Plant and Soil 147: 87-93, 1992. © 1992 Kluwer Academic Publishers. Printed in the Netherlands. PLSO 9576 Effect of mutual shading on the emergence of nodal roots and the root / shoot ratio of maize SABINE DEMOTES-MAINARD and SYLVAIN PELLERIN 1 INRA, 28 rue de Herrlisheim, B.P. 507, F-68021 Colmar, France. ICorresponding author Received 16 April 1992. Revised August 1992 Key words: mutual shading, nodal root, root morphology, root/shoot ratio, root system, Zea mays L. Abstract The effect of mutual shading on the root/shoot ratio and on the number of nodal roots of maize was studied. Plants of two varieties (Dea and LG2281) were grown in individual pots of 9 L, at three plant densities: 7.5, 11 and 15 plants m -2. A control experiment was carried out in order to study if root growth was affected by the small size of the pots. Maize plants (cv Dea) were grown at a low plant density (7.5 plants m -2) in pots of two different volumes (9 and 25 L respectively). In both experiments plants were watered every two hours with a nutrient solution. Some plants were sampled at five dates in the main experiment and the following data were recorded: foliar stage; root, stem and leaf dry weight; number of root primordia and number of emerged roots per phytomer. The final sampling date occurred at silking. Results of the control experiment showed that the root biomass was lower in small pots but the number of nodal roots per phytomer was not affected. Results of the main experiment showed that the total plant biomass and the root/shoot ratio were lower at high plant density. The number of emerged roots was strongly reduced on the upper phytomer (Ps). This reduction was mainly due to a lower percentage of root primordia which elongated. A proposed interpretation is that the number of roots which emerge on upper phytomers is controlled by carbohydrate availability. Introduction It has been clearly established that carbohydrate availability from aerial parts of the plant can affect the growth of the root system. Aresta and Fukai (1984), Buttery and Stone (198~8), Lam- bers and Posthumus (1980), Tester et al. (1986) and Vincent and Gregory (1989a) have observed on several species a decreased root biomass and total root length if plants were grown at low light intensity. In field conditions, Vincent and Gre- gory (1989b) have observed on wheat a relation- ship between the intercepted photosynthetically- active radiation (PAR) and the total root length. Consistently, Chaudhuri et al. (1990) and Del Castillo et al. (1989) have observed an increase in root biomass and total root length when plants were grown in a carbon dioxyde-enriched aerial environment. Most of these studies have also shown that the root/shoot ratio was lower at low light intensity or low CO 2 concentration. This indicates that root growth is relatively more reduced than shoot growth when plants are grown at low light intensity or low CO 2 concen- tration. In most of these studies, the growth of the root system was considered in terms of total weight or length, with little attention being paid