Non-photochemical quenching (NPQ) photoprotective processes are key for protecting photosynthetic machinery from excess light energy. Improving the regulation of NPQ in leaves during dynamic light conditions has been identified as a potential route toward increasing photosynthetic efficiency in the world’s most important crops. Prior studies have revealed intraspecific variability in other photosynthetic efficiency-related traits, but variation in photoprotective capacity within crop species, particularly those utilizing the C₄ photosynthetic pathway, is not yet well-characterized. The aim of this research project was to determine the genetic underpinnings of photoprotective traits in Sorghum bicolor and improve understanding of the balance between photosynthesis and photoprotection in dynamic light conditions.

Utilising a high-throughput chlorophyll fluorescence technique, a panel of 869 field-grown sorghum accessions was screened for multiple traits related to NPQ kinetics over two growing seasons. An ensemble approach combining genome and transcriptome-wide association studies was used to characterise the genetic architecture of NPQ in sorghum, and several high-confidence loci correlated with the observed variation were identified. These results were validated via re-screening of NPQ kinetics of selected sorghum accessions displaying contrasting genotypes at two of these loci. Sorghum’s photoprotective response was also compared and contrasted with that of three other C₄ crops and one C₄ model species, in order to contextualise photoprotection in sorghum within a broader C₄ photosynthetic phylogeny.

Subsequently, two sorghum accessions with strongly contrasting NPQ phenotypes were used to investigate the effect of photoinhibition on the coordination between the C₄ carbon concentrating mechanism and Calvin-Benson-Bassham cycle CO₂ fixation. Imbalances between these cycles during stress conditions likely represent a loss of photosynthetic efficiency in the field, potentially resulting in decreased productivity, but there presently exists minimal knowledge of the effect of photoinhibition on coordination of mesophyll/bundle-sheath cell carboxylation activity. A combined carbon isotope/leaf-level gas exchange system was developed and utilised for real-time measurement of carbon isotope discrimination during photosynthesis in high-intensity steady-state and fluctuating light conditions, to investigate the effects of high-light treatments on bundle sheath leakiness– an indicator of loss-of coordination between cell types. The results suggest that the C₄ photosynthetic apparatus is robust under both fluctuating and steady-state high light conditions and that NPQ capacity may partially explain susceptibility to photoinhibition.

This project has identified genetic loci underlying key photoprotective traits in sorghum and improved understanding of the interplay of photosynthesis and photoprotection under dynamic light conditions. The resulting knowledge of the genetic basis and physiological implications of variation in photoprotection will help guide crop improvement via both traditional breeding and biotechnology based approaches.