The dynamic nature of phloem transport in seedlings: Growth, phenology and environmental stimuli

Background/Question/Methods

Methodological limitations in measuring in vivo phloem transport remain a major challenge to understanding the dynamic nature of carbohydrate movement and allocation in living plants. To understand how translocation changes during development and how plants respond to environmental conditions, we use a novel technique that can monitor shifts in phloem velocity in ten to fifteen minute increments. This technique involves tracking the movement of a fluorescence dye (CFDA) from the source to the sink with photodiodes. To eliminate the effect of loading on velocity calculations, we bleach a small band of dye with a laser and monitor the band’s movement along the stem. This method is minimally invasive because the dye is initially membrane permeable, and once it enters cells, it is cleaved and only travels symplastically. In this study, we examine changes in phloem velocity and anatomy along with seedling growth and phenology during early seedling development. We use seedlings because of their importance in plant establishment and because they provide a simple source and sink model. We manipulate sinks by removing the apical bud at different points during development and explore the response of seedling to environmental stimuli including changes in light levels.

Results/Conclusions

A few days after cotyledons unfold, there is a pronounced shift in the relative importance of the two main sinks, resulting in more carbohydrate transport to the apical meristem than the roots. This transition is marked by an elongation of the apical bud and is reversed by its subsequent removal. After four days of reduced allocation to the roots (velocity <0.05 mm/s), young leaves begin to develop and reduce their demand for sugar from the cotyledons.  At this point, the transport rate in the hypocotyl increases reaching a velocity of the same order of magnitude as measured in adult plants (~0.2 mm/s). These developmental changes result in a greater increase in transport rate than environmental stimuli including increased light levels. This experiment demonstrates the dynamic nature of phloem transport in seedlings, and the versatility and utility of this technique for measuring phloem velocity in living plants under a variety of environmental conditions.