of tropical regions worldwide. Seeds readily germinate when fresh from the fruit but germination is slow and erratic after desiccation (Wood et al , 2000) or in the presence of the sarcotesta, which contains germination inhibitors (Lange, 1961; Chow and Lin, 1991). …”

“…However, high levels of synchronised germination of hydrated dormant seeds can be induced by brief ‘heat shock’ treatment, with an up-shift of 10 °C from an imbibition temperature of ∼25 °C proving most effective (Wood et al , 2000). This upshift in temperature could be regarded not as heat shock per se but as exposure to higher temperatures naturally present in the climatic zone of C. papaya , representing a single alternating temperature pulse.…”

“…For example, exposure of hydrated E. crus-galli seeds to 46 °C for over 24 h in the dark switched the dormancy of a proportion of the seeds from phytochrome-dependent to phytochrome-independent (Taylorson and Di Nola, 1989). Wood et al (2000) suggested that as C. papaya seeds need to be hydrated for the temperature shift to be effective, then germination is also triggered by a physiological change, possibly involving protein synthesis and the activation of heat-sensitive signalling pathways. However, studies on other species conclude that HS proteins are unlikely to be involved ( R. obtusifolius ; van Assche and van Nerum, 1997) and that protein synthesis is not likely to be the key mechanism of sensing and responding to the heat treatment ( Cucumis sativa ; Amritphale et al , 2000).…”