Speaker
Description
Introduction
The sunflower production area in Switzerland is small (6404 hectares; swissgranum, 2023) and increasingly subjected to summer water deficit that may limit sunflower performance. In order to maintain a domestic production, and even support its growth, it is important to determine drought adaptation strategies. The advancement of sowing dates in combination with very early maturing varieties is one option, but under Swiss conditions, the risk of poor establishment due to cold damage will remain high in the coming decade. Drought tolerant varieties with physiological mechanisms conferring drought tolerance at flowering and seed filling constitute the most favorable strategy. Sunflower varieties are currently evaluated on the basis of grain yield, oil content and composition, early ripening, disease resistance and lodging. There are no established indicators to assess water stress tolerance. A protocol to characterise the stress level of sunflower varieties is therefore needed to improve the varietal evaluation criteria and to consider these in the description of varieties.
Materials and methods
A field trial was conducted from April to September 2022 in Nyon, Switzerland. Eight sunflower varieties were sown and replicated four times without and with irrigation (95 mm in total during vegetative and reproductive stages). Crop management followed conventional practices applied by local farmers. Physiological mechanisms of drought tolerance were studied by means of leaf water content and simultaneous leaf and canopy temperature measurements. Five measurement sessions were performed between the end of June and the end of July 2022, covering the inflorescence emergence and ripening phase (BBCH growth stages 51-89). Leaf temperature was acquired for the youngest developed leaf using a handheld infrared thermometer (Extech IRC130) between noon and the early afternoon on days with no or little clouds. The leaf water content (LWC) was determined for the same leaf. Drone surveys using a Micasense Altum multi-spectral camera were performed at one timepoint (around noon) on the same day. Out of the six spectral bands of the Altum camera, the longwave infrared band was extracted to acquire crop temperature.
Results and discussion
Leaf water content was consistently higher under irrigated compared to non-irrigated conditions, with the largest differences on days when air vapor pressure deficit (VPD) was high, i.e. above 2.5 kPa, during measurements. Differences between varieties were also detectable on these days.
The difference between leaf and air temperature (dTm) is generally considered a proxy for leaf stomatal conductance (gs) with more negative dTM values indicating higher gs. We found that negative dTm values were only obtained under conditions of high VPD, i.e., when transpiration is more important. An ANOVA including VPD as a covariable showed a significant effect of VPD on dTm. Canopy temperature determined using drone-based image acquisition showed increasingly important differences between irrigated and non-irrigated plots. Crop temperature showed similar differences between varieties as LWC. The correlation with leaf temperature was nevertheless low except for measurements at the end of July when water stress was high.
Our results indicate that conditions of high VPD, and therefore high transpiration rates, are a prerequisite for detecting differences between varieties in response to water deficit based on leaf temperature determined using a handheld infrared thermometer. Measurements of leaf water content were more reliable and sensitive for detecting the presence of drought stress, but also required high VPD conditions for determining differences between varieties. Drone-based thermal image acquisition was less sensitive to measurement conditions and was able to assess varietal differences in drought stress responses at an earlier stage. The work performed confirms the potential of leaf water content and infrared thermography in the evaluation of water stress tolerance of sunflower varieties grown in Switzerland.
swiss granum. (2023). Surfaces Cultivées, Espèces et Variétés. https://www.swissgranum.ch/fr/chiffres/production-indigene.
| Keywords | Sunflower ; water stress tolerance ; varietal study ; leaf temperature ; leaf water content |
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