Peperomia dahlstedtii, a common indoor plant, was used for the transplanting work. The transplanting method was taught to each subject prior to the experiments so that they could work more comfortably. The experiment was carried out in a greenhouse room, where the wall was covered with a black curtain (Figure 1), and environmental conditions were maintained relatively consistent (temperature 20.8°C ± 1.4°C, mean ± SD; humidity, 57.7% ± 6.6%; illuminance 1,365.5 ± 327.9 lux).
The temperature was set at 22°C, and humidity was controlled so that it would not decrease from 50%. The lighting condition was controlled at comfortable level by hanging curtains from the ceiling and on walls to protect from direct sunlight.
An electrode was attached to the subjects’ chest in the waiting room, and they moved to the experimental room. After a 2-min rest in a seated position, they performed the given tasks, that is, transplanting houseplants or computer work, for 15 min. Heart rate variability (HRV) was measured consecutively during the task using a portable electrocardiograph (Activtracer AC-301A; GMS, Tokyo, Japan). Blood pressure and pulse rate data were collected before and after the tasks using a digital blood pressure monitoring device (HEM-1000; OMRON, Kyoto, Japan).
HRV data were calculated by averaging 1-min inter-beat (R-R) data and analyzed by means of maximum entropy methods (MemCalc, GMS, Tokyo, Japan) using the low-frequency (LF; 0.04 to 0.14 Hz) and high-frequency (HF; 0.15 to 0.40 Hz) components of the power spectrum. The HF component reflects activity of the parasympathetic nervous system, which increases in a relaxed state, and LF/(LF + HF) reflects activity of the sympathetic nervous system, which increases in a stressed state. All HRV values were log-transformed (base 10).
The feelings that the subjects experienced during the test were measured using the semantic differential method (SDM), which is a self-rating assessment. The subjects rated their feelings on a seven-point scale for three test items - ‘Comfortable’, ‘Relaxed’, and ‘Natural’ - by writing down their feelings at the moment before and after the tasks.
A paired t test was used to compare the differences in HRV values and blood pressure between the two tasks. Wilcoxon signed-rank test was used for the analysis of psychological data. Statistical analysis was carried out using the SPSS software, version 21.0 (IBM Corp., Chicago, IL, USA). In both cases, we applied one-sided tests because of the hypothesis that humans would feel more relaxed after the transplanting task. In all cases, the differences were considered statistically significant at P < 0.05.