Leaves grown in low N tend to be stunted, mainly because they don't divide and elongate. In contrast to water stress, where ABA was an importnat hormone, another class of hormones called cytokinins are involved in nutrient stress responses. Check out table 5, p. 329, which shows experimental data for Plantago. When nutrients are high, cytokinins are abundant in the shoots and roots and RGR is high. In nutrient dilute solutions, cytokinin levels dropped, inhibiting leaf growth and promoting root growth.
Less cytokinins travel to the leaf, where cell division and expansion are reduced.
Photosynthate allocation to leaves and stems therefore stops. Photosynthate is then allocated to the roots where root division and elongation are stimulated by the lack of cytokinins. This allows rots to grow and to take up more nutrients to alleviate the stress. When we talked about sun and shade leaves, we saw that sun leaves had higher protein contents than shade leaves. N concentartion from high in the canopy top to low in the shaded lower leaves.