Minerals are present in the soil in sufficient amount.
(i) Do plants need to adjust the types of solute that reach xylem.
(ii) Which molecules help to adjust this?
(i) 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 amount of P and S are carried as organic compounds. In addition, small amount of exchange of materials does take place between xylem and phloem, (ii) Mineral ions are frequently remobilised, particularly from older, senescing parts. Older dying leaves export much of their mineral content to younger leaves. Similarly, before Iqaf fall in decidous plants, minerals are removed to other parts. Elements most readily mobilised are phosphorus, sulphur, nitrogen and potassium. Some elements that are structural components like calcium are not remobilised.