Analysis of yield constraints and seasonal solar radiation and temperature limits for stable cultivation of dry direct-seeded rice in northeastern Japan

Context: Dry direct-seeded rice cultivation is an important option for farmers to reduce labor costs, particularly in northeastern Japan. However, low and variable yields limit its dissemination in this cool temperate zone. Meteorological conditions during the grain-filling period pose the major constraints on yield, but the limitations they impose on crop development remain poorly understood quantitatively.Objective: We aimed at determining factors limiting rice production in a cool region by examining the quanti- tative relationships among the sink- and source-related factors, including spikelet density, canopy cover, and solar radiation. We attempted to provide a framework to estimate the seasonal latest limit of heading date, an important determinant for suitable cropping seasons based on temperature or solar radiation conditions.Methods: We used field experimental data of four cultivars with different nitrogen regimes (3 in Morioka and 5 in Hanamaki, both in Iwate Prefecture, Japan) on dry direct-seed rice. We performed multiple regression using spikelet density and canopy solar radiation interception during grain filling to analyze their effect of grain yield. We used structural equation modeling to develop a model explaining the effects of N, climate, and cultivar maturity on grain yield through regulation of canopy cover at the heading stage and spikelet density. We derived the seasonal latest heading date for securing a certain yield level based on air temperature or solar radiation.Results: Spikelet density and canopy solar radiation interception explained 82% of the variation in grain yield. Structural equation modeling revealed that grain yield was affected more by source-related canopy cover at heading than by sink-related spikelet density. Crop nitrogen content at heading strongly affected grain yield, and its effect appeared to be stronger on canopy cover than on spikelet density. For the past 10 years, the seasonal latest heading date estimated from solar radiation for dry direct-seeded rice in Morioka was on average 2.5 days earlier than that estimated from air temperature; this suggests the importance of solar radiation in determining suitable areas for dry direct-seeded rice.Conclusions: Enhancing source capacity by considering canopy cover and solar radiation conditions during the grain-filling period is key to successful dissemination of dry direct-seeded rice in northeastern Japan. Implications: Our determination of yield constraints and of solar radiation- and temperature-limited growing seasons can assist local extension services by indicating suitable areas and cultivars for dry direct-seeded rice. The estimation of the solar radiation-limited growing season is a novel finding in this study.