Fig. 4.

Scheme showing similar whole-plant effects of water and nutrient deficits. Decreased resource availability generally increases root-to-shoot ABA export and/or decreases cytokinin export. Furthermore, shoot ABA accumulation can decrease shoot cytokinin concentration by activating cytokinin oxidase. These changes in shoot hormone content inhibit leaf growth (more rapidly and to a greater extent than photosynthesis), causing foliar sugar accumulation and its transport to roots (probably controlled by cytokinin decline) due to decreased demand of sink leaves. Sugars act as both substrates for root growth and a signal controlling auxin transport from shoots to roots and its distribution in the roots.