Design and experiment of a rotating strawberry planting device with self-supplementary lighting

Three-dimensional elevated strawberry cultivation can effectively avoid soil-borne diseases and pests to decline the soil fertility under climate change, compared with the conventional soil-based ones. However, the low planting density and uneven light distribution have limited the growth, development, and final yield of strawberries. In this study, a rotational strawberry cultivation device was developed with self-supplementary lighting, in order to improve the advantages of the conventional three-dimensional cultivation. A planting experiment was conducted to compare this device and the conventional "H"-shaped elevated strawberry cultivation frame. Firstly, the physical parameters of strawberry plants were measured to determine the structural dimensions of the planting trough frame. The feasible solution domain was calculated for the radius, number of troughs, and angle of the cultivation frame under a specific planting environment. A radius of 0.85 m, 10 troughs, and an angle of 36° were selected as the basic dimensional parameters. The main structure of the rotating disk was designed to fully meet the requirements of the rotational irrigation and split-mounted planting troughs. Meanwhile, a fan-shaped supplementary lighting device was also combined to adjust light distribution, according to the radiation behavior of the surface light sources in the space. The rotational schemes were proposed to solve the light obstruction in the different periods. The reason was that the illuminance was varied to the first decrease and then increase during the planting cycle, whereas the daily solar irradiance first gradually increases and then decreases, due to the solar trajectory in the solar longitude on different dates. In rainy and low-light weather, artificial light sources were introduced for supplementary lighting. Firstly, the photosynthetic response curve of strawberries was measured to determine the light compensation point and light saturation point of strawberries. The light intensity of artificial supplementary lighting was then provided for the subsequent sections. The orthogonal simulation experiments were carried out to optimize the key parameters of the supplementary lighting device. An optimal combination of the parameters was as follows. The lamp installation position was 300 mm away from the rotation point of the device, the lamp tilt angle was 7°, and the middle lamp position was numbered 1. Simulation results showed that the average illuminance was 11701 lx, and the light uniformity was 55.98%. Actual measurements of supplementary lighting were conducted using the optimized parameters. The results showed an average illuminance of 11926 lx and a light uniformity of 56.89%. The uniformity of light distribution was improved by 11.37 percentage points, compared with the commonly used supplementary lighting in greenhouses. A strawberry planting experiment was carried out on the whole device. There was a slight difference in the net photosynthetic rate of strawberry plants on the rotational cultivation frame, compared with the "H"-shaped frame. The rotational state effectively alleviated the "midday depression of photosynthesis" in strawberries, particularly for the light distribution. The average growth multiple of plant height in the rotational frame cultivation was 0.518, which was close to 0.537 of the conventional "H"-shaped ones. The photosynthetic parameters were also relatively close to those of the "H"-shaped frame. The cultivation performance of the rotational frame reached a level comparable to that of the "H"-shaped frame. This finding can also provide a strong reference for the three-dimensional cultivation mode of the cash crops, such as the strawberries, and the self-supplementary lighting of the devices.