Abstract
Photosynthesis is the very essence of agriculture. Previous photosynthetic and transpirational studies of onion (Allium cepa) have been limited to specific developmental stages. Our study measured photosynthesis and transpiration in sixteen plants of a single short-day cultivar over an eleven week period containing both non- and bulb inductive photoperiods. Differences in weekly means for photosynthesis, leaf conductance, water use efficiency, and intercellular CO, were highly significant. Weekly photosynthetic means increased under a non-inductive photoperiod and peaked one week after initiating a bulb inducing photoperiod. A decrease and leveling period occurred as bulbs developed followed by a decrease as foliage lodged. Weekly photosynthetic and leaf conductance means were correlated and highly significant. Water use efficiency and intercellular CO, means remained fairly constant throughout the study suggesting that photosynthesis in unstressed onions was controlled by internal mechanisms instead of stomata.