Abstract
Prolonged drought stress combined with high leaf temperatures can induce programmed leaf senescence involving lipid peroxidation, and the loss of net carbon assimilation during early stages of tree mortality. Periodic droughts are known to induce widespread tree mortality in the Amazon rainforest, but little is known about the role of lipid peroxidation during drought-induced leaf senescence. In this study, we present observations of green leaf volatile (GLV) emissions during membrane peroxidation processes associated with the combined effects of high leaf temperatures and drought-induced leaf senescence from individual detached leaves and a rainforest ecosystem in the central Amazon. Temperature-dependent leaf emissions of volatile terpenoids were observed during the morning, and together with transpiration and net photosynthesis, showed a post-midday depression. This post-midday depression was associated with a stimulation of C5 and C6 GLV emissions, which continued to increase throughout the late afternoon in a temperature-independent fashion. During the 2010 drought in the Amazon Basin, which resulted in widespread tree mortality, green leaf volatile emissions (C6 GLVs) were observed to build up within the forest canopy atmosphere, likely associated with high leaf temperatures and enhanced drought-induced leaf senescence processes. The results suggest that observations of GLVs in the tropical boundary layer could be used as a chemical sensor of reduced ecosystem productivity associated with drought stress.
Highlights
As the single largest intact tropical forest in the world, the Amazon is a key component of the global carbon cycle
High vertically resolved atmospheric concentrations of green leaf volatile (GLV) were collected within and above a rainforest ecosystem in the central Amazon during the 2010 dry season. Throughout this period (June–September 2010), widespread drought conditions were reported for the Amazon rainforest, and together with high heat anomalies, resulted in large reductions in above-ground biomass [48]
Additional experiments at the leaf-level in 2015 at the Amazon field site verified that emissions of both C5 and C6 GLVs can be induced by high temperature and desiccation stress and are associated with the loss of net photosynthesis, transpiration, and volatile terpenoid emissions, likely due to the peroxidation of photosynthetic membranes
Summary
As the single largest intact tropical forest in the world, the Amazon is a key component of the global carbon cycle. We hypothesized that during drought-induced leaf senescence in Amazon trees, the combined effect of high temperature and leaf desiccation can overwhelm antioxidant defense systems such as volatile terpenoids These abiotic stress conditions can result in the peroxidation of photosynthetic membranes, a decrease in net carbon assimilation, and the emissions of GLVs to the tropical atmosphere. To test this hypothesis, we used proton transfer reaction-mass spectrometry (PTR-MS) to quantify high vertically resolved ambient concentrations of C6 GLVs within and above a primary rainforest canopy in the central Amazon rainforest during the widespread 2010 drought. GLVs accumulated within the rainforest canopy atmosphere, potentially providing a new chemical sensor of decreased ecosystem productivity
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