Nutrio-physiological studies on the tomato plant IV. Source-sink relationship and structure of the source-sink unit

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Abstract
In tomato plants the source exceeds the sink. This conclusion is arrived at from the following facts: (a) The net assimilation rate Increases by a partial removal of leaves and decreases by a partial or complete removal of fruits, and (b) the young developing vegetative organs, such as the apex of the stem or the lateral buds, do not consume a large amount of photosynthates because these are capable of photosynthesizing from early stages of development. Except for several leaves at the base of the stem which send their photosynthates to the roots, a tomato plant is composed of several units, each of which is composed of three leaves, a truss and a bud, This unit is the source-sink unit and the photosynthates of the leaves of a unit go preferentially to the sinks within the unit. However, this source-sink unit is not an absolute one. There is an inter-units translocation of photosynthates and the extent of it depends on the condition of the plant.