Symptoms
The symptoms appear on younger leaves first. They show intercostal chlorosis with veins remaining green.
The leaf blades are small ('little leaf') and show wide open petiolar sinuses and a sharply dentated margin. Furthermore, the leaf blades appear uplifted.
At further development, the chlorotic parts of the leaves show necrotic spots, die off and the leaves roll in.
Reasons
Zinc deficiency
At high P excess, Zn will combine with phosphate into the nearly water-insoluble Zn phosphate so that the roots cannot absorb it. Phosphate absorbed in excess leads to a partial inactivation of Zn, especially its specific function in the synthesis of auxins.
Magnesium deficiency produces similar chlorosis, but starts on older leaves.