Hormone and carbohydrate regulation of defense secondary metabolites in a Mediterranean forest during drought

Abstract

Plants have evolved complex physiological mechanisms to regulate production of defense secondary metabolites (SMs) to cope with environmental stress such as drought. Yet, these mechanisms remain under-studied in mature trees, limiting our ability to understand effects of climate change on tree productivity and survival. We investigated defense SMs and their relations to growth, nonstructural carbohydrates and phytohormones across seasons, in a Mediterranean forest dominated by two tree species with different resistance to drought. Our results showed seasonality of leaf SMs, with a strong accumulation of SMs during cold and dry conditions in winter. The drought-resistant species Phillyrea latifolia had high levels of SMs in both leaves (ca. 10–20%) and stem phloem (4–6%), but experimental drought decreased SMs and soluble sugars in stem phloem in spring and summer. By contrast, the drought-susceptible species Quercus ilex had relatively low and constant levels of SMs, irrespective of drought. We further showed that P. latifolia leaves had generally higher levels of jasmonic acid and lower salicylic acid than Q. ilex leaves, potentially leading to differences in SMs and growth between the two co-occucrring species. Our study suggests that species- and organ-specific dynamics of defense SMs are driven by phytohormones and carbohydrates in mature trees, and that P. latifolia may become less resistant to abiotic and biotic stress due to reduced phloem defense SMs and carbohydrates in a drying climate.