Mass transfer and 14C translocation in detached maize leaves

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Abstract
Translocation of photoassimilates was studied in detached maize [Zea mays cv Kelvedon] leaves. When 14CO2 was 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 then 14CO2 supplied to the illuminated half, the darkened half acted as a sink and 14C photosynthate was translocated acropetally, although the amount was still considerably less than that translocated basipetally. Under these experimental conditions it took slightly less than 2 h of predarkening for the apical half to develop into a sink; there was no such lag period for the basal half. Dry weight changes under steady-state photosynthetic conditions show 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 of 14C under nonsteady-state pulse feeding and transfer of dry matter under steady-state photosynthesis suggests that part of 14C supplied to freshly darkened leaves, available for basipetal translocation, is not available for acropetal translocation. Translocation in uniformly darkened leaves is weakly basipetal.