|MadSci Network: Botany|
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 photosynthesize. 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? References 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): 1244–1249. Detecting starch in Coleus Variegated Coleus leaf stained for starch Solute Transport: Phloem Structure & Function Phloem 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.
Try the links in the MadSci Library for more information on Botany.