Sunlight-induced bark formation in long-lived South American columnar cacti

• Cuticle, epidermis and hypodermis of thirty-nine species of long-lived columnar cactus species were studied.
• Anticlinal cell divisions occurred in epidermal cells to produce the bark cambium.
• Many species exhibited damage to hypodermal cells during bark formation.

Previous research has shown that sunlight-induced bark formation (epidermal browning) on vertical surfaces of tall, long-lived columnar cactus species leads to premature death of entire plants. The bark cambium that produces bark for cactus species has been shown to involve proliferation of epidermal cells only. The purpose of this research was to understand (1) the characteristics of cuticles, epidermal and hypodermal cells, (2) the process of sunlight-induced bark formation, and (3) changes in hypodermal cells during the bark formation process for many species of long-lived, columnar cacti from South America. A wide diversity of epidermal and hypodermal cells occurred among the species studied. Epidermal depths ranged from 8.7 to 49.7 μm and involved one to several epidermal cells while the thickness of hypodermal cell layers ranged from 42.7 to 661 μm among the species studied. During bark cambium formation, each epidermal cell underwent at least one anticlinal cell division prior to numerous periclinal cell divisions since lengths of initially produced bark cambium cells were half the lengths of uninjured epidermal cells. Some cactus species exhibited hypodermal cell injuries during bark formation. This study reports: (1) quantitative measurements of cuticles, epidermal and hypodermal layers, (2) the presence of anticlinal cell divisions of epidermal cells to produce the bark cambium, and (3) the destruction of hypodermal cells during bark formation for columnar cactus species of South America. Possibly these results are specific for sunlight-induced bark formation for cactus species while other types of injuries may result in bark production by other means.