Phloem Transport Velocity Varies over Time and among Vascular Bundles during Early Cucumber Seedling Development

Abstract

We use a novel dye-tracing technique to measure in vivo phloem transport velocity in cucumber (Cucumis sativus) plants during early seedling development. We focus on seedlings because of their importance in plant establishment and because they provide a simple source and sink model of phloem transport. The dye-tracing method uses a photodiode to track the movement of a bleach front of fluorescent dye traveling in the phloem from the cotyledons (source) to the roots (sink). During early seedling development, phloem transport velocity in this direction can change 2-fold depending on vascular connectivity and the number of actively growing sinks. Prior to leaf expansion, vascular bundles attached to the first developing leaf demonstrate a decline in basipetal phloem transport that can be alleviated by the leaf's removal. At this stage, seedlings appear carbon limited and phloem transport velocity is correlated with cotyledon area, a pattern that is apparent both during cotyledon expansion and after source area manipulation. When the first leaf transitions to a carbon source, seedling growth rate increases and basipetal phloem transport velocity becomes more stable. Because bundles appear to operate autonomously, transport velocity can differ among vascular bundles. Together, these results demonstrate the dynamic and heterogeneous nature of phloem transport and underline the need for a better understanding of how changes in phloem physiology impact growth and allocation at this critical stage of development.

Cellular Subcompartments through Cytoplasmic Streaming

“…Flow-dependent subcompartments can be envisioned in other cell types where regimented streaming occurs. For example, in the plant phloem, long distance pressure-driven flow occurs through perforate sieve plates that interconnect sieve elements (Mullendore et al, 2010;Savage et al, 2013). In other plant cells, a self-organizing actomyosin network produces complex patterns of circulatory streaming (Woodhouse and Goldstein, 2013).…”

The use of laser light to enhance the uptake of foliar‐applied substances into citrus (Citrus sinensis) leaves

“…Yet, methods for delivering phloem-mobile dyes into the sieve elements can be imprecise (e.g., physical abrasion of the leaf cuticle and epidermis; Savage et al, 2013) or require additional compounds applied to the leaf surface to increase cuticle permeability, which appear to currently be the best options (Knoblauch et al, 2015). Our results in phloem transport velocity were significantly slower compared to a previous report (Savage et al, 2013); although we used a similar methodology, this variation may correspond with differences in plant species, age, organs measured, and conduit diameters. Our results show phloem transport velocity from a mature leaf to its petiole in a fully developed citrus plant, which differs considerably from the herbaceous species used in the Savage et al (2013) study.…”

Multispectral Phloem-Mobile Probes: Properties and Applications

“…Despite a lack of understanding of how these probes are loaded into the phloem, they have been used extensively in monitoring phloem transport (Knoblauch and van Bel, 1998). They have also found use in imaging symplastic pathways after unloading Roberts et al, 1997;Savage et al, 2013) and identifying symplastic domains …”