Final answer:
When a sieve tube element is injured, callose precipitates and forms a plug structure with phloem proteins to prevent fluid loss. This defense mechanism maintains the integrity of the phloem's transport system.
Step-by-step explanation:
When a sieve tube element is injured, for example by an aphid, its pressure drops, causing callose to precipitate. Together with phloem proteins, the callose forms a plug structure, which prevents the leaking of fluid from the sieve tube element. Phloem tissue, which includes sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers, is essential for transporting organic substances like sugars and amino acids throughout the plant. The perforated sieve plates at the end junctions between sieve cells allow this transport. When damaged, the sieve-tube element's ability to transport nutrients is compromised, but the callose plug acts as a defense mechanism to quickly seal off the damaged area, preserving the integrity of the phloem network.
When a sieve tube element is injured, its pressure drops, causing callose to precipitate. Together with phloem proteins, the callose forms a sieve tube occlusion, which prevents the leaking of fluid from the sieve tube element.