Distribution of current photosynthate in two Guinea grass (Panicum maximum Jacq.) cultivars

In a glasshouse experiment, different tiller categories (main, young primary, and old primary) of two Guinea grass cultivars, Mombaca and Tanzania, were C-14-labelled to investigate C translocation between tillers. In both cultivars, young primary tillers retained less radiocarbon (79%) than main (86%) and old primary (87%) labelled tillers, suggesting that the photosynthetic capacity of the young tillers exceeds their capacity either to store or use that photosynthate for growth and maintenance. In cv. Tanzania the old primary tillers translocated more photoassimilate to their daughter tillers and to the rest of the plant than cv. Mombaca, suggesting either higher vascular connectivity between tillers or stronger diurnal fluctuations in sink-source balance in Tanzania than Mombaca. For unlabelled tillers, specific activities were almost always higher in roots than in shoots, although total radiocarbon uptake by roots was reduced in younger tillers with low root mass. Where young primary tillers were labelled, the largest single repository of exported photoassimilate was the main tiller roots, indicating that daughter tillers may make an important contribution to the maintenance of older roots on mature tillers.