Leaf age, position and anatomical influences on the distribution of the secondary metabolites, homonataloin and three isomers of aloeresin in Aloe hereroensis (Aloaceae) leaves

Abstract

Twenty leaves belonging to five age groups from four different Aloe hereroensis plants were investigated by anatomical and TLC methods. These results indicated that most of the phenolic metabolites are located in the vascular bundle sheath. The homonataloin accumulates in the big inner bundle sheath parenchymatous cells and the three isomers of aloeresin isomers are mainly located in the outer bundle sheath cells. Therefore, the different content of these secondary metabolites in the various leaf parts, and different leaf ages, of Aloe hereroensis are positively related to the density of the vascular bundles. Of all 20 leaves, the top thirds had similar density of vascular bundles, as well as the highest content of the secondary metabolites. However, between the top, middle and the base leaf thirds, both the density of vascular bundles and the content of secondary metabolites were significantly different. In the same leaf, the top third has the highest density of vascular bundles, the base the lowest and the middle an intermediate density. The more exposed the younger leaves and the leaf parts to consumption, the higher the content of the secondary metabolites. This distribution of phenolic metabolites in leaves seems to relate to defense strategies of a plant.