The Phosphorus-Iron Relationship in Genetical Chlorosis.

Variegation of leaves is found widely distributed in the genera of the plant kingdom. Many forms are of great intrinsic beauty and are therefore a common feature in estates and gardens. The chlorotic areas usually appear as pale edges of varying width and regularity around and overlying an otherwise apparently normal green leaf, but very often whole leafy branches may appear colourless while others may grow out normally green, indicating that the variegated plant is in fact a chimaera. Some forms of variegation are due to a virus infection as in Abutilon striatum Thompsonii (5). Some types of virus infection cause a marked chlorosis of the leaf; sugar beet yellows being a case of such infection. In the case of June Yellows in Auchincruive Climax strawberry, no virus has so far been identified and this form of chlorosis has been suggested to be due to "genetical breakdown " (10). Julius Sachs, using his classic method of growing plants in water culture, first demonstrated that plants required iron for chlorophyll formation; the application of an iron salt to the chlorotic leaves restoring the green colour. Olsen (8) observed that the leaves of chlorotic plants invariably contained more phosphorus than did normal green plants and was of the opinion that iron in chlorotic plants was immobilized as insoluble ferric phosphate. The discovery that chlorotic leaves often contained more iron than did green leaves tended to obscure Olsen's findings. This condition was often observed in plants growing on highly calcareous soils (17), and this type of chlorosis was accordingly termed " limeinduced." Lindner & Harley (6) were able to show that in lime-induced chlorosis there existed a definite ratio between the calcium and potassium contents of the leaves, healthy green leaves having high ratios while in chlorotic leaves the ratio was invariably low. For purposes of comparison they analysed the white and green leaves of Spiraea as an example of genetical chlorosis but could find no pertinent relation in the constituents of the ash. De Kock and Strmecki (2), studying the growthpromoting effect of a lignite, obtained results which clearly indicated the existence of a phosphorus-iron ratio effect in mustard, the ratio being higher for chlorotic plants, while very low values indicated chlorosis due to iron toxicity. It has similarly been found that in mustard plants grown at pH 7.8 in the presence of various forms of iron, the trend in phosphorus-iron ratio was maintained, chlorotic plants having consistently higher ratios (1). From examination of the work of other authors