Effects of biodegradable seedling tray and sowing rate on seedling quality and yield of mechanical transplanting rice

Abstract: Mechanical transplanting is the tendency of planting mechanization for most of rice regions in China. The sowing rate of nursing seedling is usually high (90-120 g/tray) for forming seedling carpet and reducing the unplanted hill percentage, which leads to weak seedlings, excessive basic seedlings in fields, and poor heterosis representations for hybrid rice. The biodegradable seedling tray is made with bioplastic - polylactic acid (PLA, a kind of material widely used) and designed as bowl-blanket shape and mechanical transplanting with seedling tray, which could provide a new way to realize mechanical transplanting of hybrid rice under thin sowing rate model. This experiment was conducted to evaluate the applicability of biodegradable seedling tray in thin sowing and mechanical transplanting. Zhongzheyou 8 (indica hybrid rice) and Yongyou 538 (indica-japonica hybrid rice) were used as materials. The carpet seedling tray was applied as control. Four sowing rates: 30, 50, 70 and 90 g/tray were adopted (the weight of dry seeds). The nineteen-days-old seedlings under different tray and sowing rate treatments were transplanted in the plot of 30 m2 separately with 30 cm × 18 cm planting density. A fertilizer dose of 180:450:112.5 kg/hm2 of N:P:K was applied in the form of urea, calcium superphosphate, and potassium chloride, respectively. The nitrogen fertilizer was applied at soil preparation, tillering stage and panicle initiation stage in a proportion of 5:3:2. Total phosphorus fertilizer was applied at soil preparation and total potassium fertilizer was applied at panicle initiation stage. Weeds, insects and diseases were intensively controlled during the whole growing season to avoid yield loss. The other managements were uniform with high-yield cultivation. In this experiment, the emergence rate of seed, morphological characteristic of shoot and root of seedlings, root entwining force, unplanted hill percentage of mechanical transplanting, the yield and its components were investigated. Results showed that, nursing seedling with biodegradable seedling tray had significant the emergence rate increase (P<0.05), and the lower sowing rate, the higher the emergence rate compared with carpet seeding tray. The emergence rate of Zhongzheyou 8 in 30 g/tray treatment was higher by 20.57 percentage points than that of the carpet seedling tray. The seedlings roots in biodegradable seedling tray treatment were white, stubby, separate and coiled in bowls due to bowl-blanket design and the average root diameter was 8.63% greater than that of the carpet seedling tray (P<0.05). In addition, the root entwining force of seedlings with biodegradable seedling tray was greater than 100 N in all sowing rates treatments, which met the demand of mechanical transplanting. Because of bowl-shaped precision transplanting with seedling tray, the unplanted hill percentage of Zhongzheyou 8 in 30 g/tray treatment declined by 6.67 percentage points compared with carpet seedling tray. Despite the fill-grain percentage and the 1 000-grain weight of biodegradable seedling tray treatment were lower (P<0.05) than that of carpet seedling tray, but the population spikelets and yield were slightly higher. According to the performance of nursing seedlings, mechanical transplanting and yield, 70 g/tray was appropriate sowing date for biodegradable seedling tray. Our results suggested that biodegradable seedling tray transplanted with tray was beneficial for nursing strength seedlings under thin sowing model, improving mechanical transplanting quality and yield potentiality of hybrid rice.