Mineral nutrition, yield, and source-sink relationships

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 the capture of light energy, through the photosynthetic activity of leaves (i.e., source activity) and leaf area, to provide carbon and energy for the entire plant. Roots supply plants with water and nutrients from the soil. Nutrients are required for leaf growth and as integral constituents of the photosynthetic apparatus. Nutrient supply also affects photosynthesis and leaf senescence indirectly via photooxidation, hydraulic and phytohormonal signals as well as by sugar signaling. Nutrients impact respiration as constituents of the respiratory electron chain and by their influence on the efficiency of respiratory ATP synthesis. The chapter describes how photosynthate partitioning to plant organs is controlled by the capacity of these organs to utilize assimilates for growth and storage, that is, 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 plant architecture, flowering, pollination, and tuber initiation, as well as in controlling storage processes in the sink organs. Nutrient supply also modifies endogenous concentrations of phytohormones that regulate sink source relationships. The source and sink organs are physically separated. 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 described