Ontogenesis and responses to shading of Attalea vitrivir (Arecaceae) eophyll

Highlights

• The ontogenesis of Attalea vitrivir eophylls occurs below ground level.

• The basal meristem produces a petiole and a folded blade with rich vasculature.

• Shading does not affect eophyll anatomy or the presence of secondary compounds.

• A high blade growth rate compensates low photosynthetic efficiency under shading.

• Eophyll development patterns and its response to shading have adaptive value.

Abstract

The eophylls (first photosynthetic leaves) of palm trees play crucial roles in seedling establishment, although information concerning their development and ecophysiology remain scarce. This work sought to evaluate eophyll development and the effects of shading on young plants of the neotropical oil palm Attalea vitrivir. Eophyll ontogenesis was characterized, and the effects of shading levels (0, 58, 73 and 93%) on their anatomy, histochemistry, and seedling development where evaluated. Photosynthetic responses were compared between plants exposed to full sunlight or shade. After germination, the cotyledonary petiole elongates downward, which promotes the burial of the vegetative axis. Two eophylls are formed by the apical meristem, and they develop a short petiole and a much-folded blade that grows through the action of a basal meristem. The eophylls emerge below soil level and show a typical leaf structure and an intense and diverse vasculature. Shading levels do not affect blade tissue thicknesses, stomatal density, or the presence of primary and secondary metabolic compounds. A low photosynthetic performance under shade conditions is compensated by greater blade growth, so that plant development is not affected by shading. The ontogenesis of A. vitrivir eophylls below the soil surface and their response to shading contributes to seedling establishment and the adaptation of the species to seasonal and anthropized environments.

Introduction

Palm trees develop specialized types of initial leaves that play crucial roles in seedling establishment (Gunawardena and Dengler, 2006; Tillich, 2007). The first photosynthetic leaves (eophylls) are distinct from definitive leaves (metaphylls) and show wide structural diversity among Arecaceae taxa (Henderson, 2006). In-depth studies focusing on the ontogenesis, structure, and functionality of palm eophylls have been scarce (Henderson, 2006; Dransfield et al., 2008), but can contribute to the expansion of our knowledge concerning the adaptations of different species to different environments and provide useful information for palm propagation and cultivation efforts.

Light is an important modulator of plant development (Poorter, 2001; Markesteijn et al. 2007). The light regimes to which plants are exposed can influence biomass production (Poorter, 2001; Gatti et al., 2011), the synthesis of primary and secondary metabolites (Dickison, 2000; Agati et al., 2011), and leaf anatomy (Araus and Hogan, 1994; Markesteijn et al., 2007; Pereira et al., 2013). Most young palm plants are shade-tolerant and grow in the understories of tropical forests (Dransfield et al., 2008; Corrêa et al., 2019). Young plants of pioneer species that are adapted to colonizing open environments, on the other hand, are commonly exposed to high light intensities and water deficits and develop peculiar structures and distinct physiological processes that have not yet been closely studied (Araus and Hogan, 1994; Orozco-Segovia et al., 2003; Dransfield et al., 2008).

Attalea vitrivir Zona is a palm tree endemic to the Cerrado (neotropical savanna) biome in central Brazil (Lorenzi et al., 2010) and is therefore adapted to a highly seasonal climate with pronounced water deficits occurring for several months during the austral winter (Guedes et al., 2015). The species demonstrates a pioneer-behavior for occupying open areas close to water courses, where it can form dense monodominant populations (Neves et al., 2013). A. vitrivir has social importance to human populations as a food source and for construction and handicraft purposes (Lorenzi et al., 2010). It also represents a potential source of biomass for charcoal production for use in steel mills (Teixeira, 2008) and the production of biofuels (Santos et al., 2007). Information concerning seedling development in the species is still scarce (Neves et al., 2013; Guedes et al., 2015), however, but would contribute to the expansion of our knowledge concerning its reproduction and propagation.

The present work was therefore designed to characterize the ontogenesis and structure of A. vitrivir eophyll, evaluate their responses to shading, and respond to the following questions: i) how does the eophyll develop, and what are the particularities of its anatomy? ii) How does eophyll development and their responses to shading (leaf anatomy, the production of primary and secondary metabolites, and photosynthetic activity) contribute to the survival of the species in the environments in which it occurs?