Drought-induced cavitation event in xylem

Title : Drought-induced cavitation event in xylem

Abstract:

When water moves through the soil-plant-atmosphere continuum under a negative pressure, cavitation often occurs. Many studies have confirmed that air bubbles cause drought-induced cavitation in xylem. However, the spread of embolisms is poorly understood.

Cavitation involves air bubble activity in xylem sap. Studying bubble activity, including the radii of bubbles at equilibrium, the stability of equilibrium, and bubble development, leads to two types of cavitation. One is the growth of pre-existing air bubbles in conduit sap, while the other is air seeding, defined as the suction of air bubbles from an already gas-filled space.

Provided that there is an air bubble steadily in a conduit of radius r_c. If the xylem absolute pressure P_l = 0, the equilibrium bubble radius is r₀ = √(3nRT / 2πσ) is in stable equilibrium. Along with the drop of P₁ , it will expand steadily and would explode at xylem pressure P_l^* = -8σ / (9r₀) with the critical radius r = r_* = 3/2 √(nRT / 2πσ) .

If r_* > r_c , before exploding the bubble has become long shaped. In this way the bubble only expands and elongates gradually to form embolism. This is the first method of cavitation: An air bubble expands gradually.

Only if r_* > r_c , can the bubble explode P₁^* at its to form long shaped, and continues to elongate slowly. This is the second method of cavitation : An air bubble steadily expands, explodes suddenly, becoming long shaped, and continues to elongate gradually.

For air seeding in the range xylem pressure -2P₀ < P_l < P₀ (where P₀ is atmospheric pressure), if an air seed enters a conduit of radius r_c through a pore of radius r_p > 0.487 µm its radius should be r¹₀ , r₀ or r₁ in which the bubble all are in stable equilibrium. As P_l decreases, it will enlarge steadily like the growth of a pre-existent air bubbles, presenting the first or second method of cavitation but with air reservoirs.

In the range of P_l ≤ -2P₀, soon after an air seed enters a conduit through a poreof radius r_p ≤ 0.487 µm it will break up immediately, letting the conduit to be full of the seed air instantly. This is the third method of air seeding: As soon as an air seed enters a conduit, it breaks up immediately and the air in it filling the conduit instantly.

The natural drying processes of xylem slices of Platycladus orientalis L. were observed under light microscopy. Three phenomena were obtained, which correspond to the three methods of air seeding. Synchrotron X-ray microscopy was used to capture cavitation events in intact vessel elements of corn and rice leaves. In the experiments, gas instantly filling conduits was more easily obtained, which should be caused by exploding event. But when we searched for gradual expansion of bubbles, it was almost invisible. We considered that the vessel elements of the leaves of corn and rice were intact, air only entered the conduits at P_l ≤ -2P₀ through the pores of radii r_p ≤ 0.487 µm . And it was very difficult for air to enter the conduits of the sample leaves in the range of xylem pressures -2P_o < P_l < P_o , leading the absence of gradual expansion.

In drought cases for whole plant the third method of air seeding should be a priority. Thus, the spread of embolism could be understood.

Biography:

Shen Fanyi is a female professor. She was born on November 7, 1943, and obtained her Master’s degree in physics from beijing normal university. She worked as a teacher at beijing forestry university from 1982 until her retirement. During her academic career in forestry, she explored the application of physics in tree physiology and proposed that the upward movement of water in the xylem is a physical process. She further developed the theory of cavitation, which was supported by two experimental methods used to validate her findings.