Long- and medium-term effects of aridity on the chemical defence of a widespread Brassicaceae in the Mediterranean

• Chemical defence was studied in plant populations from a natural aridity gradient.
• Constitutive glucosinolate concentrations decreased with increasing site aridity.
• Less enemies or limited resources may explain the reduced defence in the desert.
• A ten-year rainfall manipulation caused no rapid evolution of defence profiles.

Aridity does not only vary along different regions but the frequency of drought is also supposed to increase in the future as a consequence of climate change. Plants may respond to changes in abiotic conditions by adjusting their investment in resources such as chemical defences against herbivores and pathogens. However, knowledge is scarce about how such investments may differ in response to long- or medium-term changes in precipitation. Biscutella didyma (Brassicaceae) grows in Israel along a steep precipitation gradient. Four populations along this gradient (arid, semi-arid, Mediterranean, and mesic Mediterranean climate) served as source populations for the current study. In two of these populations, rainfall had been manipulated for ten years, simulating dry, normal, and wet conditions. Seeds from the four populations and the rainfall manipulation treatments were collected and grown under standardised conditions to investigate whether shifts in constitutive defences in relation to the aridity conditions have occurred over long- or medium-term. Young leaves of 10-week old plants were harvested and analysed for their glucosinolate contents, the characteristic defence metabolites of Brassicaceae. Plants originating from the arid region had the lowest glucosinolate concentrations, whereas plants originating from the opposite end of the gradient had the highest, indicating local adaptation in response to long-term conditions. This pattern may be explained by lower or modified enemy pressure, or by limited resource availability in the arid region. The rainfall manipulation over ten years did not affect glucosinolate profiles of the offspring. This may indicate limited potential for rapid evolutionary response but also other factors than drought as the selective agent.