Allocation of Photosynthate to Individual Tubers ofSolanum tuberosumL.

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Abstract
Potato plants (Solanum tuberosum L.) were grown in water culture. About 14 d after tuber initiation no significant differences were found between apical and basal tuber parts in 14C-uptake and partitioning into various fractions from 14C-labelled photosynthate. Thus, the fresh weight of these tubers could be used as a parameter for the sink size. The 14C-content per tuber (sink strength) 20 h after 14CO2-supply to the foliage was significantly correlated with the tuber fresh weight. No correlation was found between the 14C-concentration of the tuber (sink activity; ct. min− g− fr. wt.) and tuber fresh weight. Consequently, tuber fresh weight (sink size) per se must have been a factor which influenced sink strength. Stolon parameters characterizing their growth prior to tuber initiation (e.g. stolon volume) and their capacity for photosynthate transport (diameter, length) were measured at the time of tuber initiation. Significant correlations were found between these stolon parameters and subsequent growth of individual tubers. Anatomical studies on the proportion of various tissues in the cross sectional area of stolons support the idea of a negative relation between growth of individual tubers and transport resistance in the phloem of the stolons. It is concluded that in the initial phase of tuber growth, mainly factors outside of the tuber determine its growth rate. In later stages of tuber growth, when the sink strength increases, the competing strength of individual tubers for photosynthate is dominated mainly by factors within the tuber itself, such as their sink size and sink activity.