Nutrient Transport in Tall Trees – An Overview of the Entire Process
Tall people are literally the giants among men, walking around with near total impunity, being given every advantage along the way. But sometimes, the blessing can also be a curse. Also, while on the topic, scientists have always wondered how tall trees accomplish the task of moving nutrients over long distances.
Trees present a critical challenge to long-distance transport because as a tree grows in height and the transport pathway increases in length, the hydraulic resistance of the vascular tissue should increase.
Although species that actively load sugars into their phloem, such as vines and herbs, can increase the driving force for transport as they elongate, it is possible that many trees cannot generate high turgor pressures because they do not use transporters to load sugar into the phloem.
Therefore, researchers at the at the Harvard University examining how trees can maintain efficient carbohydrate transport as they grow taller by analysing sieve tube anatomy, including sieve plate geometry, using recently developed preparation and imaging techniques, and by measuring the turgor pressures in the leaves of a tall tree in situ.
They also found that “the pressures that develop in the leaves of a mature red oak tree are sufficient to drive transport of sugars all the way to the roots,” said Noel Michele Holbrook, a research team member who is a professor of biology and Charles Bullard Professor of Forestry in the Department of Organismic and Evolutionary Biology at Harvard.
“We now have evidence that all plants — both small and tall — use the same mechanism to transport sugars,” Holbrook said. “And we now understand how trees can get tall without running into transport limitations associated with their size. Our research answers a multidecade debate about how sugars are transported in trees.”
The research team took extensive measurements of the structure of the sugar-conducting tubes along the length of many trees, while also measuring the hydraulic resistance in these tubes.
“To sample the phloem, one of the most delicate and easily wounded tissues in the plant, we had to cut away the outer bark. In the big stems, we did this using a hammer and a chisel, not tools that we typically use in the lab,” Holbrook said. The team also measured the pressures in the leaves of a tall tree by using a fluorescent microscope they hoisted up into the tree canopy.
“We found that the resistance to moving the sugar-rich phloem sap does not increase linearly with the transport length because the phloem transport cells in the main stem, especially toward the base, were wider and longer and also had more porous ‘sieve plates,’” Holbrook said. “Thus, the pressures needed to drive phloem transport are much lower than had been predicted.”
The team believes that their work holds important implications for food production.
“The majority of food generated by photosynthesis moves through the phloem,” she said. “If there are ways to make plants more productive in terms of having higher photosynthesis, then they will also need the ability to transport those sugars to the tissues that we eat. Thus, understanding how plants make efficient transport systems could contribute toward the development of higher-yielding crops and more productive trees.”
work on plant growth regulators and innventive steps