Development of magnesium deficiency, ionic composition and yield of faba bean (Vicia faba L.) as affected by foliar MgSO4 fertilization
Being a macronutrient with essential functions in plant metabolism, a sufficient supply of magnesium to crops is important to gain high yields and quality. In agriculture and forestry magnesium deficiency has become a problem in some areas. Besides low soil concentrations especially in light and sandy soils, also other parameters such as high concentrations of Ca2+, K+, H+, Al3+ or low soil humidity can lead to suboptimal provision of magnesium to crops. This will lead among other things, to the formation of chlorosis in the older leaves, reduced growth and the accumulation of sugars in source leaves. Instead of soil amendment, magnesium can also be applied via foliar fertilization. This can be valuable if the magnesium soil availability of magnesium is reduced or if a rapid improvement of the plants nutrient status is needed. Magnesium foliar application has been demonstrated to improve chlorophyll concentration and biomass accumulation in oregano, maize, soybean and apples (Reay et al. 1998; Szulc et al. 2001; Dordas 2009; Teklic et al. 2009). With regards to responses of yield and quality, however, studies present differential results. The impact of foliar MgSO4 application on plant physiology has also been studied scarcely. In this study, the influence of foliar MgSO4 fertilization on different aspects, such as the distribution of the major cations Mg2+, Ca2+ and K+ in different plant compartments, the transcript abundance of enzymes related to the appearance of deficiency symptoms and the formation of yield and seed quality in Vicia faba was examined. For this purpose, faba beans were grown in nutrient solution containing either sufficient (0.5 mM) or low (0.05/0.015 mM) MgSO4. Plants growing in nutrient solution containing only low concentrations of MgSO4 were subjected to foliar applications of MgSO4 with either 0 mM, 50 mM or 200 mM. Foliar application proved to change the cation relations in the apoplast and symplast in diverse manners. The rise in apoplastically bound Mg2+ led to a decrease of Ca2+ while in the symplast the risen Mg2+ concentration affected on the K+ concentration, if high concentrations of MgSO4 (200 mM) were applied. Although both cations hold important functions in plant structure and metabolism, negative effects could not be observed with regards to the other parameters measured. It was therefore proposed that these cations can replace each other partially. The concentration of chlorophyll and the photosynthesis rate proved to be enhanced by application of 200 mM MgSO4. However, the reduced transcript abundance of Mg chelatase H under magnesium deficiency was not changed by foliar magnesium application. This might indicate that under magnesium deficiency chlorophyll synthesis is reduced which is not influenced by foliar application. Instead, a reduced chlorophyll breakdown due to ameliorated magnesium supply is possible. This study revealed that the transcription of the plasmalemma H+-ATPase isoforms is differentially regulated under magnesium deficiency. However, specific functions of different isoforms have not been revealed yet. Magnesium foliar application resulted in a significant increase of seed yield of plants growing in magnesium deficient nutrient solution. This increase was mainly attributable to the increase in the number of pods per plant. Additionally, the quality of the seeds was raised due to magnesium foliar application as significantly higher protein concentrations could be found. This might be associated to the role of magnesium in nitrogen assimilation and protein synthesis.
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