2.2.1 Experiment design

The present study was performed during the 2017 and 2018 rice-growing seasons. Four different Japonica rice cultivars were selected for the assessment of the impacts of climatic variations on rice grain filling, yield, and quality. The experiments applied a randomized complete block design (RCBD) with three replications. The net plot size was 6 m × 3 m. The cultivars selected for the assessment of impacts of different climates on the grain-filling process and ultimately on grain yield of Japonica rice for this experiment were Longdao-18 (V₁), Longdao-21 (V₂), Longjing-21 (V₃), and Suijing-18 (V₄). V₁ and V₂ are late-maturing cultivars whereas V₃ and V₄ are early-maturing. These Japonica rice cultivars were selected because they have been widely cultivated in the Heilongjiang province and have high grain yield capabilities. Cultivars with different growth durations were selected as rice growth duration is critical for determining the optimum plant production. Moreover, the estimation of a change in growth duration and rate during all growth stages due to changes in environmental components is essential for developing a sustainable Japonica rice system under projected climate change at high latitudes. Any additional change in the growth attributes of Japonica rice under varying climatic conditions are well known to drastically change the pliability of crop rotation and intensify the production systems under varying farming arrangements. The local recommended seed rate of 50–60 kg ha⁻¹ was used in the rice nurseries for all varieties to attain a balanced planting density. Manual transplantation was conducted by taking 2–3 seedlings per hill at 20 × 20 row spacing and 15 × 15 plant spacing. Manual weeding was carried out three times: 15, 30, and 45 days after transplantation, respectively. A synthetic pesticide (Pendimethalin) was sprayed 8 days after transplantation under optimum field moisture conditions. Macronutrients, i.e., nitrogen (N), phosphorus (P), and potassium (K), were incorporated as basal and splitting doses at local recommended rates of 150–80–80 kg ha⁻¹, respectively. Different organic and inorganic N-fertilizer sources were involved viz. synthetic urea (nearly 46% N) and compost manufactured from poultry manure (nearly 1% N). All compost doses were incorporated as basal input during land preparation at 5 t ha⁻¹. Synthetic diammonium phosphate (DAP) containing 46% P and 18% N was used for P application. The percentages of N and P provided by compost were also calculated, with the remaining N and P provided using synthetic urea and DAP, respectively. The provision of N from DAP was also calculated and the remaining required N was satisfied using synthetic urea and applied in three balanced splits during land preparation (basal), at tillering, and during panicle initiation. To fulfill the K requirements, muriate of potash (MOP) also known as potassium chloride (KCl) was used, with 60% potash. Synthetic fertilizers for P and K were amended as basal input.