Root elongation rate is accounted for by intercepted PPFD and source‐sink relations in field and laboratory‐grown sunflower

ABSTRACT The existence of relationships between intercepted photo‐synthetic photon flux density (PPFD) and growth of individual organs is somewhat controversial. We have tested whether such relationships could account for the natural variability in elongation rates of taproot and secondary roots of sunflower (from 2 to 135 mm d−1), in field and laboratory conditions. Elongation of taproot and secondary roots was recorded daily through windows in the field. A range of PPFD was obtained by following day‐to‐day natural fluctuation for three contrasting growing periods, and by shading part of the plants under study. A parallel experiment was carried out in a growth chamber with contrasting light intensities and with a 14CO2 labelling experiment. After the two‐leaf stage, i.e. when the contribution of photosynthetic carbon became appreciable in root growth, daily root elongation rate was closely linked to the PPFD intercepted from 36 to 12 h before the measurement of root elongation. Curvilinear relationships applied to plants grown in the field as well as in a growth chamber, and to shaded plants as well as to plants 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 any secondary root apex was accounted for (r= 0.77) by the ratio of intercepted PPFD to the distance between the apex and the base of the taproot. No relationships between intercepted PPFD and elongation rate were observed before the two‐leaf stage, when the CO2 labelling experiment suggests that carbon essentially originates from the seed. Therefore, this study suggests a role for source‐sink relations in the distribution of elongation between apices and a role for carbon nutrition in day‐to‐day variations of root elongation rate. Precise mechanisms explaining this behaviour remain to be investigated.