Comparison of proximal remote sensing devices of vegetable crops to determine the role of grafting in plant resistance to Meloidogyne incognita

Proximal remote sensing devices are novel tools that enable the study of plants health status through specific characteristics, including the color or the spectrum of light reflected or transmitted by the leaves or the canopy. Among these, RGB images can provide detailed information about crop status including estimates of biomass, chlorophyll (and chlorosis) and fractional vegetation cover. The aim of this study is to compare the RGB data collected during five years (2016-2020) of four fruiting vegetables (melon, tomato, eggplant and peppers), non-grafted and grafted onto resistant rootstocks cultivated in Meloidogyne incognita infested soil in a greenhouse. The proximal remote sensing plant health status data collected were divided into three levels. Firstly, leaf pigments were measured using two different handheld sensors (SPAD and Dualex). Secondly, canopy vigor and biomass were assessed using vegetation indices derived from RGB images and the Normalized Difference Vegetation Index (NDVI) measured with a portable spectroradiometer (Greenseeker). Third, we assessed the level water stress experienced by the plant, as a consequence of the root-knot nematodes, using stomatal conductance measured with a porometer (Decagon Leaf Porometer SC-1), plant temperature with an infrared thermometer and the stable carbon and nitrogen isotope composition of leaf dry matter. Among the measured parameters, percentage carbon and percentage nitrogen exhibited the highest positive correlation (r=0.9), whereas flavonoids and NBI (Nitrogen Balance Index) showed the highest inverse correlation (r=-0.87). It was found that the interaction between treatments and crops (ANOVA) was statistically different only 4 of 17 parameters (flavonoid (p=0.002), NBI (p=0.044), NDVI (p=0.004) and CSI (Crop Senescence Index) (p=0.002). Concerning the effect of treatments across all crops, differences existed only in two parameters, which were flavonoid (p=0.003) and CSI (p=0.001). Grafted plants contained less flavonoids (x̄=1.37) and showed lower CSI (x̄=11.65) than non-grafted plants (x̄=1.98 and x̄=17.28, respectively, p=0.020 and p=0.029) when combining all five years and four crops. We conclude that the grafted plants were less stressed and more protected against nematode attack. Leaf flavonoids and the RGB index (CSI) as indicator of crop senescence were good indicators of root-knot nematode impact across multiple crops.