Blue and green fluorescence and thermal imaging in the early detection of sunflower broomrape (Orobanche cumana Wallr.)

Abstract

The holoparasitic plant Orobanche cumana Wallr. depends entirely on the sunflower to obtain its supply of water, mineral nutrients and photoassimilates. The infection becomes evident by emergence of broomrape stems around flowering time of sunflower. However, the metabolism and water flow of the host plant are modified as a result of the parasite attack earlier in the infection process. Imaging of blue-green fluorescence (BGF) induced by UV light is a sensitive tool that provides information about changes in the content of plant secondary metabolites as a consequence, among others, of biotic stresses. On the other hand, infrared thermography can show alterations on leaf temperature upon infection by plant pathogens. The objectives of this work were to analyse the effect of the infection on sunflower plants by BGF and thermal imaging and to evaluate the use of these techniques in the early detection of the infection, prior to the broomrape emergence. Germinated seeds of sunflower susceptible to O. cumana were inoculated by transplant to soil previously infested with parasite seeds. The controls were sown in non-infested soil. Pots were grown in a glasshouse at 12–22°C for 5 weeks in a completely randomized design. Blue-green fluorescence (BGF) images were acquired sequentially in developed leaf pairs along the initial four vegetative stages of all the plants. Infrared and blue and green fluorescence (F440, F520, respectively, and their ratio F440/F520) images of fully developed leaf pairs and unshaded leaves were taken twice a week during three weeks. Moreover, pigment (chlorophylls and carotenoids) contents of control and parasitized sunflower were determined spectrophotometrically at the end of the experiment. Significant decreases in F440 and F520 were observed in infected plants as early as two weeks after inoculation (wai), and they remained significant along the whole experiment. Similarly, the value of F440/F520 was lower than in the controls throughout the experiment. Concerning pigments content, significant differences between inoculated and control plants were obtained only for the fourth leaf pair. Finally, canopy temperature in infected plants was consistently higher in all measurements. The decreases in fluorescence emission in sunflower suggest a down-regulation of the secondary metabolism upon parasite infection. On the other hand, increases in canopy temperature of infected plants point to an alteration in sunflower transpiration due to stomatal closure as a response of sunflower to the infection by O. cumana. To conclude, these results show that the nondestructive techniques BGF and thermography could be used as indicators of broomrape infection in sunflower plants as early as 2 wai.