Dear Sato,

Nitrogen is an important structural and functional component of proteins, nucleic acids, ATP, coenzymes [such as NAD(P)], pigments (such as chlorophylls) etc. Hence, overall growth & Development, vigour, developmental stage and functional role to be played  determine their nitrogen levels of living systems including plants or their parts/organs (like leaves). Rubisco being the most abundant protein, it can have role in over all nitrogen level, but the nitrogen level in leaves and other organs depends of wide variety of factors other than Rubisco. Nitrogen certainly has a very important role in regulating the photosynthetic capacity as it constitutes a vital structural and functional component of (1) chlorophylls (the key molecules responsible for harnessing light energy); (2) all proteins associated with photosystems, LHCPs, etc. that generate assimilatory power; (3) ATP, NADP; (4) all enzymes associated with Calvin-Benson Cycle besides Rubisco etc.

Biochemistry is simple and is the base for understanding all biological processes. 

Dear Sato,

Nitrogen is an important structural and functional component of proteins, nucleic acids, ATP, coenzymes [such as NAD(P)], pigments (such as chlorophylls) etc. Hence, overall growth & Development, vigour, developmental stage and functional role to be played  determine their nitrogen levels of living systems including plants or their parts/organs (like leaves). Rubisco being the most abundant protein, it can have role in over all nitrogen level, but the nitrogen level in leaves and other organs depends of wide variety of factors other than Rubisco. Nitrogen certainly has a very important role in regulating the photosynthetic capacity as it constitutes a vital structural and functional component of (1) chlorophylls (the key molecules responsible for harnessing light energy); (2) all proteins associated with photosystems, LHCPs, etc. that generate assimilatory power; (3) ATP, NADP; (4) all enzymes associated with Calvin-Benson Cycle besides Rubisco etc.

Biochemistry is simple and is the base for understanding all biological processes. 

Dear Sato,

I totally agree with P. Pardha-Saradhi. In addition to the significant role of nitrogen in plants, the importance of nitrogen can also be estimated from the fact that plants in various different environments have evolved to uptake and assimilate nitrogen in a wide variety of in-organic and organic states. They can uptake nitrates, ammonia ions, urea, amino acids, proteins, amides etc.from soil, depending upon their availability. If one form of nitrogen is not present then they can compensate by utilizing the other available ones. That is why nitrogen assimilation pathways are complex. Interestingly, each of these above mention nitrogen forms (Nitrates, ammonium etc.) also effect plant processes differently. For e.g. it has been shown that plants growing on nitrates give better growth response than on ammonia. Root architecture also changes according to the form of nitrogen given. A very interesting article to read is from my University professor who has worked in this field (Do plants need nitrate? The mechanisms by which nitrogen form affects plants by M. Andrews, J.A. Raven & P.J. Lea). I hope you find your answers in this article plus more knowledge about nitrogen. Thanks 

At high light, photosynthesis is basically limited by Rubisco content in C3 plants (or Rubisco activity, but that is another story...). Rubisco is the most abundant protein in plants, and can make up to 50% of total protein. So as you can see it is the greatest sink for nitrogen in leaves, and in my experience the nitrogen and Rubisco content in a C3 leaf are linearly correlated. This shows how the plant tries to make as much Rubisco as it can, depending on nitrogen availability (or how much fertilizer you add to the soil).