S can affect N metabolism indirectly as it is part of some amino acids & co-enzymes

which are required for N metabolism

1. S and N both are component of some amino acids (Cysteine, cystine, methionine), in absence of S synthesis of those amino acid is hampered.

2. In absence of S protein synthesis is hampered.

3. N is a component of chlorophyll, although S is not a component of chlorophyll, S helps in synthesis of chlorophyll.

Very interesting discussion . Let me add my point of view by taking some exerpts from classical review by Jamal et al ( Jamal et al ., 2010, Australian J. Crop. Sci. 4:523-529 ) . Because of central role of S and N in the synthesis of proteins, the supplies of these nutrients in plants are highly inter-related. Sulphur and nitrogen relationships were established  terms of dry matter and yield in several crops. A shortage in the S supply to the crops lowers the utilization of the available soil nitrogen, thereby increasing nitrate leaching .  It has been established that for every 15 parts of N in protein there is 1 part of S which implies that the N:S ratio is fixed within a narrow range of 15:1. The N:S ratio in the whole plant in general is 20:1 . While working on barley plants,  it was demonstrated that at the whole plant level the apparent matching of supply to demand is accompanied by an apparent linkage of SO42- to NO3- uptake. Sulfur and nitrogen both are required for the synthesis of protein, therefore, the ratio of total N to total S in plant tissue can reflect the ability of N and S in protein synthesis . Thus, a change in the ratio of reduced-N to reduced-S (NR/SR), which is a reflection of the amount of S amino acids, suggests that protein metabolism has been significantly altered and has important implications for protein quality .

Sulphur availability has a role in regulating nitrate reductase, in addition to its role in regulating ATP-sulphurylase. Moreover, nitrogen availability has a role in regulating ATP-sulphurylase as well as in regulating nitrate reductase. The synthesis of cysteine as a result of the incorporation of sulphide moiety into Oacetylserine appears to be the meeting point between N-and Smetabolism. Naturally occurring thiol compounds viz., cysteine and glutathione were shown to influence nitrate reductase activity in wheat and Brassica  . It has also been reported that S is specifically involved in nitrogen fixation in legumes and S additions significantly increased N2 fixation, nodule weight plant-1, nodule weight per unit weight of root and N2-fixation per unit weight nodule. 

Kelly et al (1972, Plant and Soil , 37:395-407) suggested organic-S=0.030xTotal -N. And interestingly , because of such relationship between N and S, the ratio between the two nutrients remained constant over different geographical regions in Pinus radiate.

Please find the PDF enclosed for further reading .

• You will find in our paper portfolio a number of research results on this subject.
• N and S both are constituents of amino acids, a shortage of one or the other therefore affects the AS synthesis directly and a step further on also the AS sequences of proteins: however this is only true for storage proteins, the ones in the embryo for instance are part of its genetic identity and in so fare without and flexibility in AS composition.
• Interesting is the interactive effect of N and S on the secondary metabolism of plants, eg.g. glucosinlates in cruciferous crops: here you will find a linear correlation between S supply and glucosinolate content, but with N its different, depending from yhich nutritional level of N you are starting: with severe N shortage additional supply of N will decrease seconfary S compound concentratin because of dillution effects, coming from a sufficient supply additional N will have no effect unless your nutritional N status becomes abnormal leading to reductions in mass production in this case you will see a decrease in the secondary compund concentration: Keep in mind that its always reletive to the nutritional status you are starting with what effect you will get when altering N or S supply!

Your question: What is the mechanism behind S influencing N metabolism (utilization) in plants?

Can applying S fertilizers affect grain filling/food translocation significantly?? If yes, why and to what magnitude?

Sulfur (s) is an important for plant growth, Excess can compete with and induces nitrogen deficiency. It is necessary for chlorophyll formation, promotes nodulation in legumes, helps develop and activate certain enzymes and vitamins, It is used in the formation of amino acids and  proteins etc.

Mechanism? Some important functions of nitrogen (N) are related with it. Integration role of S with N, greatly help to utilization of N in plants. S+N= Engergy, produced better transport from xylem and phloem of the plant.

Can applying S fertilizers affect grain filling/food translocation significantly ?. Yes S encourages nodulation that improves N use and more N utilized from different proper functions of plant. Yes yield can be insured up-to 100% by integrated use of S with N in case recommended P and K also applied. 

It appears that Sulfur can affect the Nitrogen efficiency significantly.

Check out Salvagiotti, F., et al. 2009. Sulfur fertilization improve Nitrogen uptake. Field Crop Res. 113:170-177.  This is 2 year study from Argentina pampas for wheat production.

Increase in the Nitrogen efficiency was 42% without the input of Sulfur and 70% with the Sulfur input. The Increased relative efficiency of Nitrogen was increased 70% from the no Sulfur level. Authors suggest sulfur can alleviate Nitrogen contamination of water sources due to much great efficiency.

In the United States with wheat Nitrogen to Sulfur ratio is 10 to 1 and if it is substantially higher Sulfur would probably improve crop response. Late season foliar Nitrogen and Sulfur can improve the protein and baking quality in Hard Red Wheat for bread.

When trying to optimize Nitrogen Sulfur it would be ideal to utilize soil and tissue analysis to get the best optimization under each field condition.

Sulfur can be very important for legume crops and they can satisfy the Nitrogen requirements of cereal crops in rotation. Legumes can be limited by Sulfur containing amino acids and Sulfur amendment can increase the composition of these essential amino acids that are often deficient.

When thermoelectric generation was mostly coal and pollution control was not conclusive Sulfur requirements would be easily met by emissions in the Eastern United States and North Europe. With the banning of emission and coal this will suggest Sulfur deficiency will be becoming more pressing as clean up efforts take effect.

Sincere thanks to all participants. I have compiled and assimilated all your comments. 

sorry, in German (the language of science! :-) ): https://idw-online.de/de/news106695

Its an ecological and economical calculation on reduced N efficiency due to S deficiency

on a national level!