Transpiração e respostas fisiológicas de plantas jovens de Eucalyptus sob diferentes níveis de restrição hídrica

Abstract

In Brazil, eucalyptus plantations have been expanding rapidly, accounting for approximately 76% of planted forest areas. However, forest plantations are highly water-sensitive, especially in the early stages of field establishment. This characteristic highlights the importance of accurately quantifying actual water consumption by plants, not only to optimize water management but, more importantly, to understand physiological thresholds under environmental stress conditions. Therefore, the present study aimed to evaluate the performance of a transpiration estimation method and investigate the effects of water restriction on young eucalyptus plants. The research was divided into two phases. In the first phase, seedlings of the hybrid Eucalyptus urophylla × Eucalyptus grandis (clone I144) were transplanted into 120-liter pots filled with commercial substrate enriched with controlled-release fertilizer. For 147 days, substrate moisture was maintained at 90% of maximum water holding capacity (MRWC), and plant transpiration was monitored using a lysimetric platform system. In parallel, an adaptation of the Penman-Monteith equation was tested to estimate transpiration based on periodic leaf area measurements and meteorological data from an automatic station installed at the experimental center. In the second stage, five months after transplanting—when the plants had an average leaf area of 8.45 m², diameter of 3.61 cm, and height of 2.85 m—three levels of substrate water availability were applied: 100% (Treatment 1), 60% (Treatment 2), and 40% of MRWC (Treatment 3). During the water restriction period, physiological parameters such as net photosynthesis, transpiration, stomatal conductance, photosystem II quantum yield (Fv/Fm), leaf water potential, and leaf temperature were evaluated. After five days of restriction, irrigation was reestablished for all treatments, and the same parameters were reevaluated to assess plant recovery. The results of the first stage demonstrated that the proposed model for estimating daily and hourly transpiration performed well when adjusted for leaf area, as evidenced by high coefficient of determination (R²) values and low root mean square error (RMSE) and mean absolute error (MAE). Including leaf area as an independent variable was essential for accurate modeling of transpiration, especially in young trees, a critical stage of field development where reliable estimates are often scarce. From a physiological perspective, analyzed in the second stage, water restriction caused progressively deleterious effects on the young plants of the clone evaluated, compromising water consumption, transpiration, stomatal conductance, net photosynthesis, and the efficiency of photosystem II. These changes culminated in morphological damage, such as increased basal leaf abscission and increased leaf temperature, especially under severe water restriction. Overall, the results reinforce the importance of considering structural variables, such as leaf area, when modeling transpiration. Furthermore, they demonstrate the physiological vulnerability of young eucalyptus plants to water limitation, highlighting the need for more effective management strategies to ensure functional performance under stress conditions