Initial effects of UV-B radiation on stem surfaces of Stenocereus thurberi (organ pipe cacti)

Most studies on the botanical effects of UV-B radiation have been conducted over short time periods. Therefore, the consequences of long-term changes in levels of UV-B radiation to flora are poorly understood. Earlier studies with 14 species of tall, long-lived columnar cacti have shown that (1) all species are experiencing surface injuries that beg-in on equatorial-facing stem surfaces that eventually progress around stems; (2) cacti that are sun-shaded by surrounding vegetation show fewer surface injuries; and (3) these surface injuries are more prevalent now than in past times. The purpose of this study was to determine whether exposure to UV-B radiation under greenhouse exposures could induce stem surface injury symptoms similar to those of injured cacti. Daily exposures of 11 kj m(-2) of UV-B for 38 months produced a 40% decrease in the number of visible stomata of stems of Stenocereus thurberi (Engelman). This decrease in visible stomata was caused by a build-up of epicuticular waxes. Even when some stomata were visible on UV-B samples, epicuticular waxes occluded many stomata. Moreover, after 38 months of exposure to UV-B radiation, the number of epidermal cell layers increased from a normal of 1.7-1.9 to 2.3-2.6 layers, The increase in epidermal cell layers is important because scaling and barking of surfaces (surface injuries) of cacti in nature is caused by proliferation of epidermal cells. These results from controlled UV-B exposures are identical to initial microscopic events that occur to cacti under natural sunlight conditions that eventually lead to surface injuries and death of plants. These results show that UV-B radiation can alter the epidermal structure of S. thurberi and also suggest that UV-B radiation may be, at least in part, a causative agent of the surface injuries to long-lived columnar cacti in nature. (C) 2001 Elsevier Science B.V. All rights reserved.