Tuber Bulking

The deposition of dry matter, fructans, protein, and sugars is not uniformly distributed across the various tissue types of the tuber (Mazza, 1985). The highest concentration of each is in the cortex and progressively decreases inward, with the pith having the lowest concentration. The distribution of assimilates, however, is relatively equal from the proximal to distal ends of the tuber. 

The inulin-containing vacuoles also often contain vesicles (Kaeser, 1983), more so in cells adjacent to the interfascicular cambium, and decreasing in number with increasing distance from the cambium. The largest are localized within the vacuole and contain fibrous or granular material. It is thought that the vesicles form in the cytoplasm and function in the transport of sucrose into the vacuole (for additional details, see Section 10.8). 

The tubers become dormant in the early fall prior to completion of their development (Steinbauer, 1939), after which they will not sprout even under favorable conditions. The exact timing of the onset of dormancy varies among individual tubers on the plant and with production location, clone, and other factors. Large, more mature tubers are the last to enter into dormancy, with dormancy initially established in the rhizomes and smaller, younger tubers. The onset of dormancy appears to be a gradual process, such that not all of the tubers become dormant simultaneously. 

The degree of dormancy varies considerably among clones and within individual clones, such that one tuber will sprout while others may be delayed considerably. Boswell (1932) found that the time required for 50% of the tubers to sprout (145 clones), when not subjected to a cold treatment to fulfill the dormancy requirement, ranged from 54 to 200 days, with 5 to 6 months typical for most cultivars. 

Biology and Chemistry of Jerusalem Artichoke Helianthus tuberosus L. 

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The onset of tuberization entails rapid cell division and expansion, during which time the initial structural framework required for assimilate storage is established, a developmental step that occurs well before the influx of carbon during tuber bulking. While tuber initiation appears to be in part controlled by carbohydrate supply, bulking is strongly modulated by photoperiod, even in clones that are day neutral for flowering. 

Timing during the development cycle is also a critical factor. For example, during the first half of the developmental cycle, photosynthate is partitioned predominantly into stem storage sites (Incoll and Neales, 1970 McLaurin et al., 1999). However, with the onset of tuber bulking, allocation within the plant shifts dramatically. Now a major portion of new assimilate preferentially moves into the rapidly developing tubers, and assimilate previously stored in the stems begins to be recycled to the tubers. 

Approximately 20 weeks after planting, tuber bulking accelerated, with stored carbon redistributed from other organs into the developing tubers (Somda et al., 1999). The tubers continued to accumulate carbon until the final harvest, at which time they contained 93.3 mg g 1 dwt or 437.8 mgg fwt (Table 10.5). By the end of the developmental cycle the tubers accounted for approximately 68% of the total plant carbon content, compared to 28% for the stems and less than 2% for each of the other plant parts. Approximately 61.2, 65.3, and 42.7% of carbon in the leaves, stems/branches, and rhizomes, respectively, were redistributed to the tubers (or lost via respiratory or other processes). The stems/branches contributed the greatest percentage of carbon to the mature tubers (77.2%), followed by the leaves (14.7%) and rhizomes (1.7%). 

Interestingly, during tuber bulking the concentration of carbon (g kg-1 dwt) in the stems/branches and leaves remains relatively stable even though there is a pronounced decline in dry matter. This appears to be due to the other components of dry matter being redistributed at essentially the same... 

Timing is crucial when the crop is harvested for the tops. The optimum time is when tuber bulking is just beginning. This ensures that there will be sufficient propagation material left in the ground to form the next season s crop, but the majority of the ffuctans still remain in the aerial plant parts for harvest. Timing, therefore, varies widely with cultivar and production conditions. 

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