Manganese (Mn) deficiency constitutes a major plant nutritional problem in commercial crop production of winter cereals. In plants, Mn has an indispensable role in the oxygen evolving complex (OEC) of photosystem II (PSII). Hence, the consequences of Mn deficiency are reduced plant growth, and eventually substantial yield losses.
It is well known, that genotypes within plant species differ considerably in tolerance to growth under Mn limiting conditions, a phenomenon designated as Mn efficiency. However, the physiological responses reflecting the underlying mechanisms of Mn efficiency are still not fully understood.

In this PhD study, a new method for determination and characterization of metal binding in size-fractionated photosynthetic protein complexes from barley thylakoids was established. The applicability of the method was shown by quantification of Mn binding in PSII from thylakoids of two barley genotypes with contrasting Mn efficiency exposed to increasing levels of Mn deficiency. Mn loading to PSII and the amount of PSII supercomplexes was drastically reduced in response to Mn deficiency.

Notably, these responses to Mn deficiency were more pronounced in the Mn-inefficient genotype, despite having lower or similar total leaf Mn concentrations. The new method thereby facilitates studies of the internal use of Mn and other biometals in various PSII complexes as well as their relative dynamics according to changes in environmental conditions.

In addition, genotypic differences related to the extrinsic proteins PsbP and PsbQ in terms of abundance and the compartmentalization of the proteins in the chloroplast were observed, suggesting a function of the protein(s) in Mn delivery during PSII assembly and/or repair. It is concluded that Mn loading and the destabilization of PSII complexes is related to Mn efficiency in plants.