Mechanical touch and pollen load differentially regulate touch-sensitive stigma movement in Campsis radicans (Bignoniaceae)

Touch-sensitive stigmas exhibit rapid closure upon mechanical stimulation, yet the quantitative dynamics of this movement and how mechanical touch and pollen signals interact to regulate different movement phases remain poorly understood. Here, we established a “stigma kinematics” framework using high-resolution time-lapse imaging and motion-tracking analysis to quantify stigma movement in Campsis radicans (Bignoniaceae). By experimentally decoupling mechanical touch and pollen signals, we identified five distinct movement patterns determined by the presence of mechanical touch and variation in pollen load. Mechanical touch alone triggered rapid temporary closure within 1-2 min, with peak angular velocities of several hundred degrees per minute, whereas pollen deposition without mechanical touch induced only slow, gradual movement. Reopening probability was negatively associated with pollen load, with each grain of pollen increase reducing reopening probability by 0.23%. Permanent closure was governed by pollen load rather than mechanical touch: stigmas achieving permanent closure had median pollen loads 6.1 times higher than those remaining open, and each additional grain of pollen reduced permanent closure time by 0.3 min. Notably, permanent closure occurred before pollen tubes reached the ovule, indicating that closure commitment is based on signals perceived at the stigma surface. These findings reveal a functional division in which mechanical touch controls movement velocity while pollen load determines closure outcome, providing new insights into how touch-sensitive stigmas integrate mechanical and chemical signals to regulate multi-stage reproductive behaviour.