Dry matter produced and allocated to the tuber and aerial portions of yam plants (Dioscorea spp) were studied over 3 cropping seasons in the moist forest region of Nigeria, in an attempt to relate dry matter accumulation to vegetative maturity in the crop. The effect of duration of growth of yam on post-harvest dormancy was also studied. Cultivars differed both in the total amount and period of growth when most dry matter was accumulated. A higher proportion of dry matter was allocated to tubers in lower-yielding than in higher-yielding cultivars. It appears that a yam tuber is 'mature' almost as soon as it is formed.Growth analysis is a recognised method of studying productivity in crop plants.In a study of the dry matter and ash contents in different parts of the potato plant during growth, Bardyshev and Masnyi (1969) recorded the maximum amounts of dry matter at the flowering stage in some varieties and at the ripening stage in others. Similarly, Kiryukhin (1969) found that dry matter and starch yields of potato tubers were highest from late and lowest from early cultivars in Moscow Province. Gooding and Hoad (1967) made a number of growth analysis observations on the white Lisbon cultivar of Dioscorea alata to provide a basis for better understanding of how a yam grows and Sobulo (1972) studied the growth of two cultivars of white yam (D. rotundata) in Western Nigeria. Although productivity from a morphogenetic point of view means biomass of the whole plant, productivity in agriculture refers only to the harvestable portion of the plant. Thus, terms like harvest index and economic yield have been used to describe the relations between the harvestable matter and the total material produced by an economic species.Edible yams, like other root crops, store their harvestable materials in underground storage organs, which introduces a new dimension in studying productivity since there is a sink-source competition when underground portions act as a photosynthate sink as well as the above ground parts (the photosynthate source) in their requirements for photosynthates for metabolism and growth. Yield in root crops depends directly on the synthesis, transport and accumulation of dry matter. Loomis and Rapoport (1977), reviewing productivity in root crops, recognized two general patterns of partitioning of photosynthates during the growth and development of plants with underground storage organs. In phasic partitioning, as in many root and tuber crops, early growth is without competition from storage organs, and may be of extended duration, while in balanced partitioning, exemplified by the sugar beet, photosynthates are allocated to both aerial and underground parts at all times.
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