Closing the global ozone yield gap: Quantification and cobenefits for multistress tolerance

Fine roots acquire essential soil resources and mediate biogeochemical cycling in terrestrial ecosystems. Estimates of carbon and nutrient allocation to build and maintain these structures remain uncertain because of the challenges of consistently measuring and interpreting fine-root systems. Traditionally, fine roots have been defined as all roots ≤ 2 mm in diameter, yet it is now recognized that this approach fails to capture the diversity of form and function observed among fine-root orders. Here, we demonstrate how order-based and functional classification frameworks improve our understanding of dynamic root processes in ecosystems dominated by perennial plants. In these frameworks, fine roots are either separated into individual root orders or functionally defined into a shorter-lived absorptive pool and a longer-lived transport fine-root pool. Using these frameworks, we estimate that fine-root production and turnover represent 22% of terrestrial net primary production globally - a c. 30% reduction from previous estimates assuming a single fine-root pool. Future work developing tools to rapidly differentiate functional fine-root classes, explicit incorporation of mycorrhizal fungi into fine-root studies, and wider adoption of a two-pool approach to model fine roots provide opportunities to better understand below-ground processes in the terrestrial biosphere.

Recent reports show ethylene-dependent reductions in stomatal sensitivity to abscisic acid (ABA) under ozone stress. These changes reduce stomatal control of plant water loss in drying soil. Here we review evidence that ABA and ethylene, and interactions between these two stress-induced hormones, control many of the responses of intact plants to drought and ozone stress, with emphasis on effects on stomata and shoot growth. We draw attention to convergent signalling and response pathways induced by ozone and drought that can increase production of hydrogen peroxide (H(2)O(2)) and nitric oxide (NO). Stomatal responses to a wider range of stresses and developmental cues may also be controlled via the same sets of signalling pathways. Other hormones, or effectors such as xylem/apoplastic pH or changes in plant water status, also play a role in signalling within and between organs. We discuss the implications, for crops, natural ecosystems and water catchment processes, of ethylene's antagonism of the stomatal response to ABA, against a back-drop of predictions for reduced precipitation and increasing ozone pollution, as part of global climate change and increasing urbanization and industrial development.