Looks like a magnesium deficiency to me
Magnesium is the nutriment most crucial to photosynthesis. It is at the center of the chlorophyll molecule, thus giving the plant its green colour. Magnesium is also responsible for aiding the process of moving nutrients throughout the plant, especially phosphorus. It is more likely to appear in flowering as it plays a vital role in maturing the buds/flower. As a Mobile nutrient, Lower leaves first begin to yellow as veins remain dark green, creating a striped pattern. In severe cases, the leaf will dry up on the edges and curl inward.
Can be caused by:
• Improper/low pH level
• Nutrient Lockout
• Cold root temperatures (Under 18C)
For Coco: Ensure Coco has been buffered with Ca/Mg. Without buffering, Ca and Mg deficiencies are more likely to occur.
Organic Solutions:
• Dolomite Lime (Raises ph in acidic soils and has calcium and magnesium)
• Organical Magic
• langbeinite
Inorganic Solutions:
• Epsom Salts (Magnesium Sulfate) add to soil or foliar spray
• CalMag Additives