The IWT project TIPRELET – TIPburn PREvention of LEttuce Types – aims to unravel the mystery of tipburn in lettuce in order to find rational measures to prevent tipburn development.
Tipburn is generally considered to be a calcium related disorder. Calcium plays a crucial role in membrane stability and cell wall strength and, as such, a lack of calcium in expanding leaf zones can result in weakened cell walls. These latter can then easily break due to abrupt changes in leaf turgor pressure, resulting in the typical tipburn symptoms.
Low transpiring plant organs are dependent on root pressure for their supply of calcium. However, because root pressure can only be built up when transpiration is nearly absent, it could be expected that increasing air humidity during the night should have beneficial effects.
In a recent experiment, butterhead lettuce (cultivar Gardia) was grown in two greenhouse compartments of Inagro, in one of which nighttime relative humidity was increased by misting. In this compartment less tipburn was found compared to the compartment in which nighttime relative humidity was not increased (Table 1). In order to investigate the specific role of root pressure, a new technique was developed in which measurements of leaf thickness are compared with predictions of it using a mechanistic model. This model predicts diurnal variations in leaf thickness based on variations in transpiration and a concept of growth in relation to turgor. The figure below illustrates the use of this technique during a two-day period: when measurements of leaf thickness exceed the predictions during the night (24h – 30h), this difference could be attributed to root pressure. If this difference is not present, the diurnal variations in leaf thickness are entirely explained by the transpiration and growth concepts present in the model and root pressure is expected to be not present. The difference between measured and predicted leaf thickness could mathematically be translated into an absolute value for root pressure. This new method will now be evaluated in a new experiment in which nighttime relative humidity will be manipulated in order to relate the occurrence of tipburn to the occurrence of root pressure.
Figure 1: Theoretical example of how measured leaf thickness deviates from predicted leaf thickness when root pressure is being built up.
Table 1: Amount of plants in which tipburn incidence was absent, small, medium or large for two treatments on a total of 30 plants per treatment.
|
absent |
small |
medium |
large |
High RH |
16 |
2 |
11 |
1 |
Low RH |
8 |
3 |
13 |
6 |