Abstract
Canopy temperature has been proposed as a relevant variable for crop water stress monitoring. Since crop temperature is highly influenced by the prevailing climatic conditions, it is usually normalized with indices such as the crop water stress index (CWSI). The index requires the use of two baselines that relate canopy temperature under maximum stress and non-water stress conditions with vapor pressure deficit (VPD). These reference baselines are specific to each crop and climatic region. In maize, they have been extensively studied for certain climatic regions but very little is known on their suitability to be used under Mediterranean-type conditions nor their temporal stability, both diurnally and between seasons. Thus, the objective of this work was to determine the reference baselines for maize grown under Mediterranean conditions, as well as its diurnal and long-term stability. An experiment was conducted for 3 years in a maize breeding field, under well-watered and water-stressed irrigation treatments. The determined reference baselines for computing CWSI in maize have shown to be stable in the long term but markedly influenced by the meteorological variations between 10–17 h UTC (Coordinated Universal Time). These results indicate that several reference baselines should be used for CWSI computing throughout the abovementioned time interval. The CWSI values calculated for well-watered and water-stressed maize breeding plots using the reference baselines derived in this study were successfully correlated with other physiological indicators of plant water stress.
Highlights
Maize is one of the world’s major staple crops [1] and the main cereal grown by smallholder farmers in several regions of the world [2]
Regarding bpot (Equations (2) and (3)), the values previously obtained in Turkey were −0.86 and −2.66 ◦ C kPa−1, whereas the values observed in this study differed significantly since they varied within the range −1.34 to −1.85 ◦ C kPa−1 between 10–17 h (UTC) (Table 5), with −1.49 ◦ C kPa−1 being the bpot value obtained after analyzing the data together (Figure 2)
Values explained 13% and 6% more of the observed variability in gs and A, respectively, than non-water-stressed baseline (NWSB)-based crop water stress index (CWSI). These results suggest that artificial reference surfaces (ARS)-based CWSI values were slightly more accurate than the NWSB-based CWSI to monitor maize water stress, but the choice of one method or the other will be conditioned more by the level of instrumental complexity desired in the high-throughput phenotyping (HTPP) platform than by the observed differences in accuracy
Summary
Maize is one of the world’s major staple crops [1] and the main cereal grown by smallholder farmers in several regions of the world [2]. The supply of irrigation water is required in many of the producing regions since water deficit has important consequences on maize yield [3,4]. Strategies to cope with the scarcity of water resources are mainly focused on monitoring crop water status to optimize production under deficit irrigation strategies [6] and on obtaining more drought-tolerant cultivars in plant-breeding programs [7]. In both cases, crop temperature has been proposed as a reliable tool to monitor maize water status and to identify water stress-tolerant maize genotypes [6,8,9]. The development of practical irrigation applications based on crop temperature monitoring has been carried out through thermal indices that normalize its value and allow the quantification of the crop water stress level [6,10,11,12]
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