Recent evidence shows that epidermal browning of tall- stemmed, long-lived cacti is related to total time of sun exposure. Epidermal browning is much greater on the southern exposures of such cacti in the Northern Hemisphere and on northern exposures of similar species in the Southern Hemisphere. This surface browning leads to death of individual cacti. An evaluation of authentic photographs of saguaro cacti an Arizona for the past 90 years shows that epidermal browning has increased by about 4% per decade, the same percentage increase in ultraviolet (UV) radiation exposure in the United States over the past three decades. The increase in ultraviolet light exposure has been shown to be related to the depletion of the ozone layer in the stratosphere. The data base (473 cacti) of saguaros has been able to show the sequence of steps during epidermal browning. The purpose of the proposed research is to make the same series of phenological and physiological measurements on several species of cacti in Chile and Argentina, to determine if the sequence of steps of epidermal browning is similar to those of saguaros. If the sequence of steps is similar among all these cacti (both Northern and Southern hemispheres), the basis for further investigation of a possible mechanism for UV-indeed injury would be strengthened.
