Non-structural carbohydrates of immature seeds of Caesalpinia echinata (Leguminosae) are involved in the induction of desiccation tolerance

Abstract

Seeds of Caesalpinia echinata fill up to physiological maturation phase ~60 days after anthesis (DAA) in the field. These seeds are desiccation tolerant to 0.08 gH2O gDW–1 and can be stored for 2 years under freezing temperatures without losing germinability. Starch (40–50%), soluble carbohydrates (10–15%, mainly sucrose and cyclitols), in addition to traces of raffinose and stachyose detected early at maturation, are supposed to be related to the acquisition of desiccation tolerance. In the present work we demonstrate that desiccation-intolerant immature seeds (45 DAA) of C. echinata can be dried until 0.14 gH2O gDW–1 when previously soaked in polyethylene glycol (PEG) solution, maintaining high germination percentage. In contrast, seeds of 55 DAA tolerated drying until 0.14 gH2O gDW–1 without previous PEG treatment, indicating that they have already reached desiccation tolerance at this developmental stage. High-performance anion exchange chromatography analysis revealed that cyclitols and sucrose increased markedly in the embryonic axes at 45 DAA after PEG treatment, reaching levels found in embryos at 55 DAA. These results suggest that PEG treatment mimics the natural maturation drying of C. echinata seeds, changing carbohydrate metabolism and triggering processes involved in desiccation tolerance.

Changes in desiccation tolerance and respiratory rates of immature Caesalpinia echinata Lam. Seeds

“…Water potential, for its part, showed greater changes, especially in seeds dried to S10, in which PEG treatment resulted in more negative water potentials, that is, with greater water retention. This was also observed by Leduc et al (2012), who attributed this decrease to a probable increase in water binding sites, arising from PEG treatment.…”

“…However, when these seeds were treated with PEG, there was loss in both the capacity for initiating germination and for producing normal seedlings (Figures 3f, h). Induction of tolerance to desiccation by PEG in immature C. echinata seeds seems to be possible only in a short period during their maturation, as shown by Leduc et al (2012). In addition, according to the model proposed by Barbedo et al (2013), artificial processes for inducing maturation applied to isolated immature seeds can slightly change their tolerance to desiccation and their longevity, but this effect would have narrow limits that are not comparable to those obtained from maturation on the mother plant.…”

Do recalcitrant seeds really exist?

“…The study of immature orthodox seeds could be an interesting experimental issue to support this model. For example, immature seeds of Caesalpinia echinata L., which were still sensitive to desiccation at about 35 days after flowering (daa), did not increase desiccation tolerance by PEG treatments; however, by maintaining these seeds in the mother plant for 10 days more was enough to create desiccation tolerance by the same PEG treatment (Leduc et al 2012). Another way to analyse this process would be the comparison of recalcitrant seeds from different populations of the same species, from different regions or years in case the cycle, the degree-days and/or the final water germination after shedding, among others.…”

“…Additionally, seeds of these species accumulated raffi nose family oligosaccharides (RFOs), such as raffi nose and stachyose (Mello et al 2010), or polyols, such as pinitol and ciceritol (Borges et al 2006), which could also contribute to glass state formation. Some studies suggest yet that the role of sucrose and RFOs as protective molecules do not depend only on their content, but rather depend on the sucrose/RFOs ratio (Steadman et al 1996, Vandecasteele et al 2011 and with sucrose/cyclitols ratio (Leduc et al 2012).…”

A new approach towards the so-called recalcitrant seeds

“…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.…”