Skills: Measuring Phloem Transport Rates (DP IB Biology: HL)

• Hemiptera (also known as true bugs) is an order of insects that have highly-adapted mouthparts for piercing and sucking
• Many hemipterans (such as aphids) use these mouthparts for feeding on phloem sap
• This behaviour has been used by scientists to investigate transport in the phloem of plants to better understand, for example, how it occurs and the speed at which it occurs

Investigating phloem transport rates using aphid stylets

• Aphids penetrate the phloem using mouthparts known as stylets (these are a bit like microscopic pipettes)
• If the aphid is first anaesthetised, its head and body can be removed leaving the stylet still in place
• Due to the pressure of phloem sap in the sieve tube, sap will continue to flow out of the stylet, forming a drop at the end of the severed stylet
• At this point, the flow rate of the phloem sap can be measured and a sample can be taken from it to analyse its composition

Aphids can be used by scientists to investigate transport in the phloem of plants

NOS: Developments in scientific research follow improvements in apparatus; experimental methods for measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes became available

• Using radioactive isotopes (also known as radioisotopes) in scientific research only became possible after 1945 because their use was developed for work on the atomic bomb during the Second World War
• For example, carbon-14 (¹⁴C) is an isotope of carbon that is radioactive
  • It is possible to generate carbon dioxide that contains carbon-14; this would be written as ¹⁴CO₂
• It was discovered that if leaves were exposed to ¹⁴CO₂ whilst they were photosynthesising (by surrounding a leaf with a polythene bag filled with ¹⁴CO₂), the leaves would produce ¹⁴C-labelled sugars
• Using autoradiography, these ¹⁴C-labelled sugars could then be traced during translocation throughout the plant and also detected in phloem sap flowing out of aphid stylets
• These techniques enabled scientists to investigate the speed of phloem transport and how sugars are distributed between sources and sinks in plants

¹⁴C-labelled sugars can be traced during translocation throughout planta by detection in phloem sap flowing out of aphid stylets

• • Sugars are produced in sources (photosynthesising leaves) while sugars are delivered to sinks (roots, young shoots and developing seeds)