Effects of environmental control and cultivation practice on maize growth in plant factory

Plant factories can be emerging as the promising potential speed-breeding system in recent years. However, it is still lacking on the environmental parameters of the maize growth in the plant factories. In this study, to establish a regulation strategy for the indoor maize growth, a systematic investigation was made to explore the effects of the light intensity, photoperiod and cultivation practice (temperature, nutrition concentration, and pot volume) on the maize growth and development rate, plant phenotype, breeding cycle, and seed quality. A series of tests were conducted in three artificial climate rooms in Shunyi district, Beijing from May 2023 to July 2024. Maize (Zhengdan 958) was grown in pots under full-spectrum white LED panels. Five experiments were also set. 1) The light intensity was adopted with the photosynthetic photon flux density (PPFD) at three levels of 400, 700 and 1000 μmol/(m²·s) (all at a photoperiod of 16 h); 2) The photoperiod: constant of 10, 13 and 16 h photoperiod and one treatment with the variable photoperiod (12 h from four-leaf stage to tasselling while 16 h at other developmental stages); 3) Pot volume experiment: 6, 9 or 15 L; 4) Electrical conductivity (EC) in the nutrient solution: 2, 4 or 6 dS/m; 5) Temperature: day/night temperature of 27 ℃/20 ℃ or 30 ℃/23 ℃. The results showed that the duration from the sowing to the final ripe stage was not affected by the PPFD from 400 to 1000 μmol/(m²·s) in maize. The maximum yield per plant was achieved at the PPFD of 700 μmol/(m²·s) and photoperiod of 16 h daily light integral of 40 mol/(m²·day). Photoperiod affected the duration from the sowing to silking and to the final ripe stage. Among them, the 12-13 h photoperiod was promoted the flowering of maize, while the 16 h after flowering was promoted the growth and mature of the maize seeds. The plant height decreased, and then the final yield decreased, with the decrease in the pot size. Furthermore, the maize height was about 180 cm under the pot volume of 6 L, which was 30-100 cm lower than the field-grown maize. Therefore, the pot volume was utilized to effectively lower the maize height. In terms of the yield, the optimal range of EC in the nutrient solution was 2-4 dS/m in the substrate cultivation. The content of the seed protein relatively increased by 25%, whereas the content of the starch relatively decreased by 4%, with the increase in the EC from 2 to 6 dS/m. The final yield was not significantly different between the day/night temperature of 30℃/23℃ and 27℃/20℃, but the combination of 30 ℃/23 ℃ was promoted early flowering to shorten the whole growth period. The harvested maize seeds after 17 days pollination can already germinate but with a low germination percentage (<10%). The percentage of the seed germination increased linearly with the increasing days after pollination. To conclude, an optimal lighting strategy was achieved for the maize growing in the plant factory: from emergence to three-leaf stage, the light intensity of 700 μmol/(m²·s) and photoperiod of 16 h; from four-leaf stage to tasseling, the light intensity of about 900 μmol/(m²·s) and the photoperiod of 12 h; and from silking to harvest stage, the light intensity of 700 μmol/(m²·s) and photoperiod of 16 h. Under the light environment and the day/night temperature of 30℃/23 ℃, the maize can enter the silking stage on day 54, early dough stage (for early harvest) on day 75, and full ripe stage on day 90, thus achieving more than 4-5 generations a year.