ONSET OF PHLOEM EXPORT FROM SENESCENT PETALS OF DAYLILY

During senescence, petals of attached daylily (Hemerocallis hybrid cv Cradle Song) flowers lost 95% sugar and 65% dry weight over the first 24 h, with 30% of dry weight loss coming from nonsugar components. Det aching flowers did not delay senescence, but halted loss of carbohydra te and amino acid, suggesting that loss in the intact state was due to phloem export. Petal autolysis occurred mainly in the interveinal par enchyma, causing vascular strands to begin separating from the petal m ass. Such vascular strands still stained with tetrazolium and accumula ted sucrose, indicating a retained viability. Their sucrose accumulati on rates were high in comparison with those of other plant tissues, an d the accumulated product was mainly sucrose. Sucrose synthesis took p lace in the senescent petal, and sucrose was the principal sugar in ph loem exudate, whereas hydroxyproline and glutamine were the main trans port amino acids. [C-14]Sucrose applied to attached senescent flowers was rapidly translocated to other parts of the plant, particularly dev eloping flower buds. Thus, onset of phloem export allowed most of the soluble carbohydrate and amino acid in the senescing flower to be retr ieved by the plant. Additional salvaged material came from proteins an d possibly from structural carbohydrate. Over a 12-h period, the flowe r switched from acting as a strong carbohydrate sink during expansion to become a strong source during senescence. This rapid reversal offer s potential for phloem transport studies.