Which plant tissues are involved in sap exudation?

Sap exudation (bleeding) in plants involves the activity of vascular tissues, primarily the xylem and, in some species, the phloem, along with surrounding parenchyma cells that contribute to fluid pressure and composition.

1. Xylem Tissue

• Primary tissue responsible for sap exudation in most plants.
• Xylem conducts water and dissolved minerals absorbed from the soil.
• When a plant stem is cut or wounded, the positive root pressure within the xylem vessels forces xylem sap out through the wound, resulting in bleeding.
• The pressure originates from osmotic water movement into the xylem in the roots, especially when transpiration is low (night or early morning).
• Example: Exudation in grapevines or sugar maples mainly comes from xylem sap pushed upward by root pressure.

2. Phloem Tissue

• In certain plants (e.g., cucurbits, castor bean), sap also exudes from phloem tissues upon cutting.
• Phloem carries organic nutrients such as sugars and amino acids under relatively high internal osmotic pressure.
• When injured, sieve tube elements and companion cells may release sap before quickly sealing the wound using P-protein plugs and callose deposition.
• In these cases, both fascicular and extrafascicular phloem contribute to the exudate, depending on the species.

3. Parenchyma Cells

• Surrounding living parenchyma cells in xylem and phloem tissues release additional water and solutes when damaged.
• These cells help maintain osmotic balance and may contribute to the initial burst of dilute sap observed immediately after wounding.