Sugarcane for future climates: final report 2013/029
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Increasing costs of irrigation in irrigated production regions, and seasonal periods of water deficits in rain-fed production regions are impacting greatly on profitability, sustainability, and expansion of the Australian sugarcane industry. Improving crop transpiration efficiency (TE, defined as growth per unit of water used) is one strategy to help address these issues. TE measured in a wide range of sugarcane clones was found to vary about ±20% around the mean. Crop growth simulation modelling indicated a 1% variation in TE translated to about 0.5 to 1.5% variation in final cane yield in commercial production environments, highlighting the potential value of this trait. Physiological mechanisms underlying genetic variation in TE were examined. This highlighted a complication in selecting for high TE in that genetic variation in TE was found to be driven in part by genetic variation in mean stomatal conductance levels: such that genotypes with lowest conductance had higher TE on average. This means that selecting for high TE alone (and without regard to growth rates) may act to reduce overall growth rates and therefore yield. However, genetic variation in photosynthesis capacity at mean operating levels of conductance were also found, indicating that improvements in TE are possible in sugarcane without necessarily compromising growth rates. These results indicate that optimised selection indices combining measures of yield and relative canopy conductance in breeding programs could predict relative genotype performance across a range of environments with varying water limitation. This approach may have high value in early stages of selection in breeding programs. However, a current practical problem in using measurements at leaf level in commercial breeding programs was found to be high sampling error variance, and no highly repeatable parameter for predicting relative clone TE was found. This means measurements at a leaf level are too costly for screening large numbers of clones. However, rapid advances are being made worldwide in aerial image capture (using drones) and associated digital image analysis technologies. These advances mean that it may soon be possible to relatively cheaply and accurately screen clones in field trials for relative rates of canopy conductance. This technology is now being developed and assessed in SRA funded project 2016028. If the use of this technology for measuring relative canopy conductance is successfully demonstrated in SRA 2016028, then the use of optimal selection indices combining measures of canopy temperature (reflecting rates of water use) and yield, for predicting clone performance across environments with varying water limits, should be tested within commercial sugarcane breeding programs. Increasing CO2 levels increased TE but genotypes ranked similarly for TE at different CO2 levels. Selection for TE at current atmospheric CO2 levels is therefore likely to translate into sustained variety performance increases into the future, and crop simulations showed that these benefits are likely to be further amplified in those environments that become hotter and/or drier.