Proceedings of the 1 st Australian Summer Grains Conference, Gold Coast, Australia, 21 st – 24 th June 2010. This paper has been submitted as a Summary Paper and has not been reviewed or edited by the Conference Committee. Plant nutrition and nutritional value in a changing climate Roslyn Gleadow, Tim Cavagnaro, Cecilia Blomstedt, Alan Neale and John Hamill School of Biological Sciences, Monash University, 3800 Vic. Presenting author e-mail: ros.gleadow@sci.monash.edu.au Introduction. Producing enough food to meet the needs of an increasing global population is one of the greatest challenges we currently face. The world needs to produce twice the amount of food using half the resources by 2050 in order to meet demand for food for a growing population and the shift towards a higher meat diet in many parts of the world. The issue of food security is further complicated by impacts of elevated CO 2 , global warming and climate change. The problem is compounded by urban encroachment onto arable land and the increased costs (and potential shortages) of fertilisers. Plants are affected by rising CO 2 indirectly, through climate change and directly through the process of photosynthesis. While the climate adaptation debate has largely focused on yields, the nutritional quality of food is also fundamental (see Gleadow 2010). Future increases in yield must not be achieved at the expense of the nutritive value of food. Here we consider the impacts of global change on selected aspects of plant nutrition, nutrient acquisition and the nutritional quality of crop plants. CO2 fertilization effect. Inherent inefficiencies in photosynthesis are overcome when plants are grown at higher atmospheric CO 2 , promoting growth. Models of future crop yields take this into account (Lobell and Field 2008). Plants convert carbon dioxide to high energy compounds such are starch using sunlight and water. The enzyme that catalyses this process (Rubisco, Ribulose bisphosphate carboxylase/oxygenase) is the most abundant protein on earth, making up to 50% of leaf protein and about 25% of leaf nitrogen. Rubisco also catalyses another reaction that uses oxygen in a wasteful process called photorespiration, which can consume up to 20% of the energy trapped from the sun. As the concentration of CO 2 in the atmosphere increases, the CO 2 -fixing function of Rubisco is favoured over the oxygenase function. This is called C3 photosynthesis because the first product contains three carbons. Some plants overcome the problem of photorespiration by increasing the concentration of CO 2 in the vicinity of Rubisco, inserting a preliminary step in the carbon fixation process. These plants are called C4 plants because the first product is a four carbon compound, usually malate, although they still have the other C3 process as well. Fast growing summer grains such as maize and sorghum are C4 plants. Sunflowers, mung beans and soybeans are C3 plants. With rising CO 2 , C3 plants become more efficient while the gain in efficiency for C4 plants is much smaller. Yields: Experimental studies suggest that the CO 2 fertilization effect will be significantly less than is predicted by the biochemistry. In Free Air CO 2 experiments (FACE), crops are planted in the ground and then CO 2 is blown across them in a precisely controlled manner, simulating a field situation. These experiments show a modest increase in yield in plants grown in CO 2 concentrations equivalent to the year 2050, but there is a lot of variation between plant functional groups and with the supply of water and fertilizers (Ainsworth & Long 2005). C3 plants with access to plenty of fertliser show the greatest gains. C4 crops such as sorghum do not show any increase in yield under well-watered conditions. Under drought conditions, C4 plants do have higher yields. The reason for this is that CO 2 affects the stomata directly resulting in less water loss and improved water use efficiency, and is independent of the effect on Rubisco (Wall et al. 2001). We have found a huge P effect on productivity of clover grown at elevated CO 2 , but at this stage we really don’t have an explanation (unpublished data). Leaf and grain protein. Leaves of plants grown experimentally at elevated CO 2 consistently have lower levels of total N and protein. Originally this was thought to be a dilution effect from the accumulation of starch but it is now clear that less Rubisco is synthesized and that the genes controlling its production are downregulated (Gleadow et al. 1998). Very recent research indicates that the ability of plants to take up nitrate (as opposed to ammonia) may also be curtailed in another, possibly independent, response (Bloom et al. 2010). Less leaf protein translates into less grain protein. Lower leaf and grain protein has enormous implications for food security and animal nutrition generally but, to date, this is an under-appreciated side effect of rising CO 2 . A comprehensive survey of published results showed that plants grown at ‘high N’ (it s not clear what they meant by this) on average had a 10% less protein, while plants grown with limiting N