Hormonal regulation of source-sink relations to maintain crop productivity under salinity: a case study of root-to-shoot signalling in tomato

Salinity decreases crop yield first by reducing growth of assimilate-consuming sink organs and, second, by decreasing assimilate production in photosynthetically active source tissues. Although much work has focussed on controlling the accumulation of toxic ions (mainly Na+ and Cl-), the search for primary growth limiting factor(s) continues. The root, by sensing environmental constraints of the soil, may influence root-to-shoot signalling to control shoot growth and physiology, and ultimately agricultural productivity. Hormonal signals, such as cytokinins, ABA, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid and the auxin indole-3-acetic acid may coordinate assimilate production and usage in competing sinks (biomass partitioning). Hormonal regulation of source-sink relations during the osmotic phase of salinity (independent of specific ions) affects whole-plant energy availability to prolong the maintenance of growth, root function and ion homeostasis, and could be critical to delay the accumulation of Na+ or any other ion to toxic levels. This viewpoint emphasises that simultaneously maintaining growth and delaying early leaf senescence is necessary to increase crop yield in salt-affected soils.