Translocation of Photosynthate in Bulbing Onions

[14C]urea was injected into the leaf lumens of growing onion plants (Allium cepa L.) and the redistribution of photosynthate was investigated after 24 h. Injections were into either an older mature leaf or into a younger recently enlarged leaf. Most of the photosynthate was retained either in the green blade of the treated leaf or in the enlarged leaf base (scale) of that leaf. Of the relatively little photosynthate exported, most went to the innermost scales, especially during bulb enlargement. The innermost (younger) scales acquired photosynthate from both the nearby leaves and from the more remote older leaves. Relatively small levels of radioactivity were found in other leaf blades. Roots obtained small amounts of photosynthate from the oldest leaves. No label was exported to scales of outer leaves. This pattern was modified, however, by excising photosynthetic area of such leaves. Scales of intermediate age, when deprived of a local supply of photosynthate, acquired translocated material from both older and younger scales. This alteration in source of photosynthate took less than 24 h to develop.