A method has been developed for examining the effect of locally applied ions on the initiation and early extension growth of first-order laterals of wheat roots. Features of the method are the avoidance of sealing, flexibility in the positioning of treatments, and freedom to examine the root systems during growth.In experiments in which nitrate was applied 4-5 days after germination, nitrate increased the density of branching when it was placed within 10 mm of the tip of the root axis. The response was confined to this portion. Nitrate also stimulated the extension growth of the laterals, whether or not it had been present at the time of their initiation. Again the response was confined to laterals in the region supplied with nitrate even though large quantities were absorbed and translocated to the shoot.

SUMMARYA quantitative study has been made of the root systems of two varieties of barley (Hordeum vulgare L. vars. IMaris Badger and Proctor) grown in water culture under three nutrient regimes -complete nutrient supply and deficiencies of potassium and phosphorus. The weight of the root systems, and the number, length and diameter of the seminal and nodal axes and laterals were measured. From these measurements, surface area and volume were calculated, and to show changes in the relationships between these quantities an expression of the fineness of the root systems, termed the average diameter, was derived. This paper describes the root systems after 4 weeks growth.Maris Badger was more affected than Proctor by nutrient deficiency, but in both varieties the main effect was on the size of the nodal, as opposed to the seminal, root system. The number of axes and the length of the primary laterals were greatly reduced by both potassium and phosphorus deficiency but, by comparison, the mean length of the axes, the number of primary laterals per axis and the diameter of all types of root member were little affected.Deficiency of potassium reduced the length of the primary laterals to a greater extent than that of phosphorus; in addition, it completely inhibited the formation of secondary laterals. Consequently, the average diameter of the root systems of the potassium-deficient plants was greater than that of the phosphorus-deficient plants.Interactions between variety and nutrition were highly significant statistically, especially with respect to the development of laterals.

Previous papers have reported that relations between the total number, length, surface area, and volume of graminaceous root members tend to remain roughly constant during vegetative growth. Through the use of a model of the extension and branching of a seminal root of barley (Hordeum vulgare L.), which was developed for the purpose, an attempt has now been made to determine the properties of root growth responsible for the phenomenon.The present paper introduces the study and describes the model. The model takes advantage of the fact that the extension and branching of cereal root members grown in homogeneous media proceeds at approximately constant rates for lengthy periods. The overall dimensions of roots can therefore be determined by reference to formulae representing time and a limited number of properties of each type of root member.The validity of the model for the use intended is demonstrated by testing the underlying assumptions and checking the model against actual data. Other possible roles for the model are suggested.Part II of the series (Hackett and Rose 1972) reports the results and inferences from manipulation of the model.

SUMMARYRelationships between the number, length, surface area, volume and dry weight of the root members of two varieties of barley {Hordeum vulgare L. vars Maris Badger and Proctor) were examined over the first 4 weeks of growth in water culture. Plants given a complete nutrient supply were compared with plants deficient in potassium or phosphorus.Although there were large effects of age, variety and nutrition on all the root dimensions, relationships between the dimensions varied comparatively little. In consequence, effects of age, variety and nutrition on the rate of nutrient uptake per unit of root were qualitatively similar whichever dimension ofthe root system was considered.It is shown by reference to descriptions of root systems published elsewhere that constancy in relationships between root dimensions occurs in solid media also. This property of root systems, which appears to result quite straightforwardly from the contrasting morphological characteristics of root axes and laterals, may help to simplify some aspects of the study of nutrient absorption by roots.