Symptoms of potassium deficiency in citrus trees.
Potassium deficiency in citrus trees is expressed by older leaves' chlorosis-yellowing-bronzing, and reduced size.
The leaves also fold down (abaxially). The fruits show the following symptoms: they are smaller and have finer and smoother rind, they show high incidence of creasing and splitting, and tend to drop prematurely. Fruits may not fully develop color-wise, and they are lower in acidity and in vitamin C. Also, their shelf life may be reduced, due to higher sensitivity to stem-end rot, and green mold.
Since the potassium cation (K+) is rather mobile in both the xylem and the phloem transport systems of the plant, and since the plant is programmed to prefer the development of young tissues, the tendency in most plants is to extract the K from the old organs and tissues, and to pass it to the newer ones.
This is the reason for the general phenomenon that K deficiency shows up firstly on the older and more mature foliage of the plant.