Brent Cornell

    
Aphids are a group of insects, belonging to the order Hemiptera, which feed primarily on sap extracted from phloem

• Aphids possess a protruding mouthpiece (called a stylet), which pierces the plant’s sieve tube to allow sap to be extracted
• The penetration of the stylet into the sieve tube is aided by digestive enzymes that soften the intervening tissue layers
• If the stylet is severed, sap will continue to flow from the plant due to the hydrostatic pressure within the sieve tube

Extraction of Phloem Sap via an Aphid Stylet

Measuring Phloem Transport

Aphids can be used to collect sap at various sites along a plant's length and thus provide a measure of phloem transport rates

• A plant is grown within a lab with the leaves sealed within a glass chamber containing radioactively-labelled carbon dioxide
• The leaves will convert the CO₂ into radioactively-labelled sugars (via photosynthesis), which are transported by the phloem
• Aphids are positioned along the plant’s length and encouraged to feed on the phloem sap
• Once feeding has commenced, the aphid stylet is severed and sap continues to flow from the plant at the selected positions
• The sap is then analysed for the presence of radioactively-labelled sugars
• The rate of phloem transport (translocation rate) can be calculated based on the time taken for the radioisotope to be detected at different positions along the plant’s length

Example of Phloem Transport Rate Data

Factors Affecting Translocation Rate

The rate of phloem transport will principally be determined by the concentration of dissolved sugars in the phloem

The concentration of dissolved sugars in the phloem sap will be affected by:

• The rate of photosynthesis (which is affected by light intensity, CO₂ concentration, temperature, etc.)
• The rate of cellular respiration (this may be affected by any factor which physically stresses the plant)
• The rate of transpiration (this will potentially determine how much water enters the phloem)
• The diameter of the sieve tubes (will affect the hydrostatic pressure and may differ between plant species)