Unidirectional transport involves transport of food in single direction .ie, Xylem transports water and nutritients unidirectionally. That means transport of water in the xylem takes place only from root to leaves.
Bidirectional transport of food invoves upward and downward movement of food through the same channel. Phloem transports food(Photosynthates) bi directionally.
Water transported through xylem is utilised in photosynthesis and most of the water is lost through transpiration. Renewed demand for water is once again supplied through the same channel. Hence, transport through xylem is unidirectional.
In phloem, during growth of the plant,food is transported from source to sink(part of plant requires or stores food). Leaves are the usual source and storage organs are the usual sink. But the storage organs become source when new buds emerge during early spring. This process get reversed as the food stored in sink is mobilised toward the growing buds of the plant through the phloem.Thus, a reverse flow of food is required.
Hence, movement through phloem is bi-directional.