Boron is required in very small quantities — typically 0.1–0.5 ppm in hydroponic solution and 0.5–3 lbs per acre in soil. Despite that tiny dose, boron is structurally integrated into plant cell walls via cross-linking of pectin molecules. Without adequate boron, new growth becomes brittle, distorted, and prone to cracking, especially in rapidly dividing meristematic tissue at growing tips and developing fruit.
It's also the master regulator of sugar transport. Boron facilitates the loading and movement of sugars through phloem tissue — deficient plants accumulate starch in leaves while starving roots and reproductive tissues of energy. The result is poor root development, reduced yield, and the classic hollow-stem symptom in brassicas. At bloom time, boron becomes critical for pollen tube germination and elongation; low boron directly reduces pollination success and fruit set in fruiting crops.
In most species boron is phloem-immobile — once delivered to a leaf, it doesn't redistribute to new growth or developing fruit. That's why a single foliar spray rarely sustains a season-long requirement, and why timing applications to coincide with flowering or fruit set tends to matter more than total dose. A handful of species (apple, almond, celery, some brassicas) translocate boron as polyol complexes, which is why a well-timed pink-bud or full-bloom foliar spray delivers disproportionate value in those crops.
For deeper coverage, see essential micronutrients for healthier plants and major vs trace elements: why your plants need both.