Magnesium–Isotope Fractionation in Chlorophyll-a Extracted from Two Plants with Different Pathways of Carbon Fixation (C3, C4)

Abstract

Relatively few studies have been focused so far on magnesium–isotope fractionation during plant growth, element uptake from soil, root-to-leaves transport and during chlorophylls biosynthesis. In this work, maize and garden cress were hydroponically grown in identical conditions in order to examine if the carbon fixation pathway (C4, C3, respectively) might have impact on Mg-isotope fractionation in chlorophyll-a. The pigment was purified from plants extracts by preparative reversed phase chromatography, and its identity was confirmed by high-resolution mass spectrometry. The green parts of plants and chlorophyll-a fractions were acid-digested and submitted to ion chromatography coupled through desolvation system to multiple collector inductively coupled plasma-mass spectrometry. Clear preference for heavy Mg-isotopes was found in maize green parts (∆26Mgplant-nutrient 0.65, 0.74 for two biological replicates, respectively) and in chlorophyll-a (∆26Mgchlorophyll-plant 1.51, 2.19). In garden cress, heavy isotopes were depleted in green parts (∆26Mgplant-nutrient (−0.87)–(−0.92)) and the preference for heavy isotopes in chlorophyll-a was less marked relative to maize (∆26Mgchlorophyll-plant 0.55–0.52). The observed effect might be ascribed to overall higher production of energy in form of adenosine triphosphate (ATP), required for carbon fixation in C4 compared to C3, which could reduce kinetic barrier and make equilibrium fractionation prevailing during magnesium incorporation to protoporphyrin ring.

Stable Isotope Fractionation of Metals and Metalloids in Plants: A Review

“…Plants preferentially take up heavy isotopes from their Mg sources, such as hydroponic nutrient solutions ( Black et al, 2008 ; Bolou-Bi et al, 2010 ; Wrobel et al, 2020 ), phlogopite ( Bolou-Bi et al, 2010 ), soil pore water ( Tipper et al, 2010 ; Kimmig et al, 2018 ), and phytoavailable soil Mg pools ( Bolou-Bi et al, 2012 ; Opfergelt et al, 2014 ; Kimmig et al, 2018 ; Wang et al, 2020 ; Figure 3 ). In ryegrass and clover, root extracts (CaCl 2 ) of Mg revealed that heavier Mg is preferentially adsorbed to the apoplast ( Bolou-Bi et al, 2010 ).…”

“…The remobilization of Mg would require a dissociation of Mg from its major organic ligands, such as chlorophyll ( Kleczkowski and Igamberdiev, 2021 ). Theoretical and experimental studies revealed that Mg bound to chlorophyll is isotopically heavier than the bulk leaf or ionic Mg ( Black et al, 2007 ; Moynier and Fujii, 2017 ; Pokharel et al, 2018 ; Wrobel et al, 2020 ). Only one study found that Mg in chlorophyll is lighter than the bulk leaf Mg ( Black et al, 2008 ).…”

“…Combining liquid chromatography with MC-ICP-MS. This has been used to measure Mg and Cu isotope ratios in chlorophyll and proteins ( Larner et al, 2019 ; Wrobel et al, 2020 ). For structural elements, such as B and Ca, extraction techniques can provide crucial insights into isotope fractionation within plants ( Schmitt et al, 2018 ).…”

Magnesium Foliar Supplementation Increases Grain Yield of Soybean and Maize by Improving Photosynthetic Carbon Metabolism and Antioxidant Metabolism

“…However, the effects of foliar fertilization can be different between soybean and maize plants, as they have different photosynthetic pathways, C 3 and C 4 , respectively [ 16 ]. There is evidence of greater preference for C 4 plants for heavy Mg isotopes in chlorophyll-a compared to C 3 plants [ 17 ]. This result was attributed to the greater need for energy and the rate of ATP production to fix carbon in C 4 , reducing the energy barrier during the incorporation of Mg to protoporphyrin IX [ 17 ].…”

“…There is evidence of greater preference for C 4 plants for heavy Mg isotopes in chlorophyll-a compared to C 3 plants [ 17 ]. This result was attributed to the greater need for energy and the rate of ATP production to fix carbon in C 4 , reducing the energy barrier during the incorporation of Mg to protoporphyrin IX [ 17 ]. In addition, Mg foliar spray can also reduce the abiotic stress of crops by protecting the photosynthetic apparatus and activating the antioxidant defense system [ 18 , 19 , 20 ].…”