Variability of Relationships between Transpiration, Shoot and Root Metabolism, and Nutrient Uptake in Intact Plants
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Reasons are given for rejecting recent criticisms of methods used in earlier work on relation ships between transpiration and the transfer of nutrients to the shoots of intact plants. Hatrick and Bowling (1973) recently claimed that they had repeated some experi ments which Shorrocks and I described 14 years ago (Russell and Shorrocks, 1959). They obtained different results and state that our conclusions are incorrect. A comparison of the two papers, however, shows that Hatrick and Bowling over looked important differences both in objective and design between our study and theirs. Moreover, contrary to the impression which their frequent reference to us may create, Hatrick and Bowling's comments have no direct bearing on the main conclusions of our paper ; they refer only to work undertaken to test the validity of one part of our experimental approach. A brief reference to our objectives and methods is necessary to place this matter in perspective after the lapse of time. It had been suggested in several recent publications (e.g. Hylmo, 1955, 1958; Epstein, 1956a, b; Kramer, 1957; Kylin and Hylmo, 1957) that mass flow in water was an important mechanism by which nutrients could reach the shoots of intact plants from the solution external to roots. To test this hypothesis, which conflicted with much earlier work (e.g. Broyer and Hoagland, 1943), we compared the arrival of nutrients in the shoots of intact barley and sunflower plants with the loss of water from them under contrasting conditions of external nutrient supply, initial salt status, and transpira tion rate. Our conclusion that mass flow could not make an important contribution to nutrient transfer rested primarily on the fact that the apparent concentration in the transpiration stream bore no constant relationship to that in the external medium and under some circumstances exceeded it by a considerable factor. Transfer across the root was considered to depend on the expenditure of metabolic