Respostas morfofisiológicas, enzimáticas e anatômicas ao mitigar o estresse hídrico com uso de Chlorella sp. em mudas Cedrela fissilis Vell.

Abstract

The intensification of climate change has led to significant impacts on natural and productive ecosystems, demanding sustainable scientific solutions for both adaptation and mitigation particularly regarding the cultivation of native forest species with high ecological and economic value. Among these species, C. fissilis, from the Meliaceae family, stands out, currently classified as threatened with extinction due to predatory exploitation and low natural regeneration rates. This thesis aimed to evaluate the effects of foliar application of the microalga Chlorella sp. on morpho-anatomical, physiological, nutritional, and biochemical variables of C. fissilis seedlings subjected to different levels of water stress, in order to identify adaptive responses that support more resilient and ecologically sound cultivation practices. The research was conducted under controlled environmental conditions, using a randomized block design with treatments arranged in a factorial scheme consisting of four doses of Chlorella sp. (0, 2.5, 5.0, and 7.5 mL plant⁻¹) and two water retention levels (70% – adequate water condition – and 40% of the water retention capacity – water stress condition). The analyses included seedling growth, physiological attributes (CO₂ assimilation, stomatal conductance, water use efficiency, chlorophyll fluorescence), foliar macro- and micronutrient content, anatomical characterization of the leaves (epidermal, palisade, and spongy parenchyma thickness), stomatal morphometry and density, and the activity of the antioxidant enzymes superoxide dismutase (SOD) and peroxidase (POD). The results demonstrated that the application of Chlorella sp. significantly modulated the anatomical structure of the leaves, especially under water stress conditions, promoting thickening of the photosynthetic tissues—particularly the palisade parenchyma—and contributing to increased stomatal density. These structural adjustments suggest an adaptive mechanism potentially associated with maximizing photosynthetic efficiency and regulating water loss, conferring greater functional resistance to drought conditions. Intermediate doses (2.5 and 5.0 mL plant⁻¹) led to significant gains in net photosynthesis, carboxylation efficiency (A/Ci), and water use efficiency (WUE), even under water deficit, indicating a biostimulant effect of the microalga. The foliar contents of essential nutrients showed significant improvement with the use of Chlorella sp., suggesting a direct influence on mineral absorption and metabolism. Additionally, an increase in antioxidant activity of SOD and POD enzymes was observed, reflecting an enhanced capacity to respond to oxidative stress. It is concluded that the foliar application of Chlorella sp. represents a promising technology for inducing drought tolerance in Cedrela fissilis seedlings, contributing to the production of plants with improved morphophysiological and biochemical performance under adverse conditions. The use of this microalga as a bioinput aligns with the principles of sustainability and bioeconomy, presenting a viable alternative for ecological restoration programs and forest production adapted to the challenges posed by global climate change.