Linking leaf initiation to the aerial environment: when air temperature is not the whole story

The initiation of new leaves, which takes place at the shoot apical meristem, is essential for plant growth and development. Leaf initiation rate (LIR) is very sensitive to meristem temperature. However, in practice meristem temperature is hardly ever monitored and air temperature is often used instead. It can be questioned whether relating LIR solely to air temperature is valid. This thesis aims at linking LIR to the aerial environment in two main horticultural crops: tomato and cucumber. It was shown that meristem temperature often differs from air temperature, depending on other environmental factors (e.g. radiation, humidity and wind speed) and species-specific traits. LIR was solely influenced by meristem temperature even when it largely deviated from air temperature. In addition, LIR was reduced at low light levels.  Consequently, air temperature is not the whole story when relating leaf initiation to the environment.