Ontogenetic Changes in Azoxyglycoside Levels in the Leaves of Dioon edule Lindl.

Abstract

Plants have multiple strategies, including phytochemicals that protect their vulnerable tissues against pathogens and herbivores. Dioon edule, like all cycads, possess unique azoxy-type compounds, azoxyglycosides (AZGs) as a chemical defense; however, the ontogenetic variability of these compounds in this long-lived cycad is unknown. Here, we investigated the effects of plant age, sex, genotype and individual heterozygosity on AZG levels in mature leaves of wild D. edule populations from eastern Mexico. Individuals were divided into three ontogenetic stages: seedlings, juveniles and adults. We established overall leaf quality by quantifying pigments associated with photosynthesis; chlorophylla, chlorophyllb and lutein. Leaf chlorophylla levels were higher in seedlings compared to adult cycads. Plants were genotyped using 11 microsatellite markers and foliar AZG levels were quantified by HPLC. AZG levels do not correlate with plant genotype or the individual's heterozygosity. Genetic analysis identified a distinction between lowland and highland individuals; foliar AZG levels were higher in lowland adult cycads compared to highland individuals. In both populations, the highest AZG levels were found in seedlings compared to adult cycads. These young cycads are highly reliant on their few leaves since seedlings bear one or two leaves for the first years of their life and, thus, are unlikely to recover from defoliation. The results suggest that cycad leaves with a greater nutritive content and a higher value for long-term survival are better protected with higher AZG levels. Female adult cycads have higher AZG levels compared to males, suggesting that the benefits of defense couldalso be linked to reproductive costs.