Submitted to: Compact Fruit Tree
Publication Type: Abstract Only
Publication Acceptance Date: 3/21/2013
Publication Date: 5/1/2013
Citation: Glenn, D.M., Kim, Y. 2013. Stress detection in plants. Compact Fruit Tree. p. 54.
Technical Abstract: How can the status of plant stress be measured rapidly and accurately in the hundreds of trees managed within a commercial orchard? Two technologies have been developed over the past two decades that will provide useful information to detect plant stress in orchard systems: 1) Reflectance of visible and near-infrared radiation from plant canopies, and 2) measurement of canopy temperature. The reflectance red light (640-700 nm) and near-infrared radiation (700-800 nm) provide clues to photosynthetic performance. The chlorophyll molecule absorbs red light from 640 to 680 nm but when the photosynthetic process is impaired by water or nutrient stress or damage from disease or insects, the reflectance of red light increases because less red light is absorbed by the chlorophyll molecule. This response has been used to develop many stress indices used in remote sensing. One of the most common is the Normalized Difference Vegetation Index (NDVI). NDVI is commonly used in agronomic crops to detect nutrient stress and in turfgrass to detect nutrient and water stress. NDVI = (NIR- RED) / (NIR+RED) where NIR is the reflectance in the near-infrared range and RED is reflectance in the red color band of visible light. Measurement of plant temperature provides information on the ability of the plant to meet the environmental demand for water, a process termed transpirational cooling. Plant canopies can be as much as 12 degrees F below air temperature depending on the wind, temperature and relative humidity. The reduction in canopy temperature from air temperature has been established for the major agronomic crops based on the temperature, relative humidity and incoming light. Similar information can be developed for tree fruit crops to evaluate whether the trees are receiving adequate moisture, either from the soil reserves or irrigation. In our studies, we used a Trimble’s GreenSeeker NDVI sensor to scan the canopies of apple trees grown with a range of soil moisture deficits that ranged from fully irrigated to 45 percent of full irrigation. Whole tree photosynthesis was measured daily and correlated with NDVI. NDVI values alone did not predict the plant photosynthetic rate. Expressing the photosynthetic rate as a percentage of the maximum was related to NDVI values but varied with the sampling date. However, the NDVI sample date variation diminished as the water stress level diminished. This result provided a high level of confidence that trees with NDVI reflectance values greater than 0.8 had minimal to no stress. In orchard production, this is useful to the grower since any stress can reduce fruit size and quality, and the NDVI sensor is a rapid means to evaluate stress. In field studies, the NDVI sensor and infrared imaging of canopy temperature were able to discriminate as high as NDVI equals 0.23 and 3.9 degrees C between irrigated and non-irrigated trees. Our study also provided guidelines for the use of a spectral sensor to compensate for surface wetness and solar radiation changes. In summary, NDVI and infrared technologies offer promise in the rapid and early detection of plant stress in orchard systems.