Nitrogen Management to Increase Cereal Resilience to Heat.

In order to maintain the present balance with food demand, crop yields must increase substantially within the next 2-3 decades in a context in which projections suggest that higher temperatures, and more frequent events of heat shocks, will become commonplace in most regions where grain crops are produced. Deleterious effects of heat on crop yields are well documented and the occurrence of heat stresses will likely be a major constraint to achieving increased yields of major crops. Thus, agronomic and genetic strategies for increased resilience to high temperatures will be necessary. Much of the work done on this area has been focused to identify genetic sources of increased resilience and much less has been done on the crop ecology side. Nitrogen (N) fertilization is within the most common management practices used in cereal production, and frequently crops are heavily fertilized to maximize productivity. However, there have been limited efforts to elucidate to what degree the level of soil fertility may affect the magnitude of the high temperature effect on crop yield. Analyzing the likely interaction may be relevant for designing more appropriate fertilization strategies. We conducted a total of 10 different studies firstly on barley (early 2000's) then on maize (2009-2012) and more recently on wheat (2013-2016), always under field conditions, to determine whether the availability of N may be responsible for the magnitude of the yield penalty imposed by heat stress during reproductive phases (i.e. when heat waves are more likely). In general, we exposed in each experiment the crop to the factorial combination of at least two contrasting temperature treatments (unheated control; heat treatment increasing maximum temperatures during several days) and at least two contrasting N fertilization regimes (low and high soil N availabilities), and in most of the experiments we also imposed source-sink manipulations to ascertain likely causes of some responses. In general, we found that, as expected, yield and grain weight were reduced by heat stress. However, the magnitude of the penalty imposed by the heat was dependent on the N fertilization, so that the high-N crops were more sensitive to heat than the crop grown under relatively low N availabilities. Thus, we show for the first time (as far as we are aware) a generalized impact of N availability on the magnitude of the penalty imposed by heat. This implies that fertilization recommendations may need to be revised, particularly in locations in which heat waves are more likely, to consider balancing the yielding benefits of high N availability and the detrimental effect such fertilization scheme might have in the event of a heat stress.