Abstract
A catalytic manganese (Mn) cluster is required for the oxidation of water in the oxygen-evolving complex (OEC) of photosystem II (PSII) in plants. Despite this essential role of Mn in generating the electrons driving photosynthesis, limited information is available on how Mn deficiency affects PSII functionality. We have here used parameters derived from measurements of fluorescence induction kinetics (OJIP transients), non-photochemical quenching (NPQ) and PSII subunit composition to investigate how latent Mn deficiency changes the photochemistry in two barley genotypes differing in Mn efficiency. Mn deficiency caused dramatic reductions in the quantum yield of PSII and led to the appearance of two new inflection points, the K step and the D dip, in the OJIP fluorescence transients, indicating severe damage to the OEC. In addition, Mn deficiency decreased the ability to induce NPQ in the light, rendering the plants incapable of dissipating excess energy in a controlled way. Thus, the Mn deficient plants became severely affected in their ability to recover from high light-induced photoinhibition, especially under strong Mn deficiency. Interestingly, the Mn-efficient genotype was able to maintain a higher NPQ than the Mn-inefficient genotype when exposed to mild Mn deficiency. However, during severe Mn deficiency, there were no differences between the two genotypes, suggesting a general loss of the ability to disassemble and repair PSII. The pronounced defects of PSII activity were supported by a dramatic decrease in the abundance of the OEC protein subunits, PsbP and PsbQ in response to Mn deficiency for both genotypes. We conclude that regulation of photosynthetic performance by means of maintaining and inducing NPQ mechanisms contribute to genotypic differences in the Mn efficiency of barley genotypes growing under conditions with mild Mn deficiency.
Highlights
Deficiency of essential plant nutrients is a significant problem for plant production throughout the world influencing crop yields and crop quality
Chl a fluorescence was continuously measured to monitor the progression of Mn deficiency and revealed the induction of Mn deficiency below a critical threshold limit of about 15 μg Mn g−1 dry weight (DW) in barley leaves (Table 1) (Reuter et al, 1997), no visual leaf Mn deficiency symptoms could be observed in any of the treatments
The measured Fv/Fm values corresponded to Mn leaf concentrations of around 9 μg Mn g−1 DW and around 6.5 μg Mn g−1 DW under mild and moderate Mn deficiency, respectively, whereas strong Mn deficiency correlated with values of 3.8 and 5.3 μg Mn g−1 DW for Antonia and Vanessa, respectively (Table 1)
Summary
Deficiency of essential plant nutrients is a significant problem for plant production throughout the world influencing crop yields and crop quality. One of the major unsolved nutritional problems in agricultural plant production is manganese (Mn) deficiency causing substantial yield reductions and during severe winters even causing complete loss of crops (Schmidt et al, 2013; Stoltz and Wallenhammar, 2014). The deficiency is traditionally corrected by repeatedly foliar Mn applications, often without knowing the exact Mn requirement of the plants. This method is, both time-consuming and inefficient, since the rate of Mn remobilisation in plants is extremely low (Loneragan, 1988). The exact mechanisms behind Mn efficiency are, still not fully understood (Leplat et al, 2016; Schmidt et al, 2016), but have so far been related to differential root acquisition of Mn, e.g., by the exudation of organic acid anions and phytases into the rhizosphere (Rengel and Marschner, 2005; George et al, 2014), increased Mn uptake capacity by expression of high affinity Mn transporters in the root (Pedas et al, 2005, 2008), and internal utilization of Mn in the plant and processes linked to the stability and efficiency of the photosynthetic apparatus (Husted et al, 2009; Schmidt et al, 2015)
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