Heterogeneity in phloem transport within and among plant organs

Background/Question/Methods

Despite the importance of carbon allocation to plant function and in turn plant ecology, relatively little is known about the developmental, anatomical and environmental factors that impact phloem transport. As a result, most empirical studies assume that transport velocity is constant throughout the plant and singular measurements are representative of species’ phloem function. To test this assumption and examine the extent that transport varies among vascular bundles within a stem and in different plant organs, we use a novel dye tracing technique to measure phloem transport velocity. This technique involves tracking the movement of a fluorescent dye (CFDA) in the phloem either at a macroscopic scale using photodiodes or at a microscopic scale using confocal microscopy. Measurements are taken by bleaching the dye with a laser and monitoring the movement of the bleach front through the tissue. This method is minimally invasive because the dye readily enters and remains in the phloem and as a result, measurements can be made on intact, living plants. In this study, we examined variation in the transport velocity of different vascular bundles during early seedling development and a variety of organs in adult plants of several species.

Results/Conclusions

During early seedling development, phloem transport velocity depends on whether or not a bundle is connected to the first true leaf. Bundles connected to the leaf demonstrate a decline in transport to the roots during early leaf expansion (velocity drops from 0.20 to 0.05 mm/s). At this stage, vascular bundles appear to operate independently; when buds and cotyledons are cut off, they do not impact the transport velocity of bundles not directly attached to the removed organs. On a larger scale, we found consistently higher transport velocities in the petioles than in midveins of mature leaves. Together these results demonstrate the heterogenous nature of phloem transport and underline the need for a better understanding of how changes in sugar transport could impact plant growth and allocation during plant development.