Drought resistance or herbivory defense strategy? Oxalate druses function in a forage xeric species

Oxalate druse synthesis in plants helps to reduce drought stress by maintaining osmotic balance and might also act as a defence against herbivory by reducing nutritional quality. This study experimentally investigated the role of druses in Atriplex lampa under drought and herbivory treatments. We propose that both stressors trigger druse synthesis. Furthermore, if druse production is an adaptation to stress, the allocation of resources to other physiological functions should not be affected. These hypotheses were experimentally tested under greenhouse and natural field conditions. Leaves of A. lampa were collected from eight rangelands in Monte Desert in Argentina, which shared similar environmental characteristics but differed in stocking rates. The manipulative experiment in the greenhouse consisted in applying drought and herbivory treatments to A. lampa seedlings. The highest druse abundance was observed at intermediate stocking rates, suggesting resource limitation for druse synthesis at extreme stocking rates. The adaptive advantage of druse synthesis was evident only for drought stress treatment, where higher druse abundance was correlated with improved growth rates. When both stressors were combined, there was no difference in druse abundance with respect to control treatment, indicating that herbivory negatively influenced the adaptive response to drought. Druse synthesis is an adaptation to drought that is susceptible to herbivory stress.