Potassium (K) and magnesium (Mg) are mineral nutrients that are required in large quantities by plants. Both elements critically contribute to the process of photosynthesis and the subsequent long-distance transport of photoassimilates. If K or Mg is not present in sufficient quantities in photosynthetic tissues, complex interactions of anatomical, physiological and biochemical responses result in a reduction of photosynthetic carbon assimilation. As a consequence, excessive production of reactive oxygen species causes photo-oxidation of the photosynthetic apparatus and causes an up-regulation of photoprotective mechanisms. In this article, we review the functioning of K and Mg in processes directly or indirectly associated with photosynthesis. Focus is given to chloroplast ultrastructure, light-dependent and -independent reactions of photosynthesis and the diffusion of CO - a major substrate for photosynthesis - into chloroplasts. We further emphasize their contribution to phloem-loading and long-distance transport of photoassimilates and to the photoprotection of the photosynthetic apparatus.

Magnesium (Mg) deficiency in plants is a widespread problem affecting productivity and quality in agricultural systems and forestry. Although numerous studies addressed the effect of Mg deficiency on biomass and photosynthetic CO 2 assimilation, a summary evaluation of the effect of Mg supply on plant growth and photosynthesis is so far missing. We performed a systematic review and meta-analysis to collect and combine all relevant scientifically published data on the relationship between Mg nutrition and parameters that can be related to plant growth such as root and shoot biomass, harvestable yield, net CO 2 assimilation and antioxidant enzyme activities. Moreover, this data pool was used to calculate critical Mg leaf concentrations for biomass and net CO 2 assimilation for various plant species. Summarizing all studies included in our analysis, adequate Mg supply enhances net CO 2 assimilation by 140%, leading to a biomass increase of 61% compared to Mg deficient control plants. Biomass partitioning between shoot and root is not only sensitive to Mg nutrition, but highly affected by the experimental cultivation technique. If plants are grown under adequate Mg supply during initial growth stages before exposing them to Mg deficiency, the shoot-root ratio was not affected. Otherwise, the shoot-root ratio significantly decreased in contrast to Mg deficient control plants. Concentration of reactive oxygen species decreased under adequate Mg supply by 31% compared to Mg deficient plants, resulting in decreased activities of most antioxidant enzymes and metabolites under adequate Mg supply. We combined all published data relating leaf Mg concentrations to growth and found a critical leaf Mg range for dry weight between 0.1 and 0.2% which was valid for numerous crop species such as wheat, potato, rice, maize, sorghum and barley. Critical leaf Mg concentrations for net CO 2 assimilation were higher than for biomass for most species, e.g., potato, rice, citrus, and cotton. In conclusion, our evaluation can be used to identify Mg nutritional status in plants and may help to optimize fertilization strategies. It quantifies the demand of Mg for various crop and tree species for maintaining important physiological processes such as net CO 2 assimilation that is required for optimal plant growth and yield.

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Aims In water-scarce agro-environments a clear understanding of how plant nutrients like magnesium (Mg) affect plant traits related to water-use efficiency (WUE) is of great importance. Magnesium plays a crucial role in photosynthesis and is thus a major determinant of biomass formation. This study investigated the effect of Mg deficiency on leaf and whole plant water-use efficiency, δ 13 C composition, hydrogen peroxide (H 2 O 2 ) production and the activity of key enzymes involved in ROS scavenging in barley. Methods Barley (Hordeum vulgare) was grown in hydroponic culture under three different levels of Mg supply (0.01, 0.1, 0.4 mM Mg). WUE was determined on the leaf-level (leaf-WUE), the biomass-level (biomass-WUE) and via carbon isotope discrimination (δ 13 C). Additionally, concentrations of Mg, chlorophyll and H 2 O 2 , and the activities of three antioxidative enzymes (ascorbate peroxidase, glutathione reductase and superoxide dismutase) in youngest fully expanded leaves were analyzed. Results Dry matter production was significantly decreased (by 34 % compared to control) in Mg deficient barley plants. Mg deficiency also markedly reduced leaf Mg concentrations and chlorophyll concentrations, but increased H 2 O 2 concentrations (up to 55 % compared to control) and the activity of antioxidative enzymes. Severe Mg deficiency decreased biomass-WUE by 20 %, which was not reflected regarding leaf-WUE. In line with leaf-WUE data, discrimination against 13 C (indicating time-integrated WUE) was significantly reduced under Mg deficiency. Conclusions Mg deficiency increased oxidative stress indicating impairment in carbon gain and decreased biomass-WUE. Our study suggests that biomass-WUE was not primarily affected by photosynthesis-related processes, but might be dependent on effects of Mg on night-time transpiration, respiration or root exudation.

Background Nitrogen (N) and potassium (K) are two important mineral nutrients in regulating leaf photosynthesis. Studying the interactive effects of N and K on regulating N allocation and photosynthesis (P n ) of rice leaves will be of great significance for further increasing leaf P n , photosynthetic N use efficiency (PNUE) and grain yield. We measured the gas exchange of rice leaves in a field experiment and tested different kinds of leaf N based on N morphology and function, and calculated the interactive effects of N and K on N allocation and the PNUE. Results Compared with N0 (0 kg N ha − 1 ) and K0 (0 kg K 2 O ha − 1 ) treatments, the P n was increased by 17.1 and 12.2% with the supply of N and K. Compared with N0K0 (0 kg N and 0 kg K 2 O ha − 1 ), N0K120 (0 kg N and 120 kg K 2 O ha − 1 ) and N0K180 (0 kg N and 180 kg K 2 O ha − 1 ), N supply increased the absolute content of photosynthetic N (N psn ) by 15.1, 15.5 and 10.5% on average, and the storage N (N store ) was increased by 32.7, 64.9 and 72.7% on average. The relative content of N psn was decreased by 5.6, 12.1 and 14.5%, while that of N store was increased by 8.7, 27.8 and 33.8%. Supply of K promoted the transformation of N store to N psn despite the leaf N content (N a ) was indeed decreased. Compared with N0K0, N180K0 (180 kg N and 0 kg K 2 O ha − 1 ) and N270K0 (270 kg N and 0 kg K 2 O ha − 1 ), K supply increased the relative content of N psn by 17.7, 8.8 and 7.3%, and decreased the relative content of N store by 24.2, 11.4 and 8.7% respectively. Conclusions This study indicated the mechanism that K supply decreased the N a but increased the N psn content and then increased leaf P n and PNUE from a new viewpoint of leaf N allocation. The supply of K promoted the transformation of N store to N psn and increased the PNUE. The decreased N store mainly resulted from the decrease of non-protein N. Combined use of N and K could optimize leaf N allocation and maintain a high leaf N psn content and PNUE. Electronic supplementary material The online version of this article (10.1186/s12870-019-1894-8) ...

Enhancing crop water-use efficiency (WUE) is a major research objective in water-scarce agroecosystems. Potassium (K) enhances WUE and plays a crucial role in mitigating plant stress. Here, effects of K supply and PEG-induced water deficit on WUE of spring wheat (Triticum aestivum L. var. Sonett), grown in nutrient solution, were studied. Plants were treated with three levels of K supply (0.1, 1, 4 mM K + ) and two levels of PEG (0, 25%). WUE was determined at leaf level (WUE L ), at whole-plant level (WUE P ), and via carbon isotope ratio (δ 13 C). Effects of assimilation and stomatal conductance on WUE L were evaluated and compared with effects of biomass production and whole-plant transpiration (E P ) on WUE P . Adequate K supply enhanced WUE P up to 30% and by additional 20% under PEG stress, but had no effect on WUE L . E P was lower with adequate K supply, but this effect may be attributed to canopy microclimate. Shoot δ 13 C responded linearly to time-integrated WUE L in adequately supplied plants, but not in K-deficient plants, indicating negative effects of K deficiency on mesophyll CO 2 diffusion. It is concluded that leaf-scale evaluations of WUE are not reliable in predicting whole-plant WUE of crops such as spring wheat suffering K deficiency.

Downy mildew caused by Plasmopara viticola is one of the most destructive diseases of Vitis vinifera worldwide. Grapevine breeding programs have introgressed P. viticolaresistant traits into cultivated V. vinifera genotypes and launched interspecific hybrids with resistance against downy mildew. In general, pathogen infection affects primary metabolism, reduces plant growth and development and modifies the secondary metabolism toward defense responses, which are costly in terms of carbon production and utilization. The objective of this work was to evaluate the photosynthesis impairment by inducible defenses at the leaf level in V. vinifera cultivars resistant to P. viticola. Photosynthetic limitations imposed by P. viticola in susceptible and resistant grapevine cultivars were evaluated. Histochemical localization of hydrogen peroxide and superoxide and the activity of ascorbate peroxidase were assessed. Measurements of leaf gas exchange, chlorophyll fluorescence and the response of leaf CO 2 assimilation to increasing air CO 2 concentrations were taken, and photosynthetic limitations determined in cultivars Solaris (resistant) and Riesling (susceptible). The net photosynthetic rates were reduced (−25%) in inoculated Solaris plants even before the appearance of cell death-like hypersensitive reactions ("HR"). One day after "HR" visualization, the net photosynthetic rate of Solaris was reduced by 57% compared with healthy plants. A similar pattern was noticed in resistant Cabernet Blanc and Phoenix plants. While the susceptible cultivars did not show any variation in leaf gas exchange before the appearance of visual symptoms, drastic reductions in net photosynthetic rate and stomatal conductance were found in diseased plants 12 days after inoculation. Decreases in the maximum Rubisco carboxylation rate and photochemical impairment were noticed in Riesling after inoculation with P. viticola, which were not found in Solaris. Damage to the photochemical reactions of photosynthesis was likely associated with the oxidative burst found in resistant cultivars within the first 24 h after inoculation. Both chlorophyll degradation and stomatal closure were also noticed in the incompatible interaction. Taken together, our data clearly revealed that the defense response against P. viticola causes a photosynthetic cost to grapevines, which is not reversible even 12 days after the pathogen infection.

Evaluation of nitrogen (N) status by leaf color is a kind of classic nutritional diagnostic method. However, the color of leaves is influenced not only by N, but also by other nutrients such as potassium (K). Two-year field trials with a factorial combination of N and K were conducted to investigate the effects of different N and K rates on soil plant analysis development (SPAD) readings and leaf N, K, magnesium (Mg), and chlorophyll concentrations. Visual inspections in leaf greenness revealed darker green leaves with increasing N rates, while paler green leaves with increasing K rates. Data showed that SPAD readings, chlorophyll, N and Mg concentrations, and the chloroplast area increased significantly with raising N rates, while declined sharply with the increase in K rates due to the antagonistic relationships between K + and NH 4 + as well as Mg 2+. It was also probable that the increase in K promoted the growth of leaves and diluted their N and Mg concentrations. The paler leaf appearance resulting from the application of K may overestimate the actual demand for N in the diagnosis of rice N status. The strong antagonistic relationships between K + , NH 4 + , and Mg 2+ should be considered in rice production and fertilization.