Expressões da memória de hidratação de sementes: efeitos ambientais e genéticos durante a germinação de cactos da Caatinga

Abstract

Depending on the water availability where the seed was dispersed in semiarid ecosystems, the imbibition process during germination can be interrupted by the absence of water in the soil. Thus, the seed interrupts its germination process and loses the absorbed water to the environment, causing hydration and dehydration cycles (HD cycles) when water becomes available again in the environment. As a result of HD cycles, there may be an expression of seed hydration memory, which can preserve the physiological modifications resulting from discontinuous hydration and, primarily, promote adaptive advantages. Some of these advantages include, for example, increased desiccation tolerance, higher germination percentage, and enhanced speed and synchrony of the germination process. Additionally, seeds can also acquire tolerance to environmental stresses such as water deficit, thermal stress and saline stress. However, seed hydration memory expression may be influenced by genetic and/or environmental factors. Furthermore, until the results of the present study, there was no way to mathematically measure the level of hydration memory in seeds. Thus, the main objective of this doctoral thesis was to evaluate the different forms of expression of seed hydration memory in species of cacti native to the Brazilian semiarid region, as well as the creation of an index capable of measuring the level of hydration memory expressed by the seeds. To achieve this objective, this thesis was divided into five chapters structured in the format of scientific articles. Chapter 1 aimed to assess how seeds from the same population of Pilosocereus catingicola (Gürke) Byles & G.D. Rowley subsp. salvadorensis (Werderm.) Zappi (Cactaceae), produced and dispersed in different seasons of the year, express hydration memory under water deficit conditions. In this chapter, we observed that seeds from the same population, produced and dispersed at different times, express hydration memory differently during germination, with discontinuous hydration providing greater 11 acquisition of seed tolerance produced during the dry season. In chapter 2, we assessed how passing through HD cycles at different temperatures influences the acquisition of drought tolerance during the germination of Cereus jamacaru DC. subsp. jamacaru (Cactaceae) and Xiquexique gounellei (F.A.C. Weber) Lavor & Calvente subsp. gounellei (Cactaceae). In this case, we observed that the increase in temperature during the HD cycles reduces the expression of hydration memory in both species, impairing the acquisition of drought tolerance. In the third chapter, we evaluated the effects of rainfall levels in different populations on the expression of seed hydration memory during the germination of C. jamacaru subsp. jamacaru under water deficit conditions. We observed that after the HD cycles, seeds from more arid environments invest in optimizing germination time due to water limitation in the habitat, while seeds from less arid environments invest in increasing the quantity of germinated seeds. Chapter 4 aimed to propose the Germination Memory Index (GMI), a mathematical tool to quantify seed hydration memory. The fifth chapter, in turn, aimed to determine which factor (genetic or environmental) has a greater influence on the expression of seed hydration memory during the germination of C. jamacaru subsp. jamacaru under water deficit conditions. In this case, we observed that genetic factors (individual germination behavior) had a greater influence on the expression of hydration memory during the species' germination under water deficit.