Abstract
Due to the multi-target effect of biostimulants, an efficient, low-cost, and rapid approach for tracking their mode of action is required. In this paper, the effect of seed coating with thyme essential oil and the bacterium Paraburkholderia phytofirmans on plant growth as well as water stress tolerance was investigated using MultispeQ device in wheat seedling grown under controlled conditions. Both treatments enhanced photosynthesis efficiency and tolerance to water stress through reducing plant damage. Plants coated with P. phytofirmans tend to maintain photosynthesis rate, while plants coated with thyme oil tend to maintain water content and reduce energy-demanding processes. All results correlated with previous biochemical, isotopic, and molecular analysis results highlighting MultispeQ reliability.
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1 Introduction
Plant phenotyping has increasingly gained importance in agriculture as it aims at understanding their health status, thus improving their performance. Already existing expensive instruments for plant phenotyping makes them inapproachable; therefore, researchers have designed MultispeQ device as a hand-held, low-cost scientific instrument that provides noninvasive measurements of environmental parameters, leaf pigmentation, and various photosynthetic parameters. This work resolves to trace the effects of seed coating with thyme essential oil and the endophytic bacterium Paraburkholderia phytofirmans on plant growth promotion and water stress tolerance for further use of the MultispeQ in the field of biostimulant screening.
2 Materials and Methods
2.1 Plant Material, Seed Coating, and Growth Conditions
The experiment was conducted in controlled environment chamber at the Faculty of Biology of Barcelona. Seeds of widely cultivated Tunisian variety of durum wheat «Karim» were used. The coating technique consists in mixing 40 μl of the coating product (ATAS, AEGILOPS Applications, France) with 400 µl of either P. phytofirmans (108 CFU/ml) or thyme oil (5 ppm) (water was used as a control), and with 10 g of wheat seeds. Coated seeds were sown with 3 seeds/pot density (five pots for each treatment) containing a mixture of substrate: perlite (1:1, v/v). The plants were irrigated every two days with 50% Hoagland’s nutrient solution. Half of the pots were cultivated under well-watered conditions (100% pot capacity, PC), while the other half subjected a progressive water stress at 25 days post sowing, by restricting water by 10% PC every two days. Sampling occurred at the end of the experiment at 25 dps; root samples were washed under tap water, and dry weight of shoot and root samples was measured.
2.2 Spectroscopic Analysis with MultispeQ
All spectroscopic measurements were made on intact, fully expanded leaves using the MultispeQ device [1]. Measurements were taken at three time points with five replicates per factor combination (irrigation and treatment) at 2, 7, and 22 dps. These measurements were used to estimate the environmental parameters (temperature, pressure, and humidity), optical measurements (chlorophyll, photosynthetic efficiency, and the total electrochromic shift), and leaf anatomical characteristics (thickness).
2.3 Statistic Analysis
Results were analyzed statistically with ANOVA and LSD test with R studio software.
3 Results
3.1 Effect of Coating with P. phytofirmans and Thyme Oil on Root and Shoot Development Under Well-Watered and Water Stress Conditions
Both treatments enhanced root and shoot dry matter (Table 1). The effect of P. phytofirmans was far more remarkable.
3.2 Effect of Coating with P. phytofirmans and Thyme Oil on Plant Response Under Well-Watered and Water-Stressed Conditions Measured with MultispeQ
Effects of treatments were analyzed through comparison between data of stressed and non stressed control plants and data of stressed and non stressed treated plants (Fig. 1). From 2 to 7 dps, the plant reduced fraction of Photosystem II centers, quantum yield of Photosystem II, processes involved in plant damage reduction, the net CO2 assimilation rate (PN) of leaves (RFd), thickness, and increased leaf temperature, leaf temperature differential, chlorophyll, processes linked to photo-inhibition and plant damage, and the maximal quantum efficiency of photosystem II (Fv’/Fm’). Plant adaptation to stress at 22 dps included decrease of early increased processes linked to photo-inhibition and plant damage, the maximal quantum efficiency of photosystem II (Fv’/Fm’), and included increase of early decreased quantum yield of Photosystem II, fraction of Photosystem II centers, processes involved in excess energy regulation and plant damage reduction, and leaf thickness.
In well-watered conditions, both treatments increased the quantum yield of Photosystem II (Phi2, ΦII) and chlorophyll. Under stress, both of them enhanced PiNPQ, NPQt (involved in plant damage reduction), and chlorophyll and decreased PiNO (linked to photo-inhibition and unregulated harmful processes). Treatments had different mode of action concerning other parameters. Under well-watered conditions, P. phytofirmans increased ambient humidity, thickness, and the total flow of electrons from antennae complexes into Photosystem II (LEF), while thyme oil increased ambient temperature and decreased thickness and LEF. Under water stress conditions, P. phytofirmans increased quantum yield of Photosystem II, light-harvesting through increased LEF and qL and decreased thickness, while thyme oil increased thickness and decreased LEF, RFd, and ambient humidity.
4 Discussion
The measurement of MultispeQ parameters in control plants confirmed stress imposition at 7 dps and plant adaptation at 22 dps. In well-watered conditions, both treatments enhanced photosynthesis efficiency. Under water stress conditions, both of them enhanced plant tolerance to water stress through increasing processes involved in plant damage reduction and decreasing processes linked to photo-inhibition and unregulated harmful events. Treatments had different modes of action concerning other parameters. Under well-watered conditions, the growth promoting effect of P. phytofirmans was observed and characterized by increased light-harvesting, evapotranspiration, water content, and by maintaining seedling performance. Thyme oil transient effect on reduced evapotranspiration and light-harvesting correlate with our biochemical and molecular results and microscopic observations indicating induced priming via transient stress characterized by peroxidases and phenolic compounds accumulation and stomatal closure via ABA accumulation (in press). Under Water stress conditions, plants coated with P. phytofirmans tend to increase light-harvesting to maintain photosynthesis rate (phenomenon called “drought tolerance”), while plants coated with thyme oil tends to maintain water content and reduce energy-demanding processes including photosynthesis (phenomenon called “drought avoidance”) which correlates highly with our biochemical, isotopic, and molecular analysis results concerning the effect of both treatments on ABA content, on N and C abundance and discrimination, and on gene expression (in press).
5 Conclusions
MultispeQ can measure a wide range of parameters that can be used to explore the effect of biostimulants on plant physiology and response to abiotic and biotic stresses. MultispeQ data highly correlates with the existing biochemical, isotopic, and molecular analysis, and accentuates its reliability in revealing different modes of action of biostimulants with little need to go through costing and time-consuming methods.
Reference
Kuhlgert, S., Austic, G., Zegarac, R., Bonsu, I.O., Hoh, D., Chilvers, M.I., Roth, M.G., TerAvest, D. and Kramer, D.M.: MultispeQ Beta: a tool for large scale plant phenotyping connected to the open PhotosynQ Network. Royal Soc. Open Sci. 3 (2016)
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Ben-Jabeur, M. et al. (2021). MultispeQ for Tracing Biostimulants Effect on Growth Promoting and Water Stress Tolerance in Wheat. In: Ksibi, M., et al. Recent Advances in Environmental Science from the Euro-Mediterranean and Surrounding Regions (2nd Edition). EMCEI 2019. Environmental Science and Engineering(). Springer, Cham. https://doi.org/10.1007/978-3-030-51210-1_191
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DOI: https://doi.org/10.1007/978-3-030-51210-1_191
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