Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/25/2016
Publication Date: 9/12/2017
Citation: Heuschele, D.J., Pinson, S.R. 2017. Small Differences in Water Level Impacted Straighthead Ratings. Rice Technical Working Group Meeting Proceedings. p. 97. March 1-4, 2016. CDROM.
Technical Abstract: Straighthead (STHD) is a physiological disorder that results in sterile rice panicles that remain upright at maturity. Depending on the rice variety and its level of host plant resistance, yields can be reduced significantly. The underlying cause of STHD is unknown, but it has been linked to nitrogen deficiency, temperature changes at panicle development, flooding, and the addition of monosodium methanearsonate (MSMA) herbicide. US rice varieties are screened and scored for STHD resistance by inducing STHD with MSMA soil amendment in designated field nurseries. At plant maturity, rice varieties are rated for STHD severity on a scale of 0 to 9; “0” rating indicating no signs of stress with > 90% seed set and no parrot beaking or twisting of panicles, and “9” rating for plants that are stunted and do not extrude panicles. The STHD screening nursery at the Dale Bumpers National Rice Research Center, Stuttgart, AR, has been in operation for 40 plus years. Annual MSMA amendment is needed to induce STHD in the nursery. The MSMA is sprayed at a rate of 6.72kg per hectare then incorporated into the soil the day of planting. Multiple plots of known resistant and susceptible check varieties (usually ‘Zhe733’and ‘Cocodrie’, respectively) are planted across each field to verify that STHD induction is consistent throughout each experiment. Anaerobic soil conditions are critical for the induction of STHD, therefore a permanent flood is applied as early as plants are tall enough (generally 2 weeks after emergence), recharged regularly, and held on the field until all plants have matured and been rated for STHD response. During the 2015 growing season an F3 mapping population was evaluated for STHD and the parental lines were used as repeated checks in these fields. The parents were selected based on previous years of study that placed them on opposite ends of the STHD scale; ’Jefferson’ rated as resistant (0-2), and ’Grassy’ as highly susceptible due to severely stunted plant height, no emergence of panicles, and rated consistently “9” for STHD. Most rice experimental fields have a slight slope to aid in irrigation; this results in some variation in water depth within most field studies. The 2015 fields contained even wider plot-to-plot differences in flood depth due to the development of significant soil level irregularities (low spots and high spots) since the last laser leveling. Maintaining a deep flood in the STHD nursery in 2015 was particularly problematic due to short levee heights that limited the maximum flood depth at the highest point in the experimental field to approximately 7.5 cm. Fields were recharged multiple times a week to maintain a constant flood; flood depth fluctuated by approximately 3 cm between recharges. As much as one month before heading it was apparent that some Grassy plots were less stunted than others. To test the hypothesis that even small differences in flood depth, as found within a single experimental field, might be introducing non-genetic variability in STHD ratings, we began monitoring flood depth variation within the Grassy/Jefferson F3 research fields. Maximum flood depths for each Grassy/Jefferson check plot location were determined at two growth stages, each time immediately after the permanent flood was recharged; with plot averages ranging each time from 7.5 to 13.5 cm maximum flood depth. Check and test plots containing segregating Grassy/Jefferson F3 progeny were rated at maturity for STHD and plants measured for height. Chlorophyll concentrations were measured using an atLEAF hand held spectrometer on the uppermost leaves at V-4 (before permanent flood, thus before soil arsenic became highly available) and again at heading (flag leaves). A positive linear relationship between water depth and STHD ratings occurred for both Grassy and Jefferson. The confounding effect of water-depth