Abstract
Hydroponics is an excellent alternative approach for the production of seedlings, given the growing demand for fruiting trees for the reforestation or recuperation of degraded natural landscapes. In most cases, however, little is known about the optimal period for the maintenance of the seedling in the hydroponic system. Given this, we decided to investigate the hypothesis that morpho-anatomical and physiological alterations can be used to predict the optimal timing for the transplantation of the seedlings to the soil substrate, thereby guaranteeing the most cost-effective application of the hydroponic system. We selected Hymenaea courbaril L., an important Neotropical fruit tree, as the model for this study. We cultivated H. courbaril seedlings in a static hydroponic system and evaluated morpho-anatomical, physiological, and growth parameters over the course of seedling development (30, 60, 90, 120, 150, and 180 days after transplantation; DAT). We observed an interesting relationship between the increase in the density (SD) and conductance (gsw) of the stomata up to 120 DAT, which reflected higher rates of photosynthesis (A), but also a reduced efficiency in the use of water. In the subsequent intervals, the SD of the plants and the diameter of the radicular xylemic vessels elements (RVE) decreased, in an attempt to increase the efficiency of the use of this resource. We also observed an increase in the thickness of the palisade parenchyma (PP) prior to 120 DAT, which did not reflect a general increase in the thickness of the mesophyll, indicating an adjustment in the thickness of the spongiform parenchyma (SP). We also observed a progressive increase in photosynthetic efficiency up to 120 DAT, based on parameters such as the absorption flux energy per active reaction center (ABS/RC) and the photosynthetic performance index (PIABS), but after this period these indices decreased progressively. However, as the PIABS is an indicator of the plant’s tolerance, its decline was associated with an increase in the dissipation of energy (DI0/RC), which indicates that, after 120 DAT, the plant pots may become a stress factor that limit the growth of H. courbaril seedlings. The results of the present study indicate conclusively that a 120-day period is the optimum for the maintenance of the H. courbaril seedlings in the hydroponic system, and also confirm the hypothesis that the morpho-anatomical and physiological responses observed in the plants can be used to predict the ideal period for the transplantation of the seedlings, contributing to a reduction in production time of the hydroponic system.
Highlights
Worldwide, there is a growing need to increase and diversify the production of seedlings for both conventional agriculture and the recuperation of areas of natural vegetation that have been degraded by human activities
As the anatomical techniques require only small samples of tissue and are well established, the present study evaluated whether these techniques, combined with the parameters of fluorescence and gas exchange can be used to identify the ideal period for the transplantation to the soil of the seedlings of fruit trees, such as H. courbaril, raised in hydroponic systems
The stem diameter (SD), the thickness of the palisade parenchyma (PP), and the diameter of the radicular xylemic vessels elements (RVE) all increased in the H. courbaril plants up to 120 Date of Transplantation (DAT), from this point onward, the plants began to reduce their investment in these structures
Summary
There is a growing need to increase and diversify the production of seedlings for both conventional agriculture and the recuperation of areas of natural vegetation that have been degraded by human activities. In response to this question, we decided to test the hypothesis that specific morpho-anatomical and physiological responses from the plant may signal the passing of the ideal period for the transplantation of seedlings from the hydroponic system to the soil. To test this hypothesis, we chose Hymenaea courbaril L., a fruit-producing tree species that is amply distributed in the Neotropical region [10], and may be tolerant of a wide range of environmental conditions [11]
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