Epidermal browning of saguaro cacti ( Carnegiea gigantea): Surface and internal characteristics associated with browning

Abstract

In some locations of Saguaro National Monument near Tucson, Arizona, there has been a decline in saguaro cacti ( Carnegiea gigantea (Engelm.) Britt. and Rose). Injury to individual cacti is frequently characterized by epidermal browning and premature senescence. On green cactus surfaces, most epicuticular waxes flake off and stomata are not obscurred. As cactus surfaces become more yellow in color (less green), epicuticular waxes no longer flake off and the build up or sheeting of waxes obscures stomata. This sheeting eventually leads to visible scaling and barking. Epicuticular wax sheeting, scaling and barking of troughs were negatively correlated with depth of green chlorenchyma but were positively correlated with depth of discoloration of internal parenchyma. These results suggest that the first step in epidermal browning is an excessive accumulation of epicuticular waxes. Sheeting of epicuticular waxes results in scaling and barking of crests followed by these same events on troughs. Areoles deteriorate as crest scaling and barking occur. These events are well-correlated with trunk color and the depths of both the green chlorenchyma and discoloration of the internal parenchyma.