Stem diameter limits phloem conductive area plasticity in Populus tremuloides seedlings

Abstract

Carbon sources and sinks and plant growth change throughout a plant’s development. In turn, carbon transport, which occurs in the phloem, is also altered. However, due to methodological constraints there are little data on the degree to which the area available for carbon transport, the phloem conductive area, can change in response to the environment. To study the plasticity of the phloem conductive area, we grew Populus tremuloides Michx. seedlings under three different light treatments (high, medium, and low) for eight weeks. We measured their growth, leaf, and root gas exchange, measuring vascular structure using a new immunolabeling technique that labels actively transporting sieve elements. Light treatment altered vascular structure at all structural levels, including the phloem conductive area. The percent xylem conductive area was the same under all light treatments while the low light plants had a lower percent phloem conductive area. Thus, there is a factor that affects the phloem conductive area that does not impact the xylem conductive area. Stem diameter was the main driver of differences in the phloem conductive area in all light treatments, suggesting that cross-sectional area may influence the diameter and/or density of sieve elements. These results suggest that the phloem might be more labile than we previously thought when considering the actual phloem conductive area, but there are limitations to phloem plasticity.