Control of photoassimilate movement in source-leaf tissues of Ipomoea tricolor Cav.

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Abstract
Autoradiographs obtained from discs from mature, peeled Ipomoea tricolor Cav. leaves indicated that a 15-min 2 mM p−chloromercuribenzene sulfonic acid (PCMBS) pretreatment did not prevent the accumulation of 14CO2−derived 14C-photosynthate within the minor veins. However, accumulation of 14C-photosynthate within these veins was almost, or completely prevented by treatment of the leaf discs at pH 9.0. Peeled Ipomoea leaf discs were also used to study the kinetics of exogenously supplied [14C]sucrose; influx occurred by the combined operation of a saturable and a first-order kinetic mechanism. Both transport mechanisms were inhibited by a 15-min 2 mM PCMBS pretreatment or by treatment at pH 9.0. An analysis of autoradiographs obtained on leaf discs that had been fed exogenous [14C]sucrose, showed that whereas all cells of the leaf took up sucrose at pH 5.0, uptake at pH 9.0 occurred primarily into cells of the minor veins. In contrast with 14C-photoassimilate experiments, PCMBS pretreatment, at either pH 5.0 or pH 9.0, resulted in a reduction in the radioactivity within the minor veins when the label was supplied exogenously as [14C]sucrose. Kinetic experiments conducted with mesophyll cells isolated enzymatically from peeled source-leaf discs indicated that [14C]sucrose influx also occurred by a saturable and a first-order kinetic mechanism. Comparative studies between leaf discs and isolated mesophyll cells showed that mesophyll cells could account for 75% of the total uptake occurring into the peeled leaf discs. These results indicate that the effects of PCMBS and alkaline pH on net sucrose uptake by leaf tissues are not exerted solely on phloem tissues, and that some of the effects of these treatments may be attributable to an inhibition of symplasmic transport to the minor veins.