Phenotyping Genetic Diversity in Acceleration of Maize Leaf Senescence under Elevated Ozone

Exposure to elevated ozone concentrations ([O₃]) accelerates the rate of leaf senescence, decreasing the functional lifespan of leaves and reducing plant productivity. The natural molecular and biochemical components of senescence are accelerated at elevated [O₃], including up-regulation of senescence associated genes, down-regulation of photosynthetic genes, chlorophyll degradation, and loss of Rubisco protein and activity. Variation in O₃-induced senescence has been correlated to yield losses in wheat and rice, but little is known about how diverse maize genotypes, and C₄ crops in general, respond to elevated [O₃].  This study investigated phenotypes associated with leaf senescence in diverse inbred and hybrid maize lines exposed to elevated [O₃] in the field at the Free Air Concentration Enrichment (FACE) facility in Champaign, IL (www.igb.illinois.edu/soyface/). Maize was grown at ambient (~40 ppb) and elevated [O₃] (100 ppb) in 2013 (200 inbreds), 2014 (52 inbreds, 26 hybrids) and 2015 (10 inbreds, 8 hybrids).  From ~10 days after anthesis until crop maturity, markers for leaf senescence including photosynthetic rate and capacity, leaf N content and chlorophyll content were assessed on the leaf subtending the ear.  In additional to these traditional methods, non-destructive, high-throughput methods using partial least squares regression analysis of leaf reflectance spectra were tested.  Most maize lines showed signs of accelerated senescence at elevated [O₃], although some were more sensitive than others. Elevated [O₃]-induced declines in photosynthetic C assimilation preceded reductions in leaf chlorophyll content. Global metabolite and transcript abundance are being evaluated to dissect the genetic and biochemical mechanisms underlying O₃-induced senescence.