Effects of punch needle diameters and calibration methods on leaf force to punch mechanical strength

The leaf force to punch strength is one of the important indexes to reflect the mechanical resistance of the leaf, but the final result is affected by the needle diameter and calibration method. To analyze the influence of different punch needle diameters and calibration methods on force to punch measurement, the difference of leaf force to punch of 30 plants with parallel vein monocots and dicots with reticulate veins under different diameters punch needle and their calibrations with functional traits from the campus of Guangxi University were measured. The results were as follows: (1) The force to punch was calibrated based on the needle circumference, cross-section area and leaf thickness. The measured force to punch varied with needle diameter and calibration method. (2) The differences among the leaf force to punch of the three diameters punch needle were different under different calibration methods. Among them, there was no significant difference in the force to using punch needles with different diameters when calibrated by punch needles circumference and needles circumference combining the leaf thickness. When only using leaf thickness for calibration, the thicker the punch needles, the force to punch increased with needle diameters, but it decreased when only using the needle cross-section area or combining the cross-section area with the leaf thickness for calibration. In addition, the leaf force to punch of monocots was significantly higher than that of dicots. (3) As the variation range of force to punch across needle diameter was calculated for each species, the correlation between this variation range and leaf traits differed with calibration methods. Vein type, leaf density, leaf mass per area, cuticle thickness and vein density significantly influenced this range. Overall, needle diameters and calibration methods significantly affect the accuracy of leaf force to punch measurements across plant groups with different venation types. We advise carefully selecting perforation needles and calibration methods for force to punch measurements based on the research questions and studied plant groups. It is suggested that the punch needle with a diameter of 0.5 mm should be applied to the blade with large vein density and small leaf mass per area, but not to the hard leaves; the needle with a diameter of 2 mm should not be applied to the blade with large vein density and small leaf mass per area. It is more reasonable to choose the needle with a diameter of 1 mm and use its circumference for calibration in three diameters.