You definitely have a phosphorus and copper deficiency both together. Search for yourself on the net or YouTube for cannabis deficiencies and the difference between mobile and in mobile nutrients. Anyway long story short, when you flip to flower before hand typically a grower would use a preflower nutrient. One that has a small amount of nitrogen and higher than normal amounts of phosphorus and potassium. But especially the phosphorus before flowering in combination with copper, which is a micro nutrient and rare to have a problem with copper deficiency. Except in your case, your plant wants more phosphorus to set the bud sights during the stretching phase. It uses higher amounts than normal to do this along with copper, iron and silica. All three of these are mirco nutrients. N-P-K are your macro nutrients or top 3 mobile ones. So you have a phosphorus deficiency as indicated by the premature purpling between the veins of the leaves and filling out towards the edges, this is not to be confused with a magnesium deficiency which is indicated in the same patter except the plant would brown and yellow around the veins themselves which remain green and healthy. A copper deficiency is indicated by the curling of the tips and ends of the leaves. Both of these nutrients work in conjunction to create bud sights during early flower so remember always use more phosphorus before flowering and only before flower during the first and second weeks. Because what happened was, the plant tried to compensate for the lack of phosphorus and keep going with it and used up your copper as well. Again check into mobile and in mobile nutrients. Because the top 4 and only mobile nutrients N-p-k and magnesium are the most used nutrients. They are mobile meaning, they are stored in the lower and oldest leaves on the plant. So knowing the deficiency and toxicity early on will help you keep your plant healthy. Both well feed but always a bit hungry. You want to give your plant just enough N-p-k to keep it hungry but not over feed. Since the mobile nutrients are stored in the lower old leaves. You can tell if they have had to mantle nutes. The leaves around the bottom will be very dark green and curling down with a tiny brown tip on each end. If they have had too little, they will be yellowing and curling upward and not down. Feeding just right and the bottom leaves should be just a shade darker than the rest of the plant. Hope this helps you and happy growing.
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