Morphoanatomy and ecophysiology of tree seedlings in semideciduous forest during high-light acclimation in nursery

The recomposition of deforested environments demands the acclimation of seedlings in nurseries. This process induces changes in physiological, anatomical, and morphological traits of plants, favouring their establishment after transplantation to the field. The present study aimed to verify the influence of full-sun acclimation on seedling hardiness. For the purpose, leaf gas-exchange, plant anatomical and morphological parameters of three tree species [Ceiba speciosa (A. St.-Hil.) Ravenna (Malvaceae), Croton floribundus Spreng. (Euphorbiaceae), and Cecropia pachystachya Trecul (Urticaceae)], which are used for reforestation in the Brazilian Atlantic biome, were evaluated. Seedlings were grown under 40% of total PPFD (shaded control) and under full sun (acclimated) for 168 days. The acclimation process induced a higher leaf production rate in C. speciosa and C. floribundus, whereas C. pachystachya seedlings replaced their leaves quickly, irrespective of the light conditions. The newly developed leaves of all three species presented a lower area and thicker palisade parenchyma, resulting in a reduced specific leaf area. The seedlings of C. speciosa and C. pachystachya showed increases in light-saturated net photosynthesis and transpiration rates, whereas water-use efficiency generally remained unchanged in all three species. The full-sun acclimated seedlings of C. pachystachya showed a reduced relative growth rate, lower height/stem diameter (H/D) and shoot to root dry mass ratios, characteristics that may result in greater physical resistance and ability for water and nutrient uptake to support the higher transpiratory demand under full sun. The reduction of the H/D ratio also occurred in the acclimated seedlings of C. speciosa. The seedlings of C. floribundus showed few changes during acclimation, but they did not seem to be affected by excessive light. In spite of the observed differences among the three species, all of them developed hardiness characteristics, mainly related to leaf anatomy, which should favour their establishment after transplantation to the field.