Structural and histochemical changes during in vitro development of Acrocomia aculeata (Arecaceae) seedlings

Abstract

Embryo culture is important for the propagation of many species, and has proved to be efficient for overcoming seminal dormancy in Acrocomia aculeata, a neotropical palm having significant agro-industrial potential. Difficulties related to acclimatizing its in vitro cultured seedlings, however, restrict the commercial use of that technique. We therefore compared the development of A. aculeata seedlings cultivated ex vitro and in vitro to identify possible causes for its low propensity for acclimatization. Morphological, anatomical, micromorphometric, and histochemical evaluations were carried out on seedlings during both ex vitro and in vitro development. In vitro culture restricts the development of the haustorium, root system, and leaves. Restrictions of cell divisions in the epidermis and cell expansion in the parenchyma, together with the accumulation of phenolic compounds, lead to haustorium atrophy. The roots showed decreased thickness and branching, a reduced cortex, the absence of phenolic compound deposition, and early aerenchyma formation. The leaves show hypertrophied hypodermises, reduced chlorophyllous parenchyma, less carbohydrate deposition in the mesophyll, and less epicuticular wax. Those developmental limitations of the haustorium, roots, and leaves, contribute to the low acclimatization capacity of the seedlings. The incorporation into the culture medium of organic substances found in the endosperm, the hormonal stimulus of root development, as well as autotrophic culture may improve the acclimatization of A. aculeata seedlings. Acrocomia aculeata seedlings grown in vitro show haustorium atrophy, restrictions of root and mesophyll development, and limited depositions of epicuticular wax, characteristics that limit their acclimatization capacity.