ARC Centre of Excellence for Translational Photosynthesis and Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia
C4 crops such as sugarcane, sorghum and maize are important food, feed and biofuel crops. They are mostly grown in water limited areas prone to drought and high temperature. Consequently, breeding for genotypes with high transpiration efficiency (TE) is highly desirable. Two main sources of obstacles stand in the way of realising this objective. Firstly, the traits underlying high TE in C4 plants are not well dissected. In addition, selecting for high TE may inadvertently result in smaller genotypes defeating the main goal of achieving high crop yield. Secondly, physiological based screening is little explored in conventional breeding programs because physiological traits are complex, not clearly linked to underlying genetic markers, and time-consuming to measure while effective screening tools are still lacking. This study presents an overview of the photosynthetic characteristics that determine high leaf-level TE in C4 plants. Using this analysis, significant genetic variation in transpiration efficiency caused by differences in both photosynthesis capacity and stomatal responses was observed among diverse C4 grasses as well as a collection of sugarcane and sorghum genotypes from the main breeding programs in Australia and China. Approaches that can eventually serve as high throughput tools for assessing leaf level TE in C4 plants are discussed.