Infrared imaging for monitoring plant responses to water stress under different irrigation regimes

In the framework of this study greenhouse and field experiments were established at three locations comprising Germany (greenhouse), China (field), and Mexico (field) and three different crops which were grapevine, winter wheat and maize. Water status detection for advanced irrigation scheduling is frequently done by predawn leaf water potential or leaf stomatal conductance measurements. However, both methods are labor intensive and time consuming as leaf to leaf variations require much replication for reliable data and are not yet possible to be automated. The basic principle of using infrared thermography as an indicator of water stress is that plants close stomata under water stress, which results in decrease of energy dissipation and increase of canopy temperature. Infrared thermography visualizes this increase in leaf temperature and the magnitude of the rise in canopy temperature among water stressed plants is influenced by successive stress imposition. Therefore, thermography is a non-contact method and thus very fast and practical. It is capable to estimate large leave populations simultaneously and provides an overview on stomatal conductance variation and dynamics. The results of the study show that although a linear correlation existed between stomatal conductance and CWSI in maize under greenhouse conditions, in winter wheat it depends on the phenological stage of the crop. For example, a correlation of r2 = 0.44 at pre-heading and r2 = 0.77 at post-heading stage was found. This is due to the change in the baselines of some crops including wheat as the plant height varies from vegetative to reproductive growth stage. A higher canopy diffusion resistance prevails in the post-heading stage than in the pre-heading stage, resulting in cooler canopy. This indicates the importance of taking crop growth parameters into consideration like plant height, which effects canopy reflectance and other energy balance components. However, the results show that in maize, the blister stage represents the most suitable time after pre-anthesis to acquire thermal images in order to indicate different levels of stress in maize varieties. Moreover, yield data were negatively correlated (p < 0.001) with canopy temperature during anthesis and blister stage in maize and at pre-heading and post heading in winter wheat. In conclusion, independent of the locations and crops, results of the study showed that there is a considerable potential for using thermography as a new water status monitoring and irrigation scheduling tool as well as tool for rapid phenotyping.