Abstract
Phosphorus (P) is an essential macronutrient for plants with a structural role in nucleic acids, ATP, NADPH, and phospholipids as well as in regulation of metabolisms via phosphorylation. Phosphorus deficiency is a major limiting factor in global plant production, and the main source of P fertilizer (rock phosphate) is a finite and non-renewable resource, predicted to become limited in the near future. Estimating plant available P in the soil is usually associated with many uncertainties, as P is one of the least mobile nutrients in soil. Climatic conditions and soil properties significantly affect the availability of P, and the correlation between extracted P using the classic soil P extraction analyses and true plant P uptake is often poor. Consequently, there is an urgent need for better real-time estimations of P status in plants.
Chlorophyll a fluorescence is a reliable probe of photochemistry and a powerful tool for detecting multiple plant stresses including pests, heat stress, and nutrient deficiencies. The objective of this PhD study was to investigate chlorophyll a fluorescence as a new approach to reveal the P status of plants directly in the field, and to identify the underlying mechanism(s) responsible for changes in the chlorophyll a fluorescence signal. The experimental part included multiple greenhouse experiments with plants cultivated at different P levels, a wide range of bioassays for studying photosynthetic responses, and finally field trials for testing the method under natural growth conditions.
Paper I (Frydenvang et al 2015, Plant Physiology, Vol. 169, pp. 353-361) demonstrates that chlorophyll a fluorescence is a unique tool to monitor bioactive P concentrations in plants. The shape of the fluorescence transient was altered when plants suffered from P deficiency, as the so-called I-step gradually straightened and eventually disappeared, suggesting that the electron flow towards photosystem I is affected during P deficiency. The effects were specific and sufficiently sensitive to detect even latent P deficiency, allowing the development of a prediction model to estimate the P status across various plant species.
Paper II (Carstensen et al 2018, Plant Physiology, Vol. 177, pp. 271-284) explains why the chlorophyll a fluorescence signal is altered during P deficiency and presents a comprehensive biological model describing how P deficiency disrupts the photosynthetic machinery through a series of sequential events. During P deficiency, orthophosphate levels decreased in the chloroplast stroma, which reduced the rate of ATP synthase. Consequently, protons accumulated in the thylakoid lumen causing acidification, which finally decreased the oxidation of the plastoquinol pool at the cytochrome b6f complex in the electron transport chain. As a result, the flow of electrons towards photosystem I was altered, which was reflected by the I-step of the chlorophyll a fluorescence signal.
Paper III (Carstensen et al 2018, submitted to Plant and Soil) used the prediction model developed in Paper I to demonstrate that chlorophyll a fluorescence is a powerful tool for early detection of P deficiency directly in the field. The fluorescence measurements were able to detect P deficiency, and if P fertilizers were applied no later than 35 days after sowing, a significant yield reduction could be avoided. The method showed a promising opportunity for avoiding potential yield reductions caused by P deficiency when fluorescence analyses were performed during early vegetative growth.
During the project, a new handheld device for detecting P deficiency was developed by integrating the prediction model from Paper I with a fluorometer. The biological mechanisms behind the discovery were explained in Paper II, and the device was successfully tested in the field in Paper III. This PhD thesis concludes that chlorophyll a fluorescence is a promising method for revealing P status of plants, thereby providing a unique opportunity for timely detection and correction of P deficiency in agriculture, with the potential to increase P use efficiency in global plant production.
Chlorophyll a fluorescence is a reliable probe of photochemistry and a powerful tool for detecting multiple plant stresses including pests, heat stress, and nutrient deficiencies. The objective of this PhD study was to investigate chlorophyll a fluorescence as a new approach to reveal the P status of plants directly in the field, and to identify the underlying mechanism(s) responsible for changes in the chlorophyll a fluorescence signal. The experimental part included multiple greenhouse experiments with plants cultivated at different P levels, a wide range of bioassays for studying photosynthetic responses, and finally field trials for testing the method under natural growth conditions.
Paper I (Frydenvang et al 2015, Plant Physiology, Vol. 169, pp. 353-361) demonstrates that chlorophyll a fluorescence is a unique tool to monitor bioactive P concentrations in plants. The shape of the fluorescence transient was altered when plants suffered from P deficiency, as the so-called I-step gradually straightened and eventually disappeared, suggesting that the electron flow towards photosystem I is affected during P deficiency. The effects were specific and sufficiently sensitive to detect even latent P deficiency, allowing the development of a prediction model to estimate the P status across various plant species.
Paper II (Carstensen et al 2018, Plant Physiology, Vol. 177, pp. 271-284) explains why the chlorophyll a fluorescence signal is altered during P deficiency and presents a comprehensive biological model describing how P deficiency disrupts the photosynthetic machinery through a series of sequential events. During P deficiency, orthophosphate levels decreased in the chloroplast stroma, which reduced the rate of ATP synthase. Consequently, protons accumulated in the thylakoid lumen causing acidification, which finally decreased the oxidation of the plastoquinol pool at the cytochrome b6f complex in the electron transport chain. As a result, the flow of electrons towards photosystem I was altered, which was reflected by the I-step of the chlorophyll a fluorescence signal.
Paper III (Carstensen et al 2018, submitted to Plant and Soil) used the prediction model developed in Paper I to demonstrate that chlorophyll a fluorescence is a powerful tool for early detection of P deficiency directly in the field. The fluorescence measurements were able to detect P deficiency, and if P fertilizers were applied no later than 35 days after sowing, a significant yield reduction could be avoided. The method showed a promising opportunity for avoiding potential yield reductions caused by P deficiency when fluorescence analyses were performed during early vegetative growth.
During the project, a new handheld device for detecting P deficiency was developed by integrating the prediction model from Paper I with a fluorometer. The biological mechanisms behind the discovery were explained in Paper II, and the device was successfully tested in the field in Paper III. This PhD thesis concludes that chlorophyll a fluorescence is a promising method for revealing P status of plants, thereby providing a unique opportunity for timely detection and correction of P deficiency in agriculture, with the potential to increase P use efficiency in global plant production.
| Original language | English |
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| Publisher | Department of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen |
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| Publication status | Published - 2018 |