The effect of photon flux density on distribution of assimilate between shoot and storage root of carrot, red beet and radish

Abstract

Growth and 14C distribution were measured in carrot, red beet and radish exposed to different photosynthetic photon flux densities (PPFD) in controlled environments to permit a comparison with their responses to varying plant density. Growth studies revealed that red beet maintains its leaf area, shoot fresh and dry weights at a range of PPFD while its storage organ weight is markedly reduced as PPFD decreases. Experiments with 14C showed that the beet shoot apex was a strong sink for assimilates and mobilised 14C at the expense of the storage root. In carrot, leaf area and shoot fresh weight were also maintained across a range of PPFD, though shoot and storage root dry matter were reduced at lower PPFD. The distribution of 14C suggested that in carrot there was a tendency to mobilise 14C to the apex at the expense of the leaf lamina at lower PPFD. In radish, leaf area and the fresh and dry weights of both shoot and storage organ decreased as PPFD decreased, but with the storage organ affected to a greater extent. The tendency in red beet and radish to mobilise assimilate to the shoot at the expense of the storage organs at lower PPFD (in contrast to carrot) is consistent with the response of these species to increasing density.