Effects of fungicides on physiological parameters and yield formation of wheat assessed by non-invasive sensors

Abstract

Apart from fungicidal effects, some fungicide classes have been reported to induce physiological changes in crops such as increased tolerance to abiotic stress, delayed senescence of the photosynthetic leaf area and modifications in the balance of plant growth regulators. The aim of this study was to investigate the effects of different fungicidal groups on physiological parameters of wheat through the use of non-invasive sensors and imaging techniques. Experiments were conducted under field and also under disease-free conditions in the greenhouse.<br /> Under field conditions, application of the azole, carboxamide and strobilurin compounds resulted in low disease incidence. All fungicide treatments delayed the senescence of the uppermost leaf layers; treatments with longer leaf life and lower disease incidence resulted in higher chlorophyll content. The effect of the fungicides on wheat senescence was positively correlated to grain yield and the thousand-kernel weight. However, under field conditions, the presence of the main foliar pathogens of wheat influenced the green leaf area duration as well as the yield, generating a disadvantage for the fungicide treatments with low disease control efficacy.<br /> Under disease-free conditions, an effect produced by the pyrazole carboxamide fungicide bixafen was observed. Bixafen delayed the senescence of leaves and ears resulting in a significantly extended green leaf area duration compared to untreated plants. In addition, an effect produced by this compound on morphogenesis was observed. The combination of the positive effects on physiology and morphogenesis of wheat resulted in a yield advantage of bixafen-treated plants. Furthermore, bixafen had a positive effect on plant tolerance to water stress conditions.<br /> Different non-invasive sensors and imaging techniques were used and compared to measure the effects of fungicidal compounds on wheat physiology. By using ground-based optical sensors it was possible to detect the influence of fungicidal compounds in crop physiology, i.e. degradation of photosynthetic pigments, photosynthetic activity, leaf reflectance, and transpiration of plant tissue earlier than with destructive and visual methods. Chlorophyll fluorescence of leaves was useful to measure differences in the effective quantum yield of photosystem II. Reflectance measurements of wheat leaves were highly sensitive to changes in plant vitality. The spectral vegetation indices were useful to determine differences between treatments in terms of leaf senescence, pigments and water content. Digital infrared images revealed significant differences between untreated and fungicide-treated plants at different growth stages. Moreover, thermography proved to be a suitable technique for distinguishing the beneficial effects of fungicides on plant senescence under different water supply conditions. Through the use of an image analysis software program, leaf senescence differences were successfully detected, thus allowing an early detection of the effect produced by the fungicide on the senescence status of flag leaves. Using hyperspectral imaging, it was possible to study differences in the senescence status of flag leaves. Furthermore, through the analysis of hyperspectral images it was achievable to study the pattern of the senescence process in flag leaves and to determine a delay of senescence of wheat produced by fungicides.<br /> The results of this study demonstrated that non-invasive sensors and imaging techniques are excellent alternatives to conventional screening methods for detecting the beneficial effects of fungicides on plant physiology. Furthermore, among this innovative group of sensors and techniques it was spectrometry, which proved to be the most sensitive and specific method with a high potential for large-scale fungicide screening. Sensors can be incorporated in automatic and reproducible screening of new active ingredients with high efficiency and accuracy. The recent development of hyperspectral imaging techniques will improve future studies to additionally explore plant physiology with high spatial and temporal resolution.