Simultaneous Monitoring of Leaf Growth and Leaf Movement

Leaves experience predictable 24-h (diel) fluctuations in environmental factors such as light and temperature, as well as in energy availability. Accordingly, leaf growth is among the many processes regulated by the circadian clock. Despite the importance of leaf growth to the plant, it is relatively poorly understood, in large part due to the difficulty of tracking growth in leaves compared with the more simple elongation of hypocotyls and roots, for example. The shape and organization of dicot leaves makes them particularly problematic to measure over time (reviewed in Ruts et al., 2012). In addition, leaf position changes throughout the 24-h cycle, a factor that complicates measurements and is itself a growth response. Now, Dornbusch et al. (2014) report the use of laser scanning to analyze daily patterns of growth and movement in individual leaves of Arabidopsis thaliana.

Dornbusch et al. developed an algorithm to track specific points on individual rosette leaves over time in laser scanning images (see figure). They monitored leaf length to report elongation growth and leaf elevation angle to determine hyponastic movement. Importantly, the authors were able to measure these traits simultaneously but analyze them separately. This allowed them to see that leaf growth peaked in the morning, and leaf elevation angle peaked in the evening. To address leaf movement, they considered the rate of change of leaf position, finding that maximal leaf movement occurred approximately three hours after maximal leaf elongation. They also discovered that growth in the leaf blade makes an important, and early, contribution to hyponastic movement.

A laser scanning method to track leaf growth and movement of individual leaves. Point clouds from scanned images of Arabidopsis rosettes (left) are monitored for changes over time in the positions of specific points. P₀, position of meristem; P_P, position of petiole-blade junction; P_T, position of leaf tip; l and Φ, length and elevation angle of the whole leaf (l_tip, Φ_tip), petiole (l_pet, Φ_pet), and blade (l_bl, Φ_bl). (Reprinted from Dornbusch et al. [2014], Figure 1A.)

Dornbusch et al. subjected the plants to different light regimes in order to test the contributions of light quality and daylength to diel growth and movement. The results provided evidence that light-driven metabolism underlies leaf growth at dawn. This finding is in stark contrast to previous results for hypocotyls, in which light at dawn inhibits growth. Furthermore, mutant analysis revealed that the mechanisms linking the circadian clock to rhythmic growth are different in leaves than in hypocotyls.

Dornbusch et al. thus provide information on leaf growth and movement at a time-scale resolution that allows them to address important biological questions regarding the relationships of leaf growth with leaf movement, light, and the circadian clock. Their results emphasize the importance of addressing leaf growth directly. Although hypocotyl growth has been a useful model for studying rhythmicity of growth in plants, Dornbusch et al. have revealed fundamental differences between hypocotyl growth and leaf growth.