Resistência e resiliência de espécies de leguminosas à irregularidade hídricada caatinga

Abstract

Low and irregular water availability, together with high temperatures, represent the main limiting factors to seed germination, establishment, and development of plant species in semiarid ecosystems. The Caatinga, the largest tropical dryland in South America and the most extensive Neotropical nucleus of Seasonally Dry Tropical Forests, provides an exceptional setting to investigate these processes. Within this ecosystem, the Fabaceae stands out as the most abundant plant family, comprising a remarkable diversity of herbaceous and tree species that occupy a wide range of habitats. In this context, the present study aimed to elucidate how hydric irregularity and high temperatures influence germination, the breaking of physical dormancy, and seedling growth in Fabaceae species from the Caatinga. To this end, six complementary studies were conducted, integrating ecophysiological and functional approaches to understand the mechanisms underlying germination, establishment, and early development in representatives of this family. The first study investigated the morphological and germinative attributes of seeds from herbaceous and tree species occurring across distinct habitats, collected over three years characterized by contrasting rainfall regimes (510.3, 762.6, and 194.6 mm). Although some traits varied among years, such variation was not directly related to precipitation. Seeds of the different species consistently exhibited high germination percentages, reflecting a marked resilience to the contrasting rainfall conditions observed across the three sampling years. The second study evaluated the effects of temperature and moisture on the physical dormancy release of seeds from herbaceous and tree species. It was demonstrated that dormancy release is promoted by the interaction between temperature and humidity. Moreover, distinct strategies were observed between the two growth forms: herbaceous seeds tend to lose dormancy at the onset of the rainy season, optimizing the window of opportunity for their rapid life cycle, whereas tree seeds remain dormant for longer periods, gradually releasing germination over time. The third study examined the effects of a wide range of temperatures (12.5–50 °C) and water potentials (0 to −1.2 MPa) on the seed germination of herbaceous and tree species. It was found that water limitation, rather than high temperature, is the main factor constraining germination. Overall, the species exhibited high thermal tolerance and moderate resistance to water stress, coupled with short germination times (t₅₀ < 24 h). These responses were particularly pronounced in herbaceous species, likely reflecting their annual habit and short life cycle, which are adaptive traits to the brief favorable moisture periods characteristic of the Caatinga. The fourth study investigated the relationships between seed morphological traits, germination percentage and rate, and early root growth in herbaceous and tree species. Species with smaller seeds, mostly herbaceous, germinated more rapidly and exhibited greater root development, particularly under high temperatures (35 °C) and negative water potentials. These relationships highlight the predictive role of seed traits in plant establishment and reflect the coexistence of distinct adaptive strategies among species inhabiting semiarid environments. The fifth study examined the influence of drought cycles on the early development of tree seedlings and the relationships between their growth characteristics and seed traits. The species maintained a balanced biomass allocation pattern between roots and shoots, even under water deficit conditions. Moreover, it was found that larger seeds produced individuals with greater stem diameter but reduced vertical growth, relationships that persisted under recurrent drought cycles. The sixth study investigated the transgenerational effects of water deficit on seed germination and progeny growth in an annual herbaceous species. Water stress imposed on the parental generation did not alter the germination or growth of the offspring. However, variations in hormone and antioxidant concentrations were detected, suggesting subtle physiological adjustments with adaptive significance. These findings indicate that drought tolerance in Fabaceae species from the Caatinga results from well-established physiological mechanisms that enable them to withstand the extreme climatic conditions characteristic of semiarid environments. In general, Fabaceae species from the Caatinga share adaptive mechanisms that enable seed germination, seedling establishment, and plant development under the unpredictable climatic conditions typical of this ecosystem. The findings broaden our understanding of the mechanisms underlying the resilience of Caatinga Fabaceae and make a substantial contribution to the functional ecology of seeds and plants in semiarid ecosystems.