Abstract
Soil and water salinization has global impact on the sustainability of agricultural production, affecting the health and condition of staple crops and reducing potential yields. Identifying or developing salt-tolerant varieties of commercial crops is a potential pathway to enhance food and water security and deliver on the global demand for an increase in food supplies. Our study focuses on a phenotyping experiment that was designed to establish the influence of salinity stress on a diversity panel of the wild tomato species, Solanum pimpinellifolium. Here, we explore how unoccupied aerial vehicles (UAVs) equipped with both an optical and thermal infrared camera can be used to map and monitor plant temperature (Tp) changes in response to applied salinity stress. An object-based image analysis approach was developed to delineate individual tomato plants, while a green–red vegetation index derived from calibrated red, green, and blue (RGB) optical data allowed the discrimination of vegetation from the soil background. Tp was retrieved simultaneously from the co-mounted thermal camera, with Tp deviation from the ambient temperature and its change across time used as a potential indication of stress. Results showed that Tp differences between salt-treated and control plants were detectable across the five separate UAV campaigns undertaken during the field experiment. Using a simple statistical approach, we show that crop water stress index values greater than 0.36 indicated conditions of plant stress. The optimum period to collect UAV-based Tp for identifying plant stress was found between fruit formation and ripening. Preliminary results also indicate that UAV-based Tp may be used to detect plant stress before it is visually apparent, although further research with more frequent image collections and field observations is required. Our findings provide a tool to accelerate field phenotyping to identify salt-resistant germplasm and may allow farmers to alleviate yield losses through early detection of plant stress via management interventions.
Highlights
The area of agricultural land impacted by salinization and sodification is increasing globally, with more than 50% of arable land predicted to be affected by 2050 (Wang et al, 2003; Jamil et al, 2011)
We explored the retrieval of Tp from a unoccupied aerial vehicles (UAVs) thermal infrared (TIR) camera in a tomato field trial, demonstrating that it is possible to detect differences between salt-treated and control plants, which may help identify salt-tolerant tomato germplasm in future research
Tp was retrieved where green–red vegetation index (GRVI) > 0 and setting a maximum pixel threshold of Ta + 9°C. The latter condition was required because the presence of pixels with Tp > Ta + 20°C in the object-based image analysis (OBIA) plant delineation showed that objectbased methods alone are insufficient to retrieve accurate Tp, at least from the tomato plants explored
Summary
The area of agricultural land impacted by salinization and sodification is increasing globally, with more than 50% of arable land predicted to be affected by 2050 (Wang et al, 2003; Jamil et al, 2011). Global freshwater supplies are under extreme pressure, with agricultural production already accounting for more than two-thirds of freshwater use (Famiglietti, 2014; Brauman et al, 2016; Pastor et al, 2019). Irrigation with brackish water presents as an enticing option, as the targeted application of water is an effective way to close the yield gap (Licker et al, 2010; Mueller et al, 2012). The identification and breeding of cultivars with increased resilience to salt stress would provide an effective twofold solution to ensuring future food security by enabling production on marginal land and the potential to irrigate with brackish water (Morton et al, 2018)
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