I was thinking a short on K at first but Turbo is right as I was able to just barely able to see the main stem in a pic yes it does appear to be asking for phosphorus. K isn't a bad idea to give though at this stage. The plants nitro content is so good. Really love that shade of green, so healthy looking. I do see some slight blotchy chlorosis and would ask has the plant received any chlorine? If not it could use a little. Potassium Chloride would be a great additive right now.