Behavior of carbohydrates in mother and daughter bulbs of tulips (Tulipa gesneriana)

Abstract

The changes in the reserve carbohydrate constituents were observed in the mother bulb scales and bulblets of tulip plants (Tulipa gesneriana) grown in a culture solution. At the time of planting, starch was the predominant reserve carbohydrate in mother bulb scales. The starch content decreased continuously until January, but starch degradation was interrupted for about 2 months until sprouting occurred. Thereafter rapid starch consumption resumed and the starch content was depleted until anthesis. The concentration of soluble carbohydrates especially the sucrose concentration in the mother bulb scales increased rapidly in winter, with maximum values of 64 mg/g FW on January 13.

Regarding the changes in the fructosylsucrose content after planting, the content of isokestose (GF₂) increased, and that of nystose (GF₃) remained constant, while the content of pentasaccharide (GF₄) and insoluble fructan with a high degree of polymerization decreased as in the case of starch. All the fructosylsucroses were consumed rapidly after sprouting, and the content was completely depleted at anthesis. Therefore, starch and the oligofructans with a high degree of polymerization (DP>5) are considered to be reserve carbohydrate sources for the development of the roots and shoots as well as for nutrient absorption before the onset of photosynthesis. On the other hand, sucrose and isokestose converted from starch and oligofructan were considered to be temporary storage carbohydrates accumulated in the mother bulb scales during the winter season.

Regarding the behavior of carbohydrates in the bulblets, the starch content increased significantly after the removal of a flower (defloration). The sucrose concentration also increased markedly after defloration, showing a maximum value of about 20 mg/g FW. The content of the fructans with a low degree of polymerization (DP<5) which was initially negligible, increased as in the case of sucrose. On May 12, the concentration of isokestose and nystose was found to be very high, i.e. 15 and 8 mg/g FW, respectively. Thereafter the concentration decreased markedly until the maturity stage. Therefore, the low DP fructosylsucroses as in the case of sucrose are considered to be carbohydrates temporarily accumulated in the bulblets, some of which could be converted to starch as maturation proceeds.

Based on morphological observations, two types of starch granules were observed in the bulb scales, one which consisted of large granules with a thin lenticular shape and the other type which consisted of small granules spherical in the shape. The small granules were produced in the bulblets just before maturation and they disappeared immediately after planting.

Abbreviations
DMSO	=	dimethylsulfoxide
DP	=	degree of polymerization
DW	=	dry weight
Frc	=	fructose
FW	=	fresh weight
GFn	=	fructosylsucroses in which n molecules of Frc residues are combined with Glc
Glc	=	glucose
Sue	=	sucrose.

Key Words:

• tulip
• bulb
• starch
• fructan.