Title: Understanding phloem functioning in the context of water stress
Abstract: Plants take carbon dioxide out of the atmosphere and convert it into sugars through photosynthesis. Once carbon dioxide is in the form of sugar, it is then transported throughout the plant along a “sugar highway” known as the phloem. Let’s take a moment to imagine what would happen if this transportation pathway was slowed, or even blocked: in this scenario there would be a buildup of sugars in the leaves and large deficits in other parts of the plant. This build-up of sugar in the leaf would result in severely limited photosynthesis, and the deficit of sugars in other parts of the plant would result in severely limited growth. Despite this crucial role of the phloem, we currently lack an understanding of how environmental conditions alter sugar transportation through the phloem. This knowledge gap makes the pursuit of understanding how photosynthesis and plant growth will respond to climate change increasingly difficult. An equivalent situation would be trying to understand how a human body will function under various stressors, while not understanding how those stressors influence blood flow. In my dissertation, I have attempted to fill in some of our knowledge gaps about the sugar highway in the context of water stress, one of the key environmental stressors of climate change.
In Chapter 1, I studied how midday water stress influences phloem functioning in mature red oak trees at Harvard Forest (Petersham, MA), and how accumulation of sugar in the leaves over the course of the day helps keep the leaves from wilting. Surprisingly, I found that carbon movement out of the leaf in the phloem primarily occurs during the middle of the day, despite midday water stress.
In Chapter 2, I sought to understand how this midday carbon transport is happening. To do this, it was necessary to quantify the amount of sugar in the phloem. However, there was no method for quantifying sugar concentration of phloem sap for actively photosynthesizing leaves at the top of a mature, forest tree. By developing a new method using Raman Spectroscopy, I was able to quantify midday phloem sugar concentrations in leaves. I found that, for the same red oak trees as in Chapter 1, phloem sap sugar concentrations were up to 3x higher than previously thought, supporting the common hypothesis for phloem functioning (Münch Hypothesis) and challenging the hypothesis for how carbon is moved into the phloem. This finding also suggests that the phloem has a large safety buffer in the case of drought.
In Chapter 3, I furthered our understanding of phloem under water stress by performing a drought experiment using Ricinus communis. I used this species because it is a phloem “bleeder” meaning the phloem sap leaks out of the plant when cut, allowing for in-depth studies of phloem-drought dynamics. I found that phloem pressure is maintained during drought, and that phloem functioning occurs well after stomata have closed and photosynthesis has ceased. This finding challenges hypotheses suggesting that loss of phloem functioning causes permanent stomatal closure. These findings also suggest that viscosity is an important factor in phloem loss of functioning during drought.
In sum, this dissertation deepens our understanding of the “sugar highway” and how it functions under water stress, enabling more accurate parameterization of models predicting whole-plant responses to climate change.
Committee: N. Michele Holbrook (Advisor), Andrew Knoll, Paul Moorcroft, Tony Rockwell