Summer climate mediates UK wheat quality response to winter North Atlantic Oscillation

Previous studies have shown that a high North Atlantic Oscillation (NAO) index in winter is associated with better postharvest quality of the UK wheat crop. This prompted investigation of the possible mechanisms and two main hypotheses were proposed. The first hypothesis tested was that a high winter NAO index could, through higher winter temperatures, advance crop development to bring grain growth and ripening into a period of more favourable weather. This was examined by simulating wheat development in the UK from 1974 to 1999, using the AFRC wheat model and regressing wheat quality on development stage dates. No consistent relationship was found between the dates of development stages and the wheat quality variable specific weight, therefore this hypothesis was rejected. The second hypothesis tested was that a high winter NAO index could lead to sunnier and drier weather during grain growth and ripening, giving better grain quality. Two summer climate variables, cumulative sunshine between anthesis (pollen release) and the end of grain-filling, and unconditional probability of a wet day between the end of grain filling and harvest, were found to be important in determining specific weight. Prediction of these summer climate variables from the January NAO accounted for 70% of the variance of the relationship between the January NAO and specific weight. These results show that the memory of the January NAO-specific weight relationship is in the climate, and not in the crop.