词条 | Phloem loading |
释义 |
HistoryYuri Gamalei first described the different types of phloem-loading mechanisms in 1989, correlating loading strategy with the leaf's anatomy. He found that plasmodesmatal abundance in the minor veins of leaves was correlated with a plant's loading strategy. Plasmodesmata allow solutes to diffuse through the symplast. Thus, plants with a lot of plasmodesmata are generally passive loaders. Plants with few or absent plasmodesmata are usually active loaders since they do not have a way for solutes to pass through the symplast.[3] In 2001, Robert Turgeon and colleagues found that plasmodesmatal frequency is not the sole indicator of a plant's phloem-loading strategy. While the presence of abundant plasmodesmata does allow for passive, symplastic transport, it does not exclude the possibility of active, apoplastic transport. The amount of plasmodesmata in a leaf is an aspect of a species' anatomy and not necessarily a determinant of phloem-loading strategy. Plants with a polymer-trapping mechanism may have an abundance of plasmodesmata, but this trait alone does not indicate this loading strategy. Therefore, to accurately determine phloem-loading mechanisms, the type and amount of transport solute must also be identified. A plant's phloem-loading strategy can be accurately determined by combining data on solute type, solute concentration and plasmodesmatal abundance.[2] Phloem loading and growth formActive phloem loading requires less carbon, allowing carbon allocation to other sinks in the plant, such as growth. Active phloem loading allows for higher growth potential.[4] Herbaceous plants have a relatively high growth rate and many are active phloem loaders. Active loaders tend to have a high hydraulic conductivity and low solute concentrations, while passive loaders have high solute concentrations and a low hydraulic conductivity.[5] Trees also exhibit high solute concentrations and low hydraulic conductivity and thus are thought to be passive loaders. Based on early research, scientists correlated herbaceous species with active loading and trees and other woody species with passive loading. However, in 2011, Davidson et al. examined the correlation between growth from and phloem-loading strategy. They found that a loose association exists but not a definitive correlation.[6] Some trees exhibit active loading, while some herbs exhibit passive loading, and many species have mechanisms of both.[7] References1. ^{{Cite journal|last=Gamalei|first=Yuri|year=1991|title=Phloem loading and its development related to plant evolution from trees to herbs|url=|journal=Trees|volume=5|pages=50–64|via=}} 2. ^1 {{Cite journal|last=Turgeon|first=Robert|last2=Medville|first2=Richard|last3=Nixon|first3=Kevin|year=2001|title=The evolution of minor vein phloem and phloem loading|url=|journal=American Journal of Botany|volume=88|pages=1331–1339|via=}} 3. ^{{Cite journal|last=Gamalei|first=Yuri|year=1989|title=Structure and function of leaf minor veins in trees and herbs - A taxonomic review|url=|journal=Trees|volume=3 |issue=2|pages=96–110|via=}} 4. ^{{Cite journal|last=Turgeon|first=Robert|year=2010|title=The role of phloem loading reconsidered|url=|journal=Plant Physiology|volume=152 |issue=4|pages=1817–1823|via=}} 5. ^{{Cite journal|last=Fu|first=Qiushi|last2=Cheng|first2=Lailiang|last3=Guo|first3=Yangdong|last4=Turgeon|first4=Robert|year=2011|title=Phloem loading strategies and water relations in trees and herbaceous plants|url=|journal=Plant Physiology|volume=157 |issue=3|pages=1518–1527|via=}} 6. ^{{Cite journal|last=Davidson|first=Anna|last2=Keller|first2=Felix|last3=Turgeon|first3=Robert|year=2011|title=Phloem loading, plant growth form, and climate|url=|journal=Protoplasma|volume=248 |issue=1|pages=153–163|via=}} 7. ^{{Cite journal|last=Oner-Sieben|first=Soner|last2=Lohaus|first2=Gertrud|year=2014|title=Apoplastic and symplastic phloem loading in Quercus robur and Fraxinus excelsior|url=|journal=Journal of Experimental Botany|volume=65 |issue=7|pages=1905–1916|via=}} 1 : Plant physiology |
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