EPIDERMAL BROWNING OF SAGUARO CACTI (CARNEGIEA-GIGANTEA) - SURFACE AND INTERNAL CHARACTERISTICS ASSOCIATED WITH BROWNING

In some locations of Saguaro National Monument near Tucson, Arizona, t here has been a decline in saguaro cacti (Carnegiea gigantea (Engelm.) Britt. and Rose). Injury to individual cacti is frequently characteri zed by epidermal browning and premature senescence. On green cactus su rfaces, most epicuticular waxes flake off and stomata are not obscured . As cactus surfaces become more yellow in color (less green), epicuti cular waxes no longer flake off and the build up or sheeting of waxes obscures stomata. This sheeting eventually leads to visible scaling an d barking. Epicuticular wax sheeting, scaling and barking of troughs w ere negatively correlated with depth of green chlorenchyma but were po sitively correlated with depth of discoloration of internal parenchyma . These results suggest that the first step in epidermal browning is a n excessive accumulation of epicuticular waxes. Sheeting of epicuticul ar waxes results in scaling and barking of crests followed by these sa me events an troughs. Areoles deteriorate as crest scaling and barking occur. These events are well-correlated with trunk color and the dept hs of both the green chlorenchyma and discoloration of the internal pa renchyma.