Seasonal studies of vegetative buds of Helianthus tuberosus: concentration of nuclei in phase G1 during winter dormancy

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Abstract
The following vegetative buds were studied at several developmental stages during the course of the year: from November through February dormant terminal buds of subterranean tubers; in March, newly reactivated buds of young shoots; in June, terminal buds of horizontal underground stolons that will form tubers; and in July, the terminal buds of erect aerial shoots. Microdensitometric studies of DNA levels after Feulgen staining showed that during the winter period of dormancy, from November through February, the temporary arrest of growth and morphogenesis is accompanied by a concentration of nuclei in phase G1 (2C level) of a diploid cycle for all nuclei in the terminal meristems of the tubers. In March, reactivation occurs uniformly throughout the meristem without any zonal differences. The G1 phase remains predominant in the cycle and mitotic activity increases uniformly. In the meristems of young underground stolons, beginning in the month of June, signs of concentration at the 2C level again are perceptible. The nuclei of the apical meristems of erect shoots are also diploid at 2C and 4C. In the very large nuclei in the central zone of the tunica where the mitotic index is very low, the distribution of DNA levels shows that nuclei are present at all phases of the cycle. These results are discussed and compared with other species in temporary dormancy and in regard to the concept of nonpolysomatic species.