Contributed by Gerard Sapés
Every year hills and prairies change from green to yellow to welcome summer. While for us this change is synonym of vacations and fun, for plants signifies upcoming challenge. Terrestrial plants are organisms made of 90% water that live surrounded by an atmosphere that is an order of magnitude dryer than them. This difference between atmospheric and plant water content continuously pulls water out of plants and has forced them to evolve several traits such as cuticles and stomata to prevent water loss. In summer, drought strongly increases atmospheric demand and also limits ground water. Under these circumstances, the fight for water between a plant and the atmosphere generates incredibly high pressures within the vascular system of the plant that can reach in some cases up to 100 times atmospheric pressure.
Even though water loss is essential for plants to transport nutrients from roots to leaves, excessive tensions can break the water column that connects ground water to the atmosphere through the vascular system of a plant and induce an embolism in a similar fashion to the way it occurs in human beings. While for us a single embolism may be enough to kill us, plants have evolved a vascular system that provides several alternative pathways to ensure water transport when embolisms occur. Despite being resistant to embolisms, plants seem to perish when certain hydraulic threshold is reached. Specifically, gymnosperms seem to die when they lose 60% of their hydraulic conductivity while angiosperms die at higher levels (80%).
Plants have multiple ways to avoid reaching these thresholds. They can increase the efficiency of cuticles and stomata at preventing water loss or the efficiency of roots at absorbing water. But it has also been suggested that plants can prevent and repair embolisms using non-structural carbohydrate compounds (NSCs) derived from photosynthesis. It seems that plants with higher amounts of NSCs tolerate drought for longer periods of time. It is suggested that NSCs can travel through the plant and act as osmolites to drive water to areas where it is needed like embolized conduits or cells that are losing turgor but direct evidence of the relationship between carbon and hydraulics has not been established yet. Under drought, plants close stomata to avoid water loss. This strategy also prevents carbon acquisition and limits photosynthesis thus limiting the potential use of NSCs to prevent and repair embolisms. Therefore, plants face the challenge to balance carbon intake and water loss under drought to avoid reaching their hydraulic threshold either through embolism production or loss of their osmoregulation capacity.
The challenge is even greater for seedlings. First year old seedlings have undeveloped roots, thin cuticles, and small NSC pools which makes them much more susceptible to drought than adults. Under climate change, a mild increase in drought can be sufficient to prevent seedling establishment in areas where adult trees can still survive thus bringing populations to a decline and ultimately to a decrease in the forested area in places where drought is the limiting factor such as the lower tree-lines.
As a plant ecophysiologist, my project tries to provide a direct link between plant hydraulics and carbon in conifer seedlings. By studying the mechanisms through which NSCs modify the rate at which seedlings lose their hydraulic function (hydraulic conductivity and conductance) I hope to shed light on the effects that climate change will have on conifer forests of the Northwestern United States.
One of the main reasons that has prevented plant physiologists to find the link between plant hydraulics and carbon is the difficulty to accurately measure plant hydraulic conductivity and conductance. Measuring plant flow rates is a technique that is sensitive to many artifacts such as artificial embolization of conduits. Additionally, such flow rates are extremely low on seedling stems given their small size and measurements in whole root systems are extremely controversial. It has not been until recently that technology has develop systems that allow for accurate measurements of flow rates of the order of microliters per minute and these systems are expensive. Thanks to the Collaboration Challenge Research Grant I have been able to acquire a flow meter with high accuracy and establish a connection with two of the scientists leading the field of hydraulics (Dr. Craig Brodersen and Dr. Daniel Johnson) to learn the best method to measure hydraulic conductivity in stems avoiding potential artifacts.
The process of measuring has been extremely challenging. It has taken me 4 months and about 100 measurements to accurately measure hydraulic conductivity and conductance in seedlings that have been experiencing drought in a greenhouse. Due to the complexity of the method, I have not been able to capture the loss of hydraulic conductivity and conductance from the beginning of my experiments with enough accuracy (Figure 1b weeks 0 to 2). But on the other side, I successfully modified the current method to measure hydraulic conductivity and conductance in stems to be able to also measure hydraulic conductance in whole root systems, one of the key organs involved in plant hydraulics during drought. My method has proven to yield the same values of hydraulic conductivity and conductance in stems as the current method but with the advantage of being compatible with roots (Figure 1a).
Working together with scientists out of my specific field have shown me how collaborations can exponentially reduce the amount of time needed to achieve our goals in science. Without the expertise of Brodersen and Johnson, it would not have mastered the techniques needed to answer my research questions in time. For this reason, I highly recommend others to do the same. I found that, many times, the scientific community is conservative with sharing thoughts because we fear that our ideas could be stolen, but my experience have shown me that there is more to win than to lose in communicating with other minds.
Thanks to the Interdisciplinary Collaborative Network, now I have the means to design an experiment next summer to assess how NSCs influence hydraulics under drought in first year old Ponderosa pine seedlings from the lower tree-line.