MadSci Network: Botany

Re: What is the source of sugar in nongreen plant tissues

Date: Mon Oct 3 16:14:19 2005
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Botany
ID: 1128315118.Bt

Sugars are soluble and many can be transported in the phloem. Thus, sugar can be
transported to a nonphotosynthetic part of a leaf from areas of the leaf that
can photosynthesize or from other photosynthetic leaves. (see first reference) A
starch test is better to determine which parts of coleus leaves are able to

Some nongreen parts of a leaf may be able to photosynthesize. It depends on the
color. White or pale yellow parts of leaves usually cannot photosynthesize
because they lack chloroplasts. However, red, purple, orange, yellow and blue
leaves or leaf parts may contain chloroplasts that are simply masked by large
amounts of nongreen pigments, such as anthocyanins or carotenoids. Many people
cultivate plants with nongreen, but photosynthetic leaves, such as red Japanese
maple, purpleleaf plum, blue Colorado spruce and golden Vicary privet. 

All photosynthetic plants will contain sugars but the predominant sugar will
vary depending on the species. Sucrose is the main transport sugar in most
plants. Stachyose is the main sugar transported in cucumber. Other sugars found
in phloem are raffinose and verbascose. Some plants transport large amounts of
sugar alcohols such as mannitol or sorbital (as in apple).

Glucose and fructose are not translocated in phloem presumably because they are
reducing sugars and more reactive than nonreducing sugars that are translocated.
There is usually little free glucose in plants despite that glucose is usually
written as the only carbohydrate product in the summary equation for
photosynthesis. Most glucose in plants occurs as part of larger carbohydrates
such as sucrose, starch or cellulose.  

The last reference examined sugar contents of green and white parts of a
variegated coleus leaf. That study found lower sugar contents in white parts
than in green parts. Thus, with a quantitative analysis for sugars, a difference
can be detected. Did you also test green leaf tissues of coleus and corn as
control treatments?


Turgeon, Robert and Gowan, Esther. 1990. Phloem loading in Coleus blumei in the
absence of carrier-mediated uptake of export sugar from the apoplast. Plant
Physiol. 94(3): 12441249.

Detecting starch in Coleus

Variegated Coleus leaf stained for starch

Solute Transport:  Phloem Structure & Function


Hershey, David R. 2004. Avoid Misconceptions When Teaching about Plants

Pattanagul, W. and Madore, Monica A. 1999. Water deficit effects on raffinose
family oligosaccharide metabolism in Coleus. Plant Physiol 121: 987-993.

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