Abstract
AbstractSink demand was abruptly changed for an illuminated sugar beet source leaf by shading the six to ten other source leaves. Export of recently assimilated, labeled material underwent a transient increase and then returned to a steady rate approximately equal to the pretreatment rate. Uncovering the darkened leaves caused a transient decrease in export of 14C; following recovery there was a gradual decline. It remains to be established whether export of unlabeled reserves occurs in response to increased sink demand. The possibility that phloem loading increases in response to decreased sieve tube turgor was tested. Phloem loading of exogenous 14C‐sucrose increased when turgor in leaf cells was decreased by floating leaf discs on solutions with up to 1 M mannitol osmoticum. However, the increase appeared to be the result of plasmolysis of mesophyll cells possibly resulting from easier access to minor veins via the free space. Phloem loading in leaf discs continued undiminished even though sieve tube‐companion cell sucrose concentration exceeded a calculated value of 1 M. Regulation of export to meet sink demand by a direct response of phloem loading to a turgor or concentration set point does not appear to occur. Phloem loading may be promoted by the influx of water which drives mass flow, increasing phloem loading in response to increased velocity of transport.
Published Version
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