ABSTRACT
This chapter shows the aspects of physiology that give insight into the behavior of crops in the field. Studies on some of the more biochemical and biophysical aspects of physiology where onion tissues have been used for convenience are not thoroughly reviewed, and are considered only when they give some insight into physiology at the "whole-plant" level. Some features of the onion plant make it a very convenient material for physiological studies. Onion bulbing is promoted by long photoperiods. Cultivars differ in the photoperiod needed to induce bulb development. Bulbs grown under photoperiods much longer than the minimum required for bulbing are small because of the rapid cessation of leaf blade production. The terms ' 'long-day", "intermediate-day'', and ' 'short-day" can cause confusion when applied to onion cultivars. These terms are widely used by onion breeders and agronomists to describe the photoperiod requirements for bulbing of different cultivars.
