PLANT PHYSIOLOGY , Vol 109, Issue 2 557-565, Copyright © 1995 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Onset of Phloem Export from Senescent Petals of Daylily
R. L. Bieleski
The Horticulture and Food Research Institute of New Zealand Ltd., Mount Albert Research Centre, Private Bag 92-169, Auckland, New Zealand
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. Detaching
flowers did not delay senescence, but halted loss of carbohydrate and amino
acid, suggesting that loss in the intact state was due to phloem export.
Petal autolysis occurred mainly in the interveinal parenchyma, causing
vascular strands to begin separating from the petal mass. Such vascular
strands still stained with tetrazolium and accumulated sucrose, indicating
a retained viability. Their sucrose accumulation rates were high in
comparison with those of other plant tissues, and the accumulated product
was mainly sucrose. Sucrose synthesis took place in the senescent petal,
and sucrose was the principal sugar in phloem exudate, whereas
hydroxyproline and glutamine were the main transport amino acids.
[14C]Sucrose applied to attached senescent flowers was rapidly translocated
to other parts of the plant, particularly developing flower buds. Thus,
onset of phloem export allowed most of the soluble carbohydrate and amino
acid in the senescing flower to be retrieved by the plant. Additional
salvaged material came from proteins and possibly from structural
carbohydrate. Over a 12-h period, the flower switched from acting as a
strong carbohydrate sink during expansion to become a strong source during
senescence. This rapid reversal offers potential for phloem transport
studies.
