Crescimento e desenvolvimento de mudas de Annona squamosa L. em diferentes níveis de sombreamento

Abstract

The present study was conducted with the species of Annona squamosa L. fruit, under different levels of shading between May 2020 and February 2021, with an evaluation period divided into dry season (21/05/2020 to 18/09/2020) and rainy season (19/09/2020 to 05/02/2021). This research was divided into four chapters, the first, a literature review on the relevant issues to the other chapters, which originated from three experiments. In general, the experimental design used was completely randomized (CRD), using coverage of 35, 50, 65 and 80% of shading, and in full sun. The climate in the shaded environments was monitored by digital thermo-hygrometers and in full sun by the UFMT automatic weather station. In the first experiment (second chapter) the thermal requirements were determined, and the leaves number of Annona squamosa L. was estimated by the phyllochron and Wang and Engel methods with 19 repetitions. The maximum and minimum basal temperature for development was 38 and 11°C, from which the accumulated thermal sum was determined, corresponding to 4197.52; 4497.35; 4550.41; 4278.94 and 4409.83°C.day, accumulated at the end of the experiment in full sun, 35, 50, 65 and 80% of shading. The microclimatic dynamics influenced the phyllochron, with the highest value observed in full sun and 80% shading (109.8°C.day.leaf-1 ) and lowest value in 50% shading (91.74°C.day.leaf-1 ). The phyllochron method showed greater efficiency in estimating leaf emergence when compared to the Wang and Engel method. In the second experiment (third chapter) the initial growth of Annona squamosa L. seedlings in different shade levels was evaluated. Non-destructive evaluations (biometric parameters and leaf color) were carried out fortnightly during the dry and rainy seasons, starting 15 days after the treatments application, and at the end of the experiment a destructive analysis (biometrics parameters). It was found that: regardless of the seasons, seedlings grown in full sun showed chlorophyll photooxidation and the presence of photoprotective pigments; full sun and shading levels altered the dry matter partition between seedling organs; shading of 65 and 80% showed greater significance in the etiolation level of seedlings and greater specific leaf area. The results show that Annona squamosa L. seedlings have different growth and development strategies, demonstrating the ability to acclimate in contrasting light environments, which indicates a high capacity for phenotypic plasticity of the species. The third and last experiment (fourth chapter) aimed to characterize the photosynthesis of Annona squamosa L. in different shading levels. Evaluations of gas exchange and Chl a fluorescence variables were performed, both during the dry season, and during the rainy season only for the Chl a fluorescence variables. At the end of the experiment, Chl fluorescence data were collected before dawn, and throughout the day at sequential intervals of two hours, totaling five readings during the day. It was verified for the levels of shading and full sun, similarities in the variable concentration of CO2 in the substomatic chamber; as for the transpiration variables, stomatal conductance and photosynthesis, an increase was identified as the shading level was increased, with the highest % increase occurring at 80% of shading; 50% shading provided greater efficiency in the water xxi use by the seedlings. The environments of full sun and 35% promoted photoinhibition by excess light, and regardless of the environment, the seedlings underwent dynamic photoinhibition. The shading level of 50% is the most suitable for the production of Annona squamosa L. seedlings, promoting greater CO2 fixation per molecule of water lost and for presenting smaller daily variation between maximum and minimum of the maximum quantum yield.