Estimation of flowering time and its effect on grain yield of photoperiod sensitive varieties in rainfed lowland rice in Northeast Thailand

Abstract

Flowering time of photoperiod sensitive varieties is an important determinant of grain yield of rainfed lowland rice, and is affected by the growing environment. The present study aimed to quantify variety specific flowering time response to photoperiod under irrigated conditions and demonstrate the usefulness of delay in flowering observed in rainfed lowland fields as an index of severity of water deficit affecting yield. Both irrigated on-station and rainfed lowland on-farm experiments were conducted with 4 popular photoperiod sensitive varieties in Northeast Thailand. The irrigated experiment was conducted with 12 sowing times throughout a year and flowering time was quantified with prevailing daylength and temperature. Among the 4 varieties, the minimum time to flowering varied from 68 to 77 days, and maximum daylength required for panicle initiation varied from 12 hrs 21 min to 12 hrs 39 min. Flowering time of all varieties was delayed in 3–4 sowings due to cold during vegetative stage; these sowings did not coincide with farmers’ common sowing time. On-farm flowering time and yield data were obtained from 108 crops grown under rainfed lowland conditions across 8 provinces in Northeast Thailand in 2013–2015, and were used for validating flowering time and grain yield simulation based on the sequential sowing experiment. Both curvilinear and linear equations to estimate time to flowering from sowing time gave a similar result. Delay in flowering due to water deficit observed on-farm was estimated as the difference between the observed on-farm and simulated flowering time under irrigated conditions, and the estimated delay was related to grain yield reduction due to water deficit. It is concluded that simulated flowering time under irrigated conditions together with observed flowering time in farmer’s fields can be used to determine flowering delay under rainfed conditions, and the delay is an indication of the severity of water deficit reducing yield. Extension material may be developed in which estimated flowering time of key varieties under well-watered conditions may be tabulated for various sowing times and locations in the region, and then farmers can determine delay in flowering and likely yield reduction for their own and neighboring crops.