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

Abstract

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.

Keywords: Crop yield; flowering time; harvest index; inflorescence photosynthesis; nitrogen-use efficiency; phenology; resource allocation; senescence; source–sink; stay-green.

Fig. 1.

The role of inflorescence photosynthesis and senescence-dependent nutrient recycling. In cereals (left), awns and bracts (glume, palea, and lemma) play an important role in the provision of photosynthates to the grain, by uptake of external CO₂ and refixation of respired CO₂ by the bracts. In oilseeds (right), silique wall photosynthesis provides carbon for grain filling. Senescence-dependent recycling of nitrogen from the leaves is important for the formation of the photosynthetic apparatus in the inflorescence and can later be recycled from the inflorescence for grain protein synthesis. Created in BioRender. Wingler, A. (2025) https://BioRender.com/h83u256.

Fig. 2.

Proposed breeding targets for improved grain yield and quality under stress and favourable conditions. The high harvest index of modern cultivars already supports yields but can result in nitrogen limitation. Under stress conditions (left, brown), early flowering can be beneficial (stress escape). Senescence is important for nitrogen recycling, especially under low-nitrogen conditions. Enhanced inflorescence photosynthesis can compensate for the shorter vegetative photosynthetic period resulting from earlier leaf senescence. Alternatively, stay-green of leaves can improve yield under source limitation if it is associated with improved water balance. Under favourable conditions (right, green), high leaf photosynthetic activity and late senescence support high source strength but can result in sink limitation. Enhanced grain sink strength and extended post-anthesis nitrogen uptake are therefore proposed as the main breeding targets not just to support grain yield but also to overcome the trade-off between high harvest index and grain protein content.