Possibly sulfur, could be iron, but it's not nitrogen because the plant hasn't pulled any from the older tissues. Both it and the one to the left look like they could use more light and/or food. I would try a seaweed foliar, I prefer the liquid solutions, whatever you find should be of some help, along with a root feeding of such. Start with label rates, and for foliar cut those rates to 1/4 of the root feeding rate. Use a couple of drops of dish soap to ensure it sticks to the leaves, and be certain to hit the undersides of the leaves or the plant can't really use what you're giving it.