ROOT ELONGATION RATE IS ACCOUNTED FOR BY INTERCEPTED PPFD AND SOURCE-SINK RELATIONS IN-FIELD AND LABORATORY-GROWN SUNFLOWER

The existence of relationships between intercepted photosynthetic phot on flux density (PPFD) and growth of individual organs is somewhat con troversial. We have tested whether such relationships could account fo r the natural variability in elongation rates of taproot and secondary roots of sunflower (from 2 to 135 mm d-1), in field and laboratory co nditions. Elongation of taproot and secondary roots was recorded daily through windows in the field. A range of PPFD was obtained by followi ng day-to-day natural fluctuation for three contrasting growing period s, and by shading part of the plants under study. A parallel experimen t was carried out in a growth chamber with contrasting light intensiti es and with a (CO2)-C-14 labelling experiment. After the two-leaf stag e, i.e. when the contribution of photosynthetic carbon became apprecia ble in root growth, daily root elongation rate was closely linked to t he PPFD intercepted from 36 to 12 h before the measurement of root elo ngation. Curvilinear relationships applied to plants grown in the fiel d as well as in a growth chamber, and to shaded plants as well as to p lants subjected to day-to-day changes in intercepted PPFD. For a given intercepted PPFD, the taproot elongated faster than secondary roots, and secondary roots originating near the base of the taproot elongated faster than those originating near the apex. The elongation rate of a ny secondary root apex was accounted for (r = 0-77) by the ratio of in tercepted PPFD to the distance between the apex and the base of the ta proot. No relationships between intercepted PPFD and elongation rate w ere observed before the two-leaf stage, when the CO2 labelling experim ent suggests that carbon essentially originates from the seed. Therefo re, this study suggests a role for source-sink relations in the distri bution of elongation between apices and a role for carbon nutrition in day-to-day variations of root elongation rate. Precise mechanisms exp laining this behaviour remain to be investigated.