Overcoming physiological trade-offs between flowering time and crop yield - strategies for a changing climate.

Early flowering of annual plants can lead to resource limitation owing to reduced uptake of nitrogen during the reproductive phase and declining foliar photosynthesis as a result of monocarpic senescence. Low availability of accumulated resources can therefore result in a trade-off between early flowering and reproductive fitness. However, green inflorescence organs (such as siliques, pods, bracts or awns) can make considerable contributions to photosynthetic carbon gain, and in some cases provide more carbon to seed formation than the leaves. Inflorescence photosynthesis may thereby overcome the flowering time trade-off. In addition to providing photosynthates, inflorescence organs can contribute to seed nitrogen through senescence-dependent nitrogen recycling. In annual crops, breeding has resulted in increased carbon allocation to the grain and higher harvest index, but in some cases, this had led to reduced grain protein content. We discuss different breeding targets to address carbon and nitrogen limitation, dependent on the climatic environment. For environments that are prone to drought, we propose a combination of early flowering with enhanced inflorescence photosynthesis, or, alternatively, delayed senescence (stay-green) associated with improved water balance. For optimized yield and grain protein content under favourable conditions, enhanced sink strength and extended nitrogen uptake are suggested as breeding targets.