Efeitos das flutuações térmicas diárias na germinação de sementes de três espécies de Myrteae (Myrtaceae) em um cenário de aquecimento global

Abstract

The effects of climate change on biodiversity are currently being documented, but the impacts of microclimatic fluctuations (e.g., daily thermal variations in the soil) on seed germination remain poorly understood. Asymmetric thermal fluctuation, characterized by an increase in the daily minimum temperature and a reduction in thermal amplitude, has been neglected in modeling and experiments. Investigating these effects in widely distributed and ecologically representative groups, such as Myrtaceae, is essential to understanding their response to environmental changes. This study aims to investigate the germination percentage and speed of seeds from Eugenia uniflora L., Myrcia sp., and Psidium guajava L., as well as the morphology and anatomy of the vegetative structures of developing seedlings under conditions of symmetric and asymmetric daily temperature fluctuations. The seeds of the studied species were subjected to growth chambers with a 12-hour photoperiod, divided into symmetric and asymmetric conditions, with three treatments each. In the symmetric phase, the initial temperature (TS1) was 20°C (day)/25°C (night), increasing by 5°C gradually in treatments (TS2) and (TS3). In the asymmetric phase, the initial temperature (TA1) was 22°C (day)/31°C (night), with a gradual increase of 6°C and 3°C, respectively, in treatments (TA2) and (TA3). Seeds that did not germinate were subjected to the tetrazolium test to assess viability. Ten individuals from each treatment were selected for biometric and anatomical analyses. Biometric measurements included root and stem length, as well as the width and length of the third fully developed leaf. For anatomical analyses, samples of leaves, stems, and roots were fixed in FAA 50 and stored in 70% ethanol. The thickness of the adaxial and abaxial epidermis, leaf mesophyll, stem hypodermis, and root cortex were measured. All data obtained were subjected to analysis of variance (ANOVA), and when significant, the Tukey test was applied for mean comparison. Eugenia uniflora demonstrated high thermal plasticity, with a high germination percentage and healthy seedling development at higher temperatures and smaller thermal amplitudes, indicating greater tolerance to global warming scenarios. This capacity may be related to the greater amount of cotyledonary reserves reported for Eugenia L. species, which potentially favors their adaptation to variable environments. Myrcia sp. exhibited high germination rates under different thermal conditions, but seedling development was compromised by tissue necrosis, possibly due to thermal stress. In contrast, Psidium guajava showed lower germination percentages under asymmetric conditions but high seed viability, suggesting a possible dormancy mechanism as an adaptive strategy to favor germination under suitable conditions and avoid development in unfavorable environments. These differences suggest potential changes in species coexistence dynamics and their ecological interactions, such as with dispersers and predators, in future climate change scenarios. The studied species exhibited distinct adaptive responses to thermal fluctuations. Psidium guajava adopted a structural resistance strategy, with reduced germination but vigorous development. Myrcia sp. showed high germination but compromised development due to seedling necrosis. Eugenia uniflora demonstrated greater plasticity, germinating and developing well under different temperatures, with anatomical adaptations that optimize resource use. The foliar variation and increased root tissues in Eugenia uniflora and Psidium guajava suggest complementary strategies for resource absorption and environmental resilience. These findings contribute to the understanding of the ecological and physiological strategies of tropical species in response to short-term thermal variations.