Translocation of photoassimilates was studied in detached maize leaves. When14CO2was pulse fed to either apical or basal halves of uniformly illuminated, freshly detached leaves,14C translocation was strongly basipetal. If halves of leaves were predarkened and then14CO2supplied to the illuminated half, the darkened half acted as a sink and14C photosynthate was translocated acropetally, although the amount was still considerably less than that translocated basipetally. Under these experimental conditions it took slightly less than 2ensp;h of predarkening for the apical half to develop into a sink, whereas there was no such lag period for the basal half. Dry weight changes under steady-state photosynthetic conditions, however, indicate that there is no difference between the percent of dry matter translocated basipetally or acropetally, when either the apical or basal half of a leaf is illuminated and the other half darkened. This difference between translocation of14C under nonsteady-state pulse feeding and transfer of dry matter under steady-state photosynthesis suggests that part of14C supplied to freshly darkened leaves, though available for basipetal translocation, is not available for acropetal translocation. Translocation in uniformly darkened leaves is weakly basipetal.
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