Increasing AWD irrigation severity during the reproductive stage reduces yields and alters outcrossing dynamics between rice and red rice

Authors: Gealy, David

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2021
Publication Date: N/A
Citation: N/A

Interpretive Summary: Alternate wetting and drying (AWD) is a conservation irrigation system for rice in which the field soil is intermittently flooded and then allowed to dry down naturally to a predetermined level of dryness before reflooding. When properly implemented, AWD can save water and maintain high yields, and reduce emissions of greenhouse gases. However, the impact of AWD on yield and the potential for outcrossing between rice and weedy red rice needs to be better understood to help assure long-term viability of these management systems. To address this question, a common biotype of red rice and a herbicide-resistant inbred rice variety were grown together in field plots at Stuttgart, Arkansas under a conventional/continuous flood (FLD), and at “Low”, “Medium”, and “High” levels of AWD stress severity to determine their respective impacts on yield and outcrossing frequencies. Red rice yields decreased at all three levels of AWD severity in the two years of the study. Rice yield response was similar to red rice in the first year but decreased only at the High level of stress in the second year. Outcrossing in the study ranged widely from frequencies as low as 0.075% to as high as 0.72%, and tended to be lower from red rice into rice (as female) than from rice into red rice at the lowest-severity levels (FLD and Low), and vice versa at the highest-severity levels (Medium and High). This may mean that higher AWD severities will be more favorable to outcrossing in rice, and lower AWD severities more favorable to outcrossing in red rice. However, another factor, which seemed to explain the wide range in outcrossing observed in the two years of the study and was not related directly to AWD stress level, was the difference between the number of seeds produced by rice and red rice plants in each plot. When seed numbers were relatively greater for rice than red rice, outcrossing in red rice tended to increase, and when seed numbers were relatively greater for red rice than rice, outcrossing in rice tended to increase. It may be that AWD severity affects yields as well as gradients of pollen movement between rice and red rice plants, resulting in the very wide range of outcrossing rates that were observed. It should also be noted that, whatever the underlying causes, increases in outcrossing in rice are more likely to affect the economic value of the crop because those outcrossed seeds are produced on seed heads of rice, and most of them are removed from fields along with the harvested grain. Increases in outcrossing in red rice, however, may contribute more to future problems within the same field because those outcrossed seeds are produced on seed heads of the red rice, and a large proportion of them will fall back onto the soil due to the high shattering potential of red rice plants. Overall, our findings have helped clarify previously unknown and complex interactions between weedy red rice and rice that farmers should be aware of when practicing AWD irrigation.

Technical Abstract: Alternate wetting and drying (AWD) irrigation can affect yield and outcrossing between rice and red rice, but these interactions are not well understood. To address this question, yield and outcrossing were assessed in a conventional flood (T1: FLD) and three levels of AWD-stress severity in a two-year field study. “Low” (T2), “Medium” (T3), and “High” (T4) levels of severity, established by withholding water for progressively longer periods of time within AWD cycles during the reproductive stage, reduced average soil moisture levels to approximately 30 percent, 21 percent, and 15 percent soil volumetric water content (VWC; -14, -24, and -43 kPa water potential equivalents), before reflooding. Red rice yields decreased with increased AWD severity in both years, whereas rice yields decreased only at the High level (T4) in 2017 (apparently due to interceding rainfall). Outcrossing rates in the study ranged from 0.075% to 0.72%. Generally, outcrossing in rice (as female) tended to be lower than in red rice (as female) in the lowest-severity treatments (T1 and T2), and often was higher than in red rice in the highest-severity treatments (T3 and T4), suggesting that high AWD severity at the flowering stage favors outcrossing in rice over red rice. However, an important factor that appeared to explain much of the range and variability in outcrossing over the two years irrespective of irrigation treatment was the floret density difference between rice and red rice within plots. Outcrossing in rice and red rice were negatively (r=-0.56, P=0.0044) and positively (r=0.65, P=0.0006) correlated, respectively, with floret density difference. It is hypothesized that reduced densities of rice florets relative to the highly prolific red rice resulted in a relative excess of red rice florets and pollen that facilitated greater outcrossing in rice, whereas increased densities of rice florets relative to red rice resulted in a relative excess of rice florets that lead to greater outcrossing in red rice. Thus, AWD severity may have affected floret density and/or seed fill, which influenced yields as well as pollen-stigma gradients between rice and red rice, resulting in the range of the observed outcrossing rates.