It's not a case of some roots being fertilized and some roots not being fertilized. When salts are dissolved in water to form an aqueous solution, equilibrium (isotonicity) of the entire volume of solution is very quickly reached. IOW, if you have an aquarium divided in two by a water-tight partition with only a small hole through the partition, and you drop a sugar cube or teaspoon of salt into one side but not the other, the entire volume of water on both sides of the partition quickly reach a state of isotonicity, which occurs when the concentration on both sides of the container are exactly the same. Pouring salt on one side and a sugar cube on the other results in an isotonic solution of both salt and sugar.
If some of the fertilizer does not dissolve into solution, it IS possible that as the volume of soil solution in the planting decreases to the point where there is no longer continuity (of moisture) between dry and moist areas of the soil column, high, localized pockets of soil solution can develop a TDS (total dissolved solids) level high enough to cause plasmolysis (fertilizer burn), but this idea isn't supported by the visual clues we have; and, as soon as the plant is watered thoroughly, the soil solution would almost immediately (within minutes) be returned to a balanced state of isotonicity. Hypertonic soil solutions (concentration of soil solution solutes higher than cell solution solutes) results in water moving out of cells. This causes a drought response that often ends in shedding of foliage, or worse, collapse and subsequent death of cells in organs other than leaves.
Auxin, a growth regulator, is a byproduct of a plant's metabolic processes. A constant flow at a given concentration of this growth regulator across the abscission zone is required to keep an abscission layer from forming and the subsequent shedding of the leaf. An abscission layer eventually forms a wall between the leaf and plant through which water, nutrients, sap, and other biocompounds cannot pass. Too, abscission of leaves does not occur because the o/a photosynthesizing ability of the entire organism might be diminished, it occurs on a leaf by leaf basis. The plant's chemical messengers inform plant central who is and who isn't making an o/a contribution to energy reserves. Those leaves unable to create as much energy as they use are targeted and shed. Auxin plays the same roll in shedding as the consequence of a dearth of light as it does during drought stress, nutritional deficiencies of mobile nutrients, or as a result of mechanical damage. All these stress forms affect auxin synthesis.
Fruit/ reproduction is not the plant's number one priority. There is a pecking order of plant parts that call for energy (energy sinks). Energy is first allocated to respiratory function, i.e. to maintenance of living tissues, then, to production of fine roots, followed by flower and seed/fruit production, then primary growth (extension of both roots and shoots), then secondary growth (thickening), and finally, the synthesis of defensive chemicals. Since energy allocations to reproductive parts occurs only after the energy needs of the plant's existing viable organs are met, we can see plants aren't programmed to sacrifice their viability on the altar of reproduction ..... not even annuals.
FWIW, it is well documented that citrus leaves on branches bearing fruit often turn chlorotic while fruit is developing, only to self-correct once fruit is removed. This is because primary growth (extension and multiplication of leaves and branches) is subordinate to reproductive parts in the pecking order of a plant's energy sinks.
Al
