Relations between shoot to root dry weight ratio (S :RKey-words: Phaseolus vulgaris; Pisum sativum; Triticum aestivum; dry matter partitioning; macronutrients; nitrogen; pea; pinto bean; protein; wheat. INTRODUCTIONSupply of all macronutrients (N, P, S, K, Mg and Ca) can affect the partitioning of dry matter between shoot and root of higher plants. There is general agreement that shoot to root dry weight ratio (S : R) decreases when growth is limited by N supply (Andrews 1993), S supply (Clarkson, Saker & Purves 1989;Zsoldos et al. 1990;Ingestad & Ägren 1991) or P supply (Adalsteinsson & Jensen 1988;Fredeen, Rao & Terry 1989;Rufty, MacKown & Israel 1990;Ingestad & Ägren 1991;Cakmak, Hengeler & Marschner 1994). In contrast, S : R has been reported to decrease or increase with decreased growth associated with K deficiency (Asher & Ozanne 1967;Drew 1975;Steen 1984;Cakmak et al. 1994;Ericsson 1995), Mg deficiency (Will 1961;Cakmak et al. 1994;Ericsson & Kähr 1995) or Ca deficiency (Joham 1957;Ericsson 1995). There are reports for several higher plant species that S : R changes with growth/development independently of nutrient supply (Bastow-Wilson 1988;Gedroc, McConnaughay & Coleman 1996). Therefore, in some cases, the apparent response of S : R to nutrient supply may have been a growth/ontogenetic effect. Nevertheless, there are several studies which unequivocally show nutrient effects on S : R that are independent of growth/development (Andrews 1993;Cakmak et al. 1994).There is no general agreement as to the mechanism(s) of macronutrient effects on S : R. Bastow-Wilson (1988) reviewed models for the control of dry matter partitioning between shoot and root of higher plants and concluded that changes in S : R in response to a wide range of environmental factors, including macronutrient supply, conform to the Thornley (1972) model. In this model the factors which determine S : R are the supply of C and N substrates by shoot and root, respectively, transport of these substrates between shoot and root and incorporation of these substrates into plant structure. It was argued that structural growth of shoot and root is colimited by the local C and N substrate concentrations and that this growth acts as a sink for substrates to which further substrates diffuse from the points of supply. It was further argued that the rate of transport of C and N substrate from shoot to root and root to shoot, respectively, is proportional to the concentration gradient divided by a resistance. Hence, a decrease in C substrate acquisition would result in an increase in S : R while a decrease in N substrate acquisition would cause S : R to decrease.Dewar (1993) developed a model in which C, N and water interact to determine S : R. In this model it was assumed that a fraction of the N taken up by the root is translocated via the xylem to the shoot where it is transferred to the phloem. The remaining fraction is transferred directly to the root phloem. Phloem translocation of C and N downwards from shoot to root is driven by a shoot ...