Glucocapparin variability among four populations ofIsomeris arborea Nutt.

Glucocapparin (methylglucosinolate), a putative defense compound, was found to vary between desert and nondesert populations ofIsomeris arborea (Capparaceae): Plants from desert populations contained greater concentrations than nondesert plants in four of the five organs analyzed. Immature leaves at desert sites had average glucocapparin concentrations of 9.2 mg/g and 8.4 mg/g, while nondesert sites averaged 6.0 mg/g and 4.6 mg/g. Mature leaves from desert sites had average concentrations of 12.8 mg/g and 7.9 mg/g; leaves from plants at nondesert sites contained approximately one third to one half of those concentrations. A similar pattern was observed in capsule walls and seeds but not in flower buds; for these, non-desert plants contained a slightly higher concentration of glucocapparin. Our studies show that nitrogen and glucocapparin concentrations fluctuate throughout the year and contribute to the observed variability among populations during any particular season. Glucocapparin may fluctuate seasonally as much as 37% in immature leaves and 78% in mature leaves. In a controlled experiment, glucocapparin concentration varied inversely with nitrogen fertilizer treatment. The plants treated with fertilizer lacking nitrogen ranged from 10.1 mg/g to 10.9 mg/g glucapparin, which was roughly twice the concentration of those supplied with 20 mM nitrogen in the fertilizer.