Short-term Effects of Heat-girdles on Source Leaves of Vicia faba: Analysis of Phloem Loading and Carbon Partitioning Parameters

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Abstract
Petiole heat-girdle treatments (followed by a 5 min 14CO2 assimilation) were performed on mature leaves of Vicia faba, in order to assess their effect on the partitioning of photo-assimilates to the minor vein phloem. Whole leaf autoradiographic evidence indicated a high leaf-to-leaf variation in the image intensity over the minor veins (relative to the mesophyll/epidermal background) in both control and heat-girdled groups of leaves. The average degree of minor vein labelling in heat-girdled leaves, however, was found to be significantly lower than that in controls. Comparative assessment of vein labelling was based on microscopic density readings of silver grains over veinal and interveinal regions in autoradiographic images. Investigations into the cause of this alteration in vein labelling indicated no involvement of an inhibition of apoplasmic phloem loading, as both heat-girdled and control leaves of Vicia were shown to have comparable minor vein uptake of exogenously supplied 14C-sucrose. Heat-girdling was shown, however, to increase significantly the partitioning of recently fixed carbon into the insoluble (mainly starch) fraction relative to the ethanol-soluble fraction, within 12 min of the treatment. We suggest that this carbon partitioning change can primarily account for the change in vein labelling, since an increase in the insoluble fraction would result in (1) more 14C-activity remaining in the leaf mesophyll and (2) less 14C-activity going into the mesophyll export pool, and thus, less 14C-sucrose being transferred to the minor vein region. Additionally, although leaf export was completely halted in heat-girdled leaves, 14C-activity was found within the major veins as far as the point of petiole heat-girdling (following a 5 min assimilation and 4 h chase). Apparently, continued (but limited) solution flow within the sieve elements is maintained by transport pathway unloading within the treated leaves.