Iron Transport in Pea Plants.

Iron has long been recognized as an essential element for plant growth, but there has been little effort to determine how iron moves in normal, green plants. Attention has focused on chlorosis, which has frequently been ascribed to iron immobilization or inactivation rather than to iron deficiency (5, 6, 30, 31, 32). However, recent evidence shows a clear relationship between total iron content and chlorophyll content (2, 20, 21, 29). Previous evidence to the contrary can often be explained as a failure to wash leaves prior to analysis (20), or a failure to maintain a constant supply of iron (21). Viewing iron chlorosis as an iron deficiency emphasizes the need to study iron uptake and transport rather than iron inactivation. Ion uptake from a nutrient solution into the roots and to the shoots of a plant has been described as an active process, dependent upon the metabolic activity of the root cells (4, 7, 34), and as a passive process dependent upon mass flow in the transpiration stream (11, 17, 18, 19, 22). The opposing viewpoints were recently reviewed by Russell and Barber (33). Many of the relations between transpiration and ion transport were elucidated by Broyer and Hoagland (7). The important feature of their conclusion is that it postulates an active secretion into the xylempossibly against a concentration gradient. On the other hand, Hylmo's (17) extensive experiments showed a close correlation between transpiration and salt uptake, and led him to believe that such metabolic processes are of little importance, especially for polyvalent ions. The evidence that Ca+ + is very slowly accumulated by excised barley roots (26) appeared to support this contention and suggested the possibility that iron moves passively across the root in the transpiration stream. Schmid and Gerloff (35) have shown that a naturally occurring iron chelate could account for iron movement in the xylem at pH's which would otherwise cause it to precipitate. But how does iron, at these same pH's, get into the plant in the first place? The unique problems which are posed by iron's insolubility at biological pH's have led many investigators to conclude that plants growing in the soil may take up iron from insoluble particles (9,12, 15). A similar phenomenon may occur in solutiongrown plants, though in this case it has generally been implied that iron is taken up from a soluble phase. For example, Glauser and jenny (12), who are strong proponents of a contact mechanism for nutrient uptake from soils, stated that their percolating nutrient solution at pH 6.1 contained enough dissolved iron to satisfy the needs of their plants. Rather than dissolved iron, it is possible they were dealing with colloidal iron which would also have passed through their percolating system. In other cases, the passage of nutrient solution-iron through filter paper (28, 38) has encouraged the belief that more iron remains in solution than calculations based on the solubility product of Fe (OH)3 predict. However, there is no evidence to substantiate this belief. This study was undertaken to answer two of the basic questions concerning iron transport in green plants: A, Is the passage of iron from a nutrient solution to the shoots a passive process, or is it dependent upon the metabolic activity of the root? B, Do plants growing in inorganic nutrient solutions take up iron from insoluble iron particles?