Acclimation of Growth

An integral part of a plant’s acclimation to water deficit caused by drought or high salt—and essentially an avoidance strategy—is the reduction of leaf area relative to biomass. In this way the loss of water via transpiration is reduced and the water status of the plant is improved. Indirectly this strategy also lowers the risk of overheating that accompanies the closing of stomata. Unfortunately for a plant, this watersaving strategy comes with the cost of potentially lower reproductive success because fewer resources can be accumulated during the vegetative stage to produce viable seeds.

The other problem is the risk of being outgrown by fastergrowing competitors. It therefore represents one of the fundamental challenges for plants to find the right balance between investment in stress tolerance on the one hand and growth on the other hand (Fig. 6.15).

Fig. 6.15. The trade-off between stress tolerance and maintained growth. In response to water limitation, stress avoidance and tolerance mechanisms are activated to ensure survival in case the stress is prolonged or becomes more severe, resulting in growth limitation and a potential competitive disadvantage. On the other hand, acclimative mechanisms exist that allow continued growth in situation where the stress is less severe. (Modified from Claeys and Inze (2013))

Accordingly, a wide natural variation exists between species, and even within species, with respect to the thresholds of water supply that trigger a strong reduction or even halt of leaf growth. Depending on the extent of the stress, different strategies can be successful. Continued growth can be beneficial in comparatively mild water limitation scenarios but detrimental when a plant is exposed to a longer drought.

A second acclimation of growth under conditions of water scarcity is the stimulation of root growth in order to improve the water uptake capacity and to access additional water resources.

This strategy is viable as long as there is soil water available. Under more severe drought, root growth also becomes inhibited. The plasticity of roots in response to drought goes beyond the size. The root system architecture—that is, the combination of the primary root length, lateral root formation, and root hair density and length, as well as root diameters—is highly flexible and can be adjusted in response to environmental fluctuations (Chaps. 2 and 7).

It is one of the major principles of plant stress tolerance that growth reduction under unfavourable conditions is not simply a consequence of disturbance or damage but, rather, an active modulation of resource utilisation (Chap. 2). The slowing of growth is a result of balanced hormonal control. Molecular understanding of underlying mechanisms has developed rapidly and is summarised in this chapter with regard to drought.