Seed dormancy is an adaptive trait that allows plants to delay germination until environmental conditions become favorable for seedling establishment and growth [1, 18]; in arid ecosystems, such conditions are generally associated with rainfall events, which are highly variable and largely unpredictable [19, 20]. In the plant family Caricaceae, seed dormancy -particularly in the form of PD- is a common characteristic [3]. Our findings reveal that over 95% of C. chilensis seeds are dormant at maturity and that this dormancy can be partially alleviated by reducing seed MC or treating the seeds H2SO4, GA3, or KNO3. These results suggest that, as predicted, seeds of the Austral papaya have non-deep PD.
Germination was 77% higher in ultra-dry than in fresh seeds. This result reveals that C. chilensis seeds can be classified as orthodox, as a reduction of 97% of their MC did not reduce survival. There are conflicting reports in the literature about the effect of reducing MC on seed viability and germination among Caricaceae. For example, Ellis and collaborators [21] found a loss of seed viability when the seed MC of Carica papaya was reduced to < 7%. However, in another experiment with the same species, a reduction of MC to 5% either improved germination or did not affect it, depending on the seed lot [22]. Similar to our results, ultra-drying promoted germination in Vasconcellea quercifolia [7]. Reducing seed MC can promote germination via several mechanisms. For example, it can alter seed water potential [23]. In this regard, seeds with lower MC have more negative matric water potentials than seeds with higher MC; as a result, they absorb water more quickly [24]. In contrast, solution uptake by fresh or fully imbibed seeds is driven by differences in osmotic potential between the seed and the surrounding solution [25]. Decreasing MC can also alter the hormonal balance in seeds, reducing levels of abscisic acid (ABA) and increasing gibberellins (GA) [26–28]. These two hormones regulate seed germination in opposite manners: ABA promotes seed dormancy and inhibition of germination, whereas GA promotes germination [29].
A reduction in seed MC is likely to occur naturally in C. chilensis habitat because the species produces mature fruits between August and October, prior to the onset of summer. Since both temperature and the gradual loss of MC are integrated over time to alter the depth of the dormancy [2], the hot and dry conditions of the Mediterranean summer in central Chile likely contribute to the gradual release of dormancy release throughout the season. Consequently, seeds can germinate in the field at the onset of the winter rain pulses [30] and take advantage of the moister soils and warmer temperatures during the spring to promote seedling establishment. Nonetheless, given that germination patterns and seed dormancy can vary geographically [31] and that we collected seeds from a population located towards the southern end of C. chilensis’ distribution in Chile, which has drier and warmer summers, it will be informative to confirm if seeds from other localities show similar behavior.
Dormancy was released by treating C. chilensis seeds with H2SO4, GA3, or KNO3. The seed coat of Caricaceae is multiplicative, consisting of a fleshy outer layer known as the sarcotesta, enclosing a hard, lignified portion of the testa [32]. Hence, scarification with H2SO4 can enhance water absorption and uptake, essential for initiating the germination process, by breaking down the seed coat [33]. Gibberellins are a family of plant hormones that control many aspects of plant growth, including germination [2]. GA3 stimulates germination through several mechanisms that include the production of hydrolytic enzymes that weaken the seed coat, mobilization of nutrient reserves, and stimulation of plant embryo expansion and hypocotyl elongation [28, 34]. Numerous studies have documented enhanced germination rates in Carica papaya with the application of GA3 [35]. Similarly, GA3 promotes germination in a few species of Vasconcellea [7, 8]. However, apart from these findings, there is currently a lack of information regarding the effects of GA3 in other Caricaceae species. Finally, KNO3 can break seed dormancy and promote germination in at least two ways. First, it has hygroscopic properties; when seeds are imbibed in a KNO3 solution, they absorb moisture from the surrounding environment, weakening the seed coat. Second, it inhibits ABA synthesis while stimulating the synthesis of GA, which promotes germination by enhancing the growth potential of the embryo and overcoming the mechanical barriers imposed by the testa [2].
One of the major impediments to the potential use of wild species germplasm for habitat restoration is the need for more knowledge about techniques for breaking seed dormancy and caring for germinating seeds [36]. Thus, improving our understanding of seed biology is crucial to restoring a broader and more representative range of species [37]. Chile is one of 115 countries that has subscribed to restoration commitments [38]. However, germination requirements and dormancy breaking of seeds of most non-tree species in Chile remain poorly known. This lack of knowledge hinders the propagation of threatened plants and is recognized as a bottleneck for fulfilling Chile’s restoration commitments [17]. This study contributes to filling this gap with the aim of promoting more extensive use of species diversity in ecological restoration and helping the conservation of endangered plant species.