MONITORING PHLOEM UNLOADING AND POST-PHLOEM TRANSPORT BY MICROPERFUSION OF ATTACHED WHEAT GRAINS

Phloem unloading and post-phloem transport in developing wheat (Tritic um aestivum L.) grains were investigated by perfusing the endosperm ca vities of attached grains. Relative unloading ratio (RUR) and the rate of sucrose release into the endosperm cavity (SRR) were calculated, r espectively, from C-14 import and from sucrose washout from the cavity . RUR and SRR continued at or near in vivo rates over a wide range of cavity sap osmolality (90 to approximately 500 milliosmolal) and sucro se concentration (14-430 mM) and for long times (29 h). These are much greater ranges than have been observed for the endosperm cavity in vi vo (230-300 milliosmolal, and 40-120 mM, respectively), indicating tha t neither the cavity sap osmolality nor sucrose concentration are cont rolling factors for the rate of assimilate import into the cavity. The maintenance of in vivo transport rates over a wide range of condition s strongly implicates the role of transport processes within the mater nal tissues of the wheat grain, rather than activities of the embryo o r endosperm, in determining the rate of assimilate import into the gra in. RUR was decreased by high concentrations of sucrose and sorbitol, but not of mannitol. By plasmolyzing some chalazal cells, sorbitol app eared to block symplastic transport across the crease tissues, but nei ther sucrose nor mannitol caused plasmolysis in maternal tissues of at tached grains. The inhibition of RUR by KCN and carbonyl cyanide m-chl orophenyl (CCCP) and the continued import of sucrose into grains again st its concentration gradient suggest that solute movement into the en dosperm cavity might occur by active membrane transport. However, the evidence is weak, since KCN and CCCP appeared to act primarily on same aspect of symplastic (i.e. nonmembrane) transport. Also, sucrose coul d move from the endosperm cavity into the maternal tissues (i.e. oppos ite to the normal direction of sucrose movement), suggesting that tran smembrane movement in the nucellus may be a reversible process. Pressu re-driven flow into the grain could account for movement against a con centration gradient.