The multi-scale nature of leaf growth - fluctuations, timescales and adaptation

A growing leaf is a fascinating system. It increases its area by orders of magnitudes while its elements – the cells- are not controlled by a central control system. Under these conditions it is highly nontrivial that leaves succeed in growing "properly" to the desired shape (in most cases flat). In order to achieve this, growth must be regulated locally, i.e., by some effective rheology.

We try to expose the characteristics of the growth field dynamics by measuring the evolving leaf surface with high spatio-temporal resolution. We find that the field is not smooth. It consists of sharp variation in growth rate and directionality, which include extensive shrinkage events as part of normal growth. We identify dominant time and length scales as well as qualitative differences between growth during day and night.

We monitor giant variations in the growth rate due to environmental changes, such as transitions in illumination. These variations most likely result from stomatal activity, which involves overshooting and non-uniform spatial distribution. We, thus, suggest that stomatal activity is a central element in growth regulation.

In another set of experiments, we measure the effect of mechanical stress on leaf deformation and growth. The measured effective rheology is viscoelastic with time varying parameters. We provide evidences for the effect of stress on growth at different time scales. The effects include passive viscoelastic deformation at short times, growth correlated with stress during intermediated times and indications for tissue remodeling in response to extended application of mechanical stress.