Radiorespirometric study of the utilization of exogenous sucrose, glucose and fructose by germinating apple pollen

The sequence of intensities with which pollen tubes utilize tested sugars from their 0.3 M solutions, is the following: sucrose >glucose >invert sugar > >fructose. The same sequence is maintained in sugar-agar media with the exception of the first two hours of incubation when sucrose, glucose and fructose are all utilized at a practically equal rate. During this period no pollen tubes were formed in the presence of fructose, in common with the sugar-free control, while in the presence of sucrose they reached a length of up to 450 µ. If sucrose was used as carrier for radioactive sugars, fructose-14C was utilized up to 12 times and glucose-14C as much as 6 times more actively than sucrose-14C. When sucrose + glucose or sucrose + fructose (in molar ratio 1 : 1) were used the pollen tubes utilized sucrose-14C at a slower rate than the corresponding monosaccharide and also at a slower rate than observed with sucrose alone. When glucose or fructose was used as carrier for sucrose-14C . the carbon dioxide-14C production by the pollen tubes was (during several time intervals of the experiment) several tens per cent higher than when sucrose served as carrier. Fructose is utilized preferentially from a medium with invert sugar. It thus appears that the capacity of the pollen enzymic systems for incorporating the sugars tested into their respiratory pathways is as follows: fructose >glucose > sucrose, this sequence being the opposite to that found for the intensity of their growth effect and to that established for the rate of their utilization, unless present in combination. The specific growth effect of sucrose cannot thus be due primarily to the rate of its absorption or to the intensty of its utilization. The rapid rate of utilization of sucrose alone is due to the more intense growth in the presence of sucrose. The results obtained further suggest that sucrose is utilized above all via its inversion when the fructose component is respired preferentially.