Epidermal browning of saguaro cacti ( Carnegiea gigantea): physiological effects, rates of browning and relation to sun/shade conditions

Abstract

There has been increased morbidity of saguaro cacti ( Carnegiea gigantea) in some portions of the Sonoran Desert. Injury to individual cacti is characterized by accumulations of epicuticular waxes that obstruct stomata and lead to eventual browning of surfaces. The purpose of this research was to determine physiological effects of obstructed stomata, rates of epidermal browning, and the relationship between epidermal browning and sun/shade conditions of stem surfaces. Nocturnal acid accumulation was positively correlated with visible stomatal density and negatively correlated with characteristics of epidermal browning. Numbers of chloroplasts per cell were higher for green cacti compared with yellow or browning stems. Over a two-year period, thickness of the green chlorenchyma decreased 1.8 mm while the thickness of internal discoloration increased 2.7 mm. Rib crest scaling and barking increased markedly along with a deterioration of spines and areoles over the period. Cacti with sun-exposed south-facing surfaces had 19% greater internal discoloration than cacti at least partially shaded by vegetation. On average, sun-exposed cacti had 20% more surface injuries on crests and trought on south-facing surfaces compared with similar surfaces of shaded saguaros. Results reported herein support the hypothesis that morbidity of some saguaros is occurring at a measurable rate and is related to sun exposure.