Sunlight-induced bark formation on current-year stems of Euphorbia plants from South Africa

Species of Euphorbia from South Africa have sunlight-induced bark injuries on stems visually identical to sunlight-induced injuries on more than twenty species of long-lived columnar, candelabra cactus species of the Americas. Injury ratios on stems on both groups of plants were similar to ratios of direct sunlight exposure on stem surfaces averaged throughout the annual cycle at many latitudes. Injuries on Euphorbia species occurred on current-year stem tissues. Stem tissues of Euphorbia species have a single-celled epidermis. Some species had a 2- to 3-celled hypodermis. Sunlight-induced bark formation only involved anticlinal followed by periclinal cell divisions of epidermal cells. No other cells were involved in bark formation. Many Euphorbia species have crests and troughs while others have prominent tubercles. Experimental results showed that prominent crests provided shading to concave troughs on some Euphorbia species and these species had less sunlight-induced barking. In contrast, other Euphorbia species that do not have prominent crests have little self-shading and these species had more sunlight-induced barking. Sunlight-induced bark formation occurred during a one year of sunlight exposure on Euphorbia stems while similar barking injuries occur on cactus stems only after several decades of sunlight exposure. These sunlight-induced injuries on young stem tissues of Euphorbia stems may result since current-year stems of Euphorbia plants had thinner cuticles and thinner epidermal/hypodermal layers compared with stem surfaces of cactus plants.