Abstract
This chapter describes the role of nutrients in regulating plant processes underlying yield formation. The yield of crop plants is controlled by biomass production and its partitioning to harvested plant organs. Biomass production is dependent on photo-synthetic activity of leaves, i.e. source activity, and leaf area, i.e. source size. Nutrients are directly required for leaf growth and as integral constituents of the photosynthetic apparatus. Nutrient supply indirectly controls photosynthesis and leaf senescence via photooxidation, hydraulic and hormonal signals as well as by sugar signalling. Nutrients also affect respiration as constituents of the respiratory electron chain and by their influence on the efficiency of respiratory ATP synthesis. The chapter further describes how photosynthate partitioning to harvested plant organs is controlled by the ability of these organs to utilize assimilates for growth and storage, i.e. their sink strength and how this is influenced by nutrient supply. Nutrients play an important role in regulating sink formation, for example by their effects on flowering, pollination, and tuber initiation, as well as in controlling storage processes in the sink organs. Nutrient supply also modifies endogenous concentrations of phytohormones, which, in turn, regulate sink-source relationships. In higher plants source and sink organs are physically separated from one another. Therefore, long-distance transport of photosynthates and nutrients in the phloem from source to sink is essential for growth and plant yield. The principles of phloem loading of assimilates at source sites, phloem transport and phloem unloading at the sink sites are also described.