Variation in leaf gall pigmentation in Cassava (Manihot esculenta, Euphorbiaceae) and its associations with structural host traits

Abstract

Cassava (Manihot esculenta, Euphorbiaceae) hosts various gall-inducing organisms, such as Iatrophobia brasiliensis (Cecidomyiidae, Diptera), which induce both white‒green and purple-leaf galls. In this study, we aimed to understand the relationship between gall structure and histochemistry with the occurrence of purple coloration in galls. Therefore, nongalled leaves of the 3rd node (n = 5) and mature white–green and purple galls (n = 5 each) were collected. Fragments of the median region of leaves (petiole and leaf blade) and of the galls were evaluated fresh and/or included in Historesin. Starch, reducing sugars, lipids, proteins, phenolics and alkaloids were histochemically evaluated in the galls. The galls assume the same pigmentation from petioles (white‒green or purple) of each M. esculenta individual, which are the mandatory intraspecific traits that determine its variation. Both white–green and purple galls have a uniseriate epidermis, with a thin cuticle and visually rare stomata. The cortex is exclusively parenchymatic and comprises an outer and inner cortex. The vascular bundles are collateral. The purple galls have subepidermal layers containing larger cells that act as sites for pigment accumulation alongside the epidermis. The occurrence of reducing sugars in the outer cortex was exclusive to purple galls, same for proteins and alkaloids in vascular bundles for white–green galls. The purple phenomenon of M. esculenta galls appears to be related to intraspecific variations in the plant species, resulting from a metabolic continuum with the petiole. Anthocyanin production seems to be sustained by the influx of sugars into the outer cortex of purple galls.