THE SHOOT AND ROOT COMPETITION FOR CARBON ASSIMILATION IN THAI CASSAVA (Manihot esculenta Crantz of cv. Kasetsart 50)

It is well known that cassava shoot and root development are limited
by carbon assimilation between shoot and root growth. Though the storage root
appears two months after planting, leaf biomass appears to be the dominant
investment. Normally, cassava plants should decrease their investment in shoots at
6 MAP. If the shoots have preference over tuberous root growth in the competition
for carbon assimilates for too long, it will affect final yield. In our study, the KU50
variety showed root preference after 4 months of planting (MAP). Interestingly, the
shoot and root competition of the KU50 variety still occurs even in the mid-stage
(5-8 MAP) and late-stage (9-12 MAP) of the growth cycle, though it is not so
obvious. Mid-stage fluctuation of biomass of root and shoot ratio can be up and
down in the range of 5–10%, and late-stage root and shoot ratio are changed in the
range of 3–5% depending on the plant age. The results of a field study with various
water treatments also revealed that the carbon partitioning ratio between shoot and
root biomass is a phenotypic trait in the KU50 variety, though environmental factors
such as water stress had a minor effect on the so-called partitioning before 4 MAP.
This trend is considered to be phenotypic traits which are heavily influenced by
variety and will be a good key factor for plant breeding programs and estimation of
harvesting time for most benefits for farmers.