Abstract
A multiple epidermis in the green stems of Parkinsonia species has been described; however, there are disagreements among authors with regard to origin and tissues involved. The aims of this study were to identify the origin and development of the epidermis and cortex of P. praecox and relate these to possible adaptations to arid environments. Samples from new branches, to the stem, were removed and prepared using two embedding techniques. Our results show that the epidermis is simple, but periclinal divisions start far from the apical meristem. The inner derivatives maintained periclinal and anticlinal divisions, which are interpreted as meristematic, promoting continuous cell renewal. Wax and cutin deposits suggest that a special epidermis is present; however, it does not correspond to a multiple one. The cortex has four distinct regions. As the branch circumference increases, modifications occur in the first, second, and fourth regions, and sclerenchyma with abundant prismatic crystals develops. The third region maintains its identity with abundant chloroplasts. The occurrence of crystals and chlorenchyma improves structural stiffness, increases the reflectivity of plant surfaces, and facilitates the recycling of respiratory CO2. Abundant waxes, chlorenchyma, sclerenchyma, and crystals in the stems may be adaptations of P. praecox to xeric environments.
Highlights
Green stems are a convergence present in different families that grow in open dry forest (Gibson, 1983)
The third (3rd) region is composed of 3–8 layers of smaller isodiametric parenchyma cells with chloroplasts occupying 19% of the total diameter, and the fourth (4th) region is a single layer of parenchyma cells in which a prismatic crystal begins to differentiate
We propose to name this a special epidermis instead of multiple epidermis, because when divisions begin, the cuticle has already been developed and divisions are far from the protoderm
Summary
Green stems are a convergence present in different families that grow in open dry forest (Gibson, 1983). In numerous species from desert communities, photosynthetic stems contribute to carbon assimilation and maintenance of metabolic activities (Nilsen, 1995; Ávila et al, 2014; ÁvilaLovera and Tezara, 2018). These green stems may be covered by well-developed cuticles or a delayed formation of the periderm. In species with green stems the epidermis is preserved even after the secondary growth takes place, as in Parkinsonia species (Gibson, 1983; Roth, 1963, 1981; Roth and Delgado, 1968a; Armitage and Howe, 2018). A multiple epidermis was described in P. aculeata (Roth and Delgado, 1968a) and P. torreyana [=C. torreyanum] (Roth, 1963)
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