Phenotyping Leaf Biochemical and Physiological Responses to Ozone in Diverse Field-Grown Maize Using Hyperspectral Leaf Reflectance

Genetic resources for exploring physiological and agronomic variation in maize are well-established, but there is a field phenotyping bottleneck that limits the rapid and accurate measurement of important traits. Ozone is a damaging air pollutant, known to accelerate leaf senescence and decrease photosynthetic capacity, which ultimately decreases yield. However, measuring biochemical and physiological traits associated with senescence and photosynthesis is slow and laborious. Here, we addressed this challenge by using hyperspectral reflectance spectroscopy (HRS) to rapidly survey leaf reflectance spectra from over 200 diverse inbred and hybrid maize lines exposed to ambient (~40 ppb) and elevated (~100 ppb) ozone at the Free Air Concentration Enrichment (FACE) field site in Champaign, IL. Partial least squares regression (PLSR) modeling was used to estimate leaf chlorophyll content, percent nitrogen, sucrose content, specific leaf area (SLA) and maximum photosynthetic capacity (V_max). PLSR models accurately predicted phenotypes, and there was significant genotypic variation for chlorophyll (15.2-31.2 µg cm^-2), nitrogen (3.2-4.3 %), sucrose (3.8-5.7 µmol cm^-2), SLA (20.6-29.6 mm² mg^-1) and V_max (18.8-41.1 µmol m^-2 s^-1) within the tested germplasm grown in ambient conditions. Exposure to elevated ozone concentrations decreased chlorophyll by 63.8%, nitrogen by 22.3%, sucrose by 17.5% and V_max by 46.8% in the most sensitive genotypes, and increased SLA by 21.2%. There was agreement among years in lines that were responsive to ozone. These results demonstrate the feasibility of using HRS to resolve genotypic variation in maize response to ozone stress, and suggest the approach could be applied to numerous biotic and abiotic stresses.