Stem surface injuries of 20 species of succulent Euphorbia (Euphorbiacae) from South Africa

Abstract

Cactus and succulent Euphorbia plants have green stems that are exposed to sunlight over long periods. Previous research demonstrated that barking injuries occur to stem surfaces of 20 species of cacti in the Americas. For these 20 cactus species the amounts of injuries were related to annual amounts of sunlight exposure on surfaces. Cacti with many injuries had high mortality rates. Data herein show that stem surfaces of 20 species of Euphorbia in South Africa have identical surface injuries as for cacti. Euphorbia plants at 30°S show a mean ratio of scale and bark injuries of 3.5:1 of equatorial-facing to polar-facing surfaces, a ratio is similar to the 4:1 of annual sunlight exposure ratio at that latitude. Results show that Euphorbia stems have injuries on younger tissues than cactus species. Thus, Euphorbia stems are more sensitive than cactus stems. Euphorbia species with most stem injuries had smooth (non-undulate) surfaces, a one-celled epidermal cell layer only, cortex cells that abutted the epidermis, and initial injuries that involved only epidermal cells. This is the first report of identical stem surface injuries to Euphorbia species that are identical to surface injuries to cactus species worldwide. Moreover, the amounts on stem injuries on Euphorbia and long-lived, columnar cactus species are highly correlated with relative amounts of sunlight on stem surfaces over the annual cycle. These results suggest that these species are good bio-indicators of annual sunlight exposure.