Modified Paddy Ecosystem on Radiation Use Efficiency, Thermal Indices, Growth and Yield Performance of Sali Rice in Assam, India

A two-year field experiment was conducted during 2016 and 2017 to assess the influence of microclimatic and thermal indices on Sali rice growth and yield attributes under a modified paddy-ecosystem. The study involved two crop establishment methods (C₁: System of Rice Intensification [SRI], C₂: Conventional), three transplanting dates (D₁: 26 June, D₂: 10 July, D₃: 25 July), and four hill densities (H₁: 33 hills m⁻², H₂: 25 hills m⁻², H₃: 20 hills m⁻², H₄: 16 hills m⁻²). Treatments were arranged in a factorial split-plot design with three replications, where crop establishment methods and transplanting dates were assigned to main plots and hill densities to subplots. Results revealed that the SRI method markedly enhanced crop performance, recording the highest radiation use efficiency (RUE) of 1.37 g MJ⁻¹ and heat use efficiency (HUE) of 2.48 kg ha⁻¹ °d⁻¹, outperforming the conventional method. Early transplanting on 26 June (D₁) also improved microclimatic utilization, registering an RUE of 1.22 g MJ⁻¹ and HUE of 2.16 kg ha⁻¹ °d⁻¹. Hill density significantly influenced both indices, with the widest spacing (25 × 25 cm; 16 hills m⁻²) achieving the highest RUE (1.30 g MJ⁻¹) and HUE (2.36 kg ha⁻¹ °d⁻¹). Pooled analysis across years indicated that SRI produced significantly higher grain yield (57.13 q ha⁻¹), straw yield (61.43 q ha⁻¹), and harvest index (48.15%) than the conventional method. Similarly, early transplanting (26 June) resulted in superior grain (56.51 q ha⁻¹) and straw yields (62.00 q ha⁻¹) with a harvest index of 47.60%. Among hill densities, 16 hills m⁻² yielded the highest grain (56.75 q ha⁻¹) and straw yield (61.83 q ha⁻¹) with a harvest index of 48.01%. Correlation analysis of pooled data revealed a strong positive association of grain yield with leaf area index (LAI), chlorophyll content at all growth stages, and both RUE and HUE at harvest, indicating that enhanced microclimatic efficiency contributed substantially to yield improvement.