In plant phenotyping, direct measurements describe physical properties of the measured object, such as dimensions, morphology, or reflectance in certain wavelength ranges. In short, these are the information that derives from operating non-invasive sensors as directly measured parameters. In a next step, one can calculate indirect information out of such parameters. Indirect measurements address growth and biomass accumulation together with physiological traits that indicate stress and disease responses, water and nutrition relations, or photosynthetic capacity. For indirect measures, mathematical models are necessary that transform the measured data into the desired information. They are frequently sample specific and require calibration experiments to establish model calcualtions.
Non-invasive measurements with cameras and scanners capture a broad variety of traits in above-ground plant parts such as shoots, leaves, flowers, seeds or fruits. Different sensor types address the electromagnetic spectrum comprising the visual light and beyond and thereby record a broad range of properties. These can be used to describe growth processes, developmental stages, or physiological responses. Usually, aboveground parts of the plants can be accessed without interfering too much with the plants, particularly when using sensor-to-plant platforms.
Non-invasive measuring organs that usually grow below ground, e.g. roots or tubers, is more challenging than measuring the above-ground parts. To makes these visible, specific experimental setups are used that enable exposing such organs to cameras, e.g. the use of transparent pots or media on plates. However, all such growing situations are quite artificial and the measured properties are at least in part result of the plants responding to the experimental environment.
So far, phenotyping of below-ground organs is mainly conducted with RGB-cameras. However, other types of cameras can also deliver meaningful data, particularly when exploring stress responses.
Germination tests are central tools in seed testing, providing data on germination percentages and germination speed. Moreover, non-invasive characterisation of seeds and their germination enables categorising seeds before sowing or after harvest. Measures of physiological properties provide insights into the quality of the seeds and add value to applied seed testing processes.
Phenotypic data relating to non-plant organisms is of great interest because many of them are plant-associated and can be beneficial or pathogenic to crop plants and might also take influence on the quality of the harversted yield. Thus, many of them could endanger food production or plant-derived materials such as wood.
Besides measuring dimensions and morphology, other traits such as movement, feeding or physiological characteristics can be measured. Such data is used for analysing the interaction of organisms with plants, and for developing pesticides and other protective agents.