Positive pressure in xylem and its role in hydraulic function

Area of Science:

• Plant Physiology
• Xylem Transport
• Biophysics

Background:

• Xylem sap transport typically occurs under negative pressure (tension).
• However, many vascular plants exhibit positive xylem pressure, particularly before leaf flush or during diurnal cycles.
• The origins and mechanisms of this positive pressure are not fully understood.

Purpose of the Study:

• To review and synthesize current knowledge on positive xylem pressure in plants.
• To discuss and compare mechanistic models explaining positive xylem pressure.
• To evaluate the hydraulic functions of positive xylem pressure and recommend future research.

Main Methods:

• Literature review and synthesis of existing research.
• Comparative analysis of mechanistic models for positive xylem pressure.
• Evaluation of experimental evidence for hydraulic functions.

Main Results:

• Positive xylem pressure can be generated by physical forces, osmotic exudation, or hydraulic pressure in associated parenchyma cells.
• These mechanisms may interact and can be modeled cohesively.
• Positive xylem pressure is implicated in tissue rehydration, water storage refilling, and embolism repair.

Conclusions:

• Understanding positive xylem pressure is crucial for comprehending plant water relations.
• Further research, employing methods minimizing cutting artifacts, is needed to elucidate its precise mechanisms and functions.
• Distinguishing between physical and biological drivers of positive xylem pressure remains a key challenge.