Magnesium deficiency induced responses in crop physiology: impacts on photosynthesis, light utilization, and photoprotection
von Setareh Jamali Jaghdani
Datum der mündl. Prüfung:2021-08-20
Erschienen:2021-11-18
Betreuer:Dr. Merle Tränkner
Gutachter:Dr. Merle Tränkner
Gutachter:Prof. Dr. Klaus Dittert
Dateien
Name:Setareh Jamali Jaghdani_Dissertation.pdf
Size:9.76Mb
Format:PDF
Description:magnesium deficiency, photoprotection, photosynthesis, plant nutrition
Zusammenfassung
Englisch
Intensive agricultural production without compensation of macro- and micro-nutrients that are taken up by the plants or leached out from the soil by rain, leads to nutrient deficiency in soils and plants. Subsequently, it leads to nutritional deficiencies in livestock and humans. Magnesium (Mg) is one of the most vital macro-nutrients that is taken up by the plants from the soil. Mg is the core element of chlorophyll pigments, which are in chloroplasts where photosynthesis takes place. Mg is required for several enzymes and enzymatic activities. One of the most important enzymes in photosynthesis that requires Mg for its activation is Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Photosynthates are then transported from source organs (leaves) to sink organs (younger leaves, fruits, and seeds). Therefore, Mg is required for adequate dry matter (DM) production. Due to the high requirement of Mg in chloroplasts, photosynthetic reactions can respond more sensitively to Mg scarcity. Moreover, under Mg deficiency, the level of reactive oxygen species (ROS) formation is enhanced. ROS are toxic and can lead to cell death. Nevertheless, plants have developed photoprotective mechanisms to overcome the excessive ROS production. Non-photochemical quenching (NPQ) is known as one of the photoprotective mechanisms, where excessive absorbed energy is dissipated as heat. There have been several studies with various plants that investigate the influence of Mg deficiency on DM formation, shoot and root growth, ROS concentration, and ROS scavengers’ activity. However, a critical Mg concentration that is determined to be influential on each of the mentioned parameters was not introduced. In this Ph.D. project, the influence of Mg deficiency on photosynthetic parameters, photoprotection, and physiology of wheat, sunflower, barley, and spinach which were grown hydroponically, is studied. A narrow range of Mg deficiency, including seven deficiency treatments was induced in chapter 2, where the range was narrowed down to three in chapter 3 and 4. The Mg deficiency had a significant influence on decreased CO2 assimilation in all the mentioned crops except wheat. Reduced assimilation rate is frequently associated with reduced activities of enzymes involved in CO2 fixation such as Rubisco and Rubisco activase. Moreover, the decreased level of photosynthates’ translocation from source organs to sinks, and the photosynthates’ accumulation in source organs, has a negative impact on Rubisco activity. In this regard, the measured shoot and root DM in spinach plants revealed the reduced translocation of photosynthates which included root growth reduction under Mg deficiency. The chlorophyll fluorescence measurements provide a relatively good information about photosynthetic efficiency. Some of the parameters that were measured by this method are maximum quantum efficiency (Fv/Fm), photochemical quenching (qP), NPQ, and electron transport rate (ETR) to name a few. qP was unaffected in sunflower, wheat, and spinach. Whereas it was significantly decreased in barley. qP is an indicator of the proportion of the open reaction centers. Hence, the reduction in its value reflects the increased number of closed reaction centers, and moreover a decrease in the capacity of electron transportation and subsequently a decrease in ETR. ETR was reduced substantially in sunflower and barley, whereas it was unaffected in wheat and spinach. The decrease in ETR damages the whole electron transport chain from photosystem II (PSII) to photosystem I and hence reducing the assimilation rate. Mg deficiency resulted in reductions in Fv/Fm in sunflower, barley, and wheat. Reductions in Fv/Fm can be an indicator of a damage in the photosynthetic apparatus. Moreover, a decline in Fv/Fm can indicate photoinhibition of PSII due to a photooxidative damage. Photooxidative damage is a result of excessive ROS formation. In order to investigate the photoprotection efficiency and photooxidative damage levels, gene expression of ROS scavenging enzymes was analyzed in barley and spinach. In barley the expression levels of catalase, glutathione reductase, and superoxide dismutase were increased, whereas no increase in ascorbate peroxidase expression was observed. In spinach no increase in the expression level of any of the mentioned genes was observed. The level of ROS formation under Mg deficiency is also dependent on the light intensity available at the growth condition. Higher light intensities enhance the ROS formation under Mg deficiency. Violaxanthin (Vx) cycle is known as one of the most important photoprotective mechanisms in high-light acclimated plants and contributes to the prevention of excessive ROS formation by NPQ. The pigments involved in violaxanthin (Vx) cycle were quantified to study the impacts of Mg deficiency on photoprotection in barley and spinach. Vx is converted to zeaxanthin (Zx) via antheraxanthin (Ax). The increase in the VAZ (Vx+ Ax+ Zx)-pool size under Mg deficiency in barley indicated a requirement for photoprotection against oxidative damage. However, the increase in VAZ-pool size was served as a NPQ-independent function. In line with this, no increase in the NPQ value by chlorophyll fluorescence measurements was observed. The Mg deficiency did not induce any increase in NPQ value or VAZ-pool size in spinach. Therefore, we could not confirm any photooxidative stress in spinach plants under Mg deficiency. In overall, this study contributes to better understanding of influences of Mg deficiency on photosynthetic and photoprotective mechanisms. It provides insights and helps to close the knowledge gaps that exist regarding the critical Mg supply for certain physiological and biochemical mechanisms. Furthermore, it improves implementation of fertilization strategies.
Keywords: Magnesium deficiency, photoprotection, photosynthesis, plant nutrition