Effect of increasing severity of alternate wetting and drying (AWD) irrigation stress on yield and outcrossing potential of rice and weedy red rice

Authors: Gealy, David

Submitted to: Proceedings of Southern Weed Science Society
Publication Type: Proceedings
Publication Acceptance Date: 1/27/2020
Publication Date: 1/30/2020
Citation: Gealy, D.R. 2020. Effect of increasing severity of alternate wetting and drying (AWD) irrigation stress on yield and outcrossing potential of rice and weedy red rice. Proceedings of the Southern Weed Science Society 73rd Annual Meeting, January 27-30, 2020, Biloxi, Mississippi. 73:199.

Interpretive Summary:

Technical Abstract: Alternate wetting and drying (AWD) is a water-conserving irrigation system for rice that, when imposed with limited stress, can produce yields as high as in a conventional flood (FLD). In more severe and stressful forms, however, yields can be reduced significantly and interactions between rice and weedy red rice can be altered unpredictably. An understanding of the influence of AWD stress severity on the complex yield interactions and potential outcrossing interactions in red rice-infested rice fields is lacking. Therefore, a two-year field study was conducted to assess yields and outcrossing in red rice and rice plants in a conventional FLD and three levels of AWD stress severity. Plots consisted of a single red rice row planted between four, 18-cm-wide rice rows on each side, were 7.6 m in length, and were drill-seeded into separate irrigation bays. “Low”, “Medium”, and “High” levels of stress severity, established by withholding water progressively longer from the irrigation bays while imposing AWD cycles during the reproductive stage, reduced average soil moisture levels to ~30%, 21%, and 15% volumetric water content (VWC), respectively, before reflooding, and the resulting plant canopy temperatures were inversely correlated with these VWC levels. Compared with the FLD, High stress severity reduced leaf photosynthesis rates by nearly 50%, which recovered to near-normal levels after reflooding. The red rice row and the three adjacent rice rows on each side were harvested and composited. Yields of red rice were always higher than rice when expressed as Kg/ha or Kg/m row. In the conventional FLD, yields were similar both years. In AWD plots, red rice yields decreased with increased AWD severity in both years, however, rice yields decreased only at the High stress level in the second year, 2017. High stress severity reduced yields of rice and red rice by as much as 50% and 60%, respectively. Outcrossing, as determined from six SSR molecular markers derived from seed from rice plants and red rice plants growing in the same plots, was variable for the different irrigation levels and years, ranging from as low as 0.075% for Medium stress-red rice in 2016 to as high as 0.72% (a 10X increase) for the same treatment in 2017. Outcrossing in rice plants trended lower than in red rice plants in the lowest-severity AWD treatments (FLD and Low), but often trended higher than in red rice plants in the highest-severity treatments (Medium and High). In the first year, when soil VWC was highly depleted during the flowering synchronization period, outcrossing tended to increase with AWD stress severity in rice plants while decreasing with stress severity in red rice plants. In an attempt to better understand the dynamics of cross pollination in rice and red rice plants over both years and all irrigation levels, the numbers of rice and red rice seed/plot were calculated from yield/plot and weight/seed for each genotype. These seed numbers/plot were used to infer the number of fertile florets in rice and red rice plots that potentially had been available for outcrossing during the flowering period. The calculated difference, (no. rice seed/plot) – (no. red rice seed/plot), explained much of the broad range and variability in outcrossing rates among AWD severity levels and between years, in that outcrossing rates in rice and red rice plants were negatively and positively correlated, respectively, with this difference in the apparent number of fertile rice and red rice florets within plots. From these results it can be inferred that within plots, lower densities of rice florets relative to red rice resulted in an excess of red rice florets and pollen, which facilitated greater outcrossing in rice plants, whereas higher densities of rice florets relative to red rice resulted in a relative excess of rice florets, which led to greater outcrossing in