Question

Which of these statements about transport in plant is incorrect?

• A. The direction of movement in the phloem can be upwards or downwards, i.e., bi-directional.
• B. Phloem sap is mainly water and sucrose, but other sugars, hormones and amino acids are also transported or translocated through phloem.
• C. Xylem transports only inorganic nutrients while phloem transports only organic materials
• D. The chief sinks for the mineral elements are the growing regions of the plant, such as the apical and lateral meristems, young leaves, developing flowers, fruits and seeds, and the storage organs

Answer 1

Answer: (C)

Phloem sap is mainly water and sucrose, but other sugars, hormones, and amino acids are also transported or translocated through the phloem. Food, primarily sucrose, is transported by the vascular tissue phloem from a source to a sink. Usually, the source is understood to be that part of the plant which synthesizes the food, i.e., the leaf, and sink, the part that needs or stores the food. But, the source and sink may be reversed depending on the season, or the plant's needs. Sugar stored in roots may be mobilized to become a source of food in the early spring when the buds of trees, act as a sink; they need energy for growth and development of the photosynthetic apparatus. Since the source-sink relationship is variable, the direction of movement in the phloem can be upwards or downwards, i.e., bi-directional. An analysis of the xylem exudates shows that though some of the nitrogen travels as inorganic ions, much of it is carried in the organic form as amino acids and related compounds. Similarly, small amounts of P and S are carried as organic compounds. In addition, a small amount of exchange of materials does take place between xylem and phloem. Hence, it is not that we can clearly make a distinction and say categorically that xylem transports only inorganic nutrients while phloem transports only organic materials