Exceptional flooding tolerance in the totipotent recalcitrant seeds ofEugenia stipitata

Abstract

Eugenia stipitata occurs along rivers in Western Amazonia and produces berry-type fruits with economic potential. Its large recalcitrant (i.e. desiccation-intolerant) seeds have been proposed as a model to study seed stress response, as no apparent differentiation between the embryonic axis and the fused cotyledons are visible. Here, the longevity of submerged seeds was analysed with a view to understanding adaptive mechanisms to seasonal flooding. Submerged seeds began germinating after 2 months. After 1 year, 87 and 96% total germination was reached when seeds were submerged under a water column of 6 cm (where seedlings could emerge from under the water) and 26 cm (where seedlings could not reach the water surface), respectively. Seedling morphology was altered underwater, with short internodes and rudimentary leaf blades, and when submersion was terminated, seedlings transplanted to nursery conditions recovered a normal phenotype. Furthermore, when seedlings were detached from the seeds, the 'resown' seeds produced a second, normal seedling within 9 months. Concentrations of the antioxidant glutathione, which was measured as a stress marker, increased with submersion time in water. Seeds that had developed roots and shoots underwater had higher concentrations of glutathione disulphide than non-germinated seeds, suggesting that the flooding stress was more intense for seedlings than seeds, although more oxidizing cellular redox environments are also consistent with the conditions required for differentiation. Submergence underwater is recommended for storage of the recalcitrant seeds of E. stipitata for up to 1 year.

A new approach towards the so-called recalcitrant seeds

“…It should be noted, however, that they would also be limited effects, evidently not comparable to the effective complementation of seed maturation in the mother plant. An example would be the inductions of tolerance to desiccation or cold when harvested seeds are submitted to some type of stress (Leduc et al, 2012;Bonjovani and Barbedo, 2014).…”

“…Among the groups that persist in the area, the panel that was demonstrated up to two decades ago did not change much afterward (Figure 1). The majority of the studies are basically focused on desiccation tolerance Corsato et al, 2012;Chapell et al, 2015;Amoedo and Ferraz, 2017;Gasparin et al, 2017;Subbiah et al, 2017;Chandra and Keshavkant, 2018;Plitta-Michalak et al, 2018), identification of storage behavior (Comin et al, 2014;Joshi et al, 2015;Vicente et al, 2016; 2017; Plitta-Michalak et al, 2018), analysis of deterioration and evaluation of techniques to extend the storage period (Barbedo and Cicero, 2000;Andréo et al, 2006;Bonjovani and Barbedo, 2008;Ballesteros et al, 2014;Lan et al, 2014;Parkhey et al, 2014;Garcia et al, 2015;Ming-Yue et al, 2015;Umarani et al, 2015;Araldi et al, 2016;Hossel et al, 2016;Félix et al, 2017), and rare studying other aspects such as seed maturation (Avila et al, 2009;Lamarca et al, 2013b;Mata et al, 2013;Schultz et al, 2014;Araldi and Coelho, 2015;Shibata and Coelho, 2016;Souza et al, 2018a, b), whose importance will be discussed later, and ecological (Ramlall et al, 2015;Vaz et al, 2016;Porto et al, 2018) or germinative aspects (Moothoo-Padayachie et al, 2016). An analysis, albeit superficial, of studies developed in Brazil and in the world shows small differences.…”

“…The process of formation and maturation of these seeds happens in about a year, but from the second third of the cycle, these seeds have the capacity to germinate and form seedlings. The capacity of seeds to survive for long periods in submerged conditions, avoiding water loss and death by desiccation, is also an interesting strategy adopted by some species (Figure 2E), such as Inga vera (Okamoto and Joly, 2000) and Eugenia stipitata (Calvi et al, 2017). Nevertheless, certainly one of the most interesting strategies is that developed by Eugenia species of the Myrtaceae family ( Figure 2F).…”

“…The second important source of variation would be the phenotypic, i.e., the environment changing the standard behavior of the species. These variations would be responsible for small differences between seeds within the same species, leading to the different results obtained in different works with the same species, such as in Caesalpinia echinata (Borges et al, 2005;Mello et al, 2011;Leduc et al, 2012), Aesculus hippocastanum (Daws et al, 2004), Eugenia pyriformis (Lamarca et al, 2011;Scalon et al, 2012;Lamarca et al, 2016), and Inga vera (Lamarca et al, 2013a;Bilia et al, 1999;Barbedo and Cicero, 2000;Lamarca and Barbedo, 2015). It is important to note that the further away from the extreme of orthodoxy the species is, the more the environment may influence it at each cycle.…”

“…Because evolution does not cease, this same response surface, which today represents small environmental variations within the species, on an evolutionary scale could also be interpreted as the accumulation of variations that resulted, or will result, in the formation of a new seed behavior of a new species. In addition, the Z-axis, i.e., the axis of the influence of environmental variations, could also be seen as the result of the application of artificial induction techniques, such as tolerance to desiccation or cold, already demonstrated for some species (Bonjovani and Barbedo, 2008;Leduc et al, 2012;Kalemba and Ratajczak, 2018). Even dormancy could be seen as one more of the lines, or response surfaces, with possibilities of changes depending on the environment, regardless of the separation into orthodox and recalcitrant.…”