Relationship Between Internode Length and the Rate of Increase of the Collar Height During Rapid Internode Elongation in Sorghum: Understanding Elongation of Internode Enclosed by a Leaf Sheath.

Sorghum (Sorghum bicolor Moench) stems are the mainly harvested organ used for feed and for biofuel production. To produce a higher stem yield, it is necessary to elucidate internode growth constituting the stems of plants grown in the field. However, internode elongation can not be observed directly from outside because the internodes are enclosed by leaf sheaths. The collar height, measured as the height from the soil surface to the collar of the youngest expanded leaf, is apparently one measurement parameter reflecting stem elongation. This study was conducted to clarify the importance of sorghum collar height. Sorghum cultivars "Kazetachi", ultra-late maturity and with large difference in internode length aboveground, were used. The collar height and the number of fully expanded leaves were recorded for every week. Internode lengths were measured by removing leaf sheaths during the growing season and at harvest. In this study, the (n)-th internode (IN n) was defined as the internode between the (n+1)-th node and the (n)-th node where the (n)-th leaf is attached. Considering the growth curve of internodes based on the number of expanded leaves found in previous reports, the internode length data of "Kazetachi" show that the rapid elongation period of IN n is the time when the (n+2)-th leaf had just fully expanded. This result demonstrates that the rapid internode elongation period can be predicted by the number of expanded leaves. The internode lengths shown on the calendar day, when the internode rapidly elongated, and rate of increase of the collar height a day at that time were synchronized. Then, the rate of increase of the collar height was greater when the internode, longer at harvest, was elongating. These results suggest that the rate of increase of collar height reflects the internode length, which increases rapidly at that time.