Seed Coat Unloading inPisum sativum—Osmotic Effects in Attached versus Excised Empty Ovules

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Abstract
Experiments were undertaken with embryo-less ovules of Pisum sativum to study the influence of apoplastic osmolality on seed coat import and seed coat unloading.11CO2 pulse labelling along with collimated monitoring of plant tissues were used with attached ovules to measure continuously and simultaneously total pod import, import into a modified ovule and photo-assimilate washout from the seed coat of the ovule into a flow-through bathingsolution. Our results indicated that seed coat import was immediately affected by a change in the applied bathing solution osmolality, with a decrease in osmolality lowering seed coat import and an increase in osmolality increasing import. 11C-photo-assimilate washout from attached ovules was found to respond in a similar manner to the apoplastic osmolality. However, the osmotic effect on 11C-washout was a delayed response and it appears that the majority of this observed response was due to the alteration in seed coat tracer import. Further experiments with 14C-labelled, excised seed coat halves (i.e. no further import) supported this hypothesis by demonstrating that seed coat unloading (measured as 14C-photo-assimilate washout) was actually enhanced at a low solution osmolality. PCMBS had no effect on seed coat import or washout in attached, modified ovules, suggesting that photo-assimilate unloading from seed coats of Pisum does not involve a carrier protein. Studies of the spatial distribution of imported 14C in Pisum seed coats further suggest that this unloading, into the apoplast, occurs from non-phloem cell types, and that the movement of photo-assimilates from the sieve elements to the terminal unloading site occurs via symplastic transport.