Vessel-length determination using silicone and air injection: are there artifacts?

Abstract

Xylem vessels are used by most angiosperm plants for long-distance water and nutrient transport. Vessel length is one of the key functional traits determining plant water-transport efficiency. Additionally, determination of maximum vessel length is necessary for correct sample collection and measurements in hydraulic studies to avoid open-vessel and cutting-under-tension artifacts. Air injection and silicone injection (BLUESIL RTV141A and B mixtures) are two widely used methods for maximum vessel length determination. However, the validity of both methods needs to be carefully tested for species with different vessel lengths. In this study, we tested the air-injection and silicone-injection methods using eight species with different vessel lengths: short (<0.5 m), medium (0.5-1 m) and long (>1 m). We employed a novel approach using RTV141A injection without the RTV141B hardener as a reference method because RTV141A cannot penetrate inter-vessel pit membranes and is not prone to hardening/solidification effects during the injection process. The results revealed that the silicone-injection method substantially underestimated the maximum vessel length of all eight species. However, the air-injection method tended to overestimate the maximum vessel length in five out of eight species. The ratio of underestimation of the silicone-injection method was higher for species with longer vessels, but the overestimation of the air-injection method was independent of the vessel length. Moreover, air injection with different pressures-ranging from 40 to 300 kPa-resulted in comparable results. We conclude that the conventional silicone-injection method can underestimate the vessel length, whereas the air-injection method can overestimate the maximum vessel length, particularly for long-vessel led species. We recommend RTV141A-only injection for determining the maximum vessel length, and it can also be used to validate the use of the air-injection and conventional silicone-injection methods for a given species.

Keywords: lianas; maximum vessel length; silicone injection; xylem anatomy.