Abstract: 【Objective】The main purpose of this study was to recover the optimal strategy for growth area expansion and output increasing of interplanting rapeseed in rice, a triple cropping pattern, through evaluating the effects of a symbiotic period on both seedling growth and yield development, which would shed light on desirable symbiotic period determination in South China.【Method】Ganyouza 5 and Fengyou 730, two elite rapeseed cultivars, were employed for a field experiment with 6 symbiotic period treats during the 2012-2014 growing season, the 6 symbiotic period treats were as followed separately: 12 d, 10 d, 7 d, 5 d, 3 d, 0 d (CK).【Result】(1) An extended symbiotic period could thoroughly promote the seedling density. A dramatic reduction of seedling density occurred along with a longer symbiotic period in the late growth period. Concurrently, the seedling rate and mature plant rate were increased at the initial stage but deteriorated under a further extended symbiotic period. The optimal mature plant rate was a 5-day symbiotic period treatment. Compared with the control (0-day symbiosis), the seedling rate and mature plant rate were raised with 0.97% and 5.98% at a 5-day symbiotic period but lessened by 19.2% and 19.35% at a 12-day symbiotic period, respectively. (2) Under a longer symbiotic period, seedling growth, number of total leaves, number of green leaves, crown diameter, and the dry weight of individual plants were gradually enhanced, whereas the leaf area index and dry matter of population plants were primitively increased and then subsequently fell. (3) A symbiotic period was highly related to yield components of interplanting rapeseed in rice including primary branches, siliques per plant, siliques on main inflorescence, seeds per silique, 1000-grain weight, dry matter weight per plant, and yield per plant. Especially, a significant positive correlation was observed between the symbiotic period and the primary branches, siliques per plant, siliques on main inflorescence, 1000-grain weight, yield per plant, and dry matter weight per plant, but a substantial negative correlation occurred for seeds per silique. (4) The yield per plant of interplanting rapeseed in rice was enhanced, whereas the yield per area was heightened initially and diminished later under a longer symbiotic period, and the optimal rapeseed yield could be approached at 5-day symbiotic period. Comparing with the control (CK), the average yield increasing rate of the symbiotic period treatment was 20.48% and 22.35% respectively within two rounds of field experiments. (5) The duration of a symbiotic period could affect the developing progress of rapeseed. The life period of the control (0-day symbiosis) was shorter than the symbiotic treats, and the difference was apparently occurred from seedling to bolting stages, but disappeared after the squaring stage.【Conclusion】Extension of a symbiotic period favors seedling density and individual development of interplanting rapeseed in rice. However, a lower seedling rate and mature plant rate under a longer symbiotic period as well as low seedling density and weak individuals under a shorter symbiotic period could result in yield reduction. Our results indicated that 5-days was the most optimal local symbiotic period within a favorable regime of 3 to 7 days to higher yield under interplanting rapeseed in rice in red soil regions. A longer or shorter symbiotic period would be detrimental to high rapeseed yield formation in South China where a triple cropping system is very popular.

Key words: rapeseed, rice, symbiosis period, interplanting rapeseed in rice, seedling traits, yield