Loss of red rice seed dormancy in response to fall flooding and burial depth

Red rice (Oryza sativa L.) is one of the most spread weeds that infests rice production in many regions of the Southern United States. This weed is characterized by different morphological and physiological traits that permit to distinguish different populations. Red rice seeds show a variable level of dormancy which contribute to its persistence in the soil seed bank. Duration of dormancy varies according to the populations and seed storage conditions after the dispersion. Seed longevity seems to be not causal correlated with dormancy degree, but many studies brought to contrasting results. Several environmental factors can affect seed longevity such as soil depth and soil moisture, but also the seed characteristics play an important role. The aim of the study was to investigate red rice dormancy loss and longevity of 6 red rice populations and 1 rice cultivar buried in the soil. Seeds were collected in 3 counties: Lonoke, Lincoln and Desha in the Arkansas rice area. The burial experiment was carried out in two sites: Stuttgart and Kibler , AR. Red rice seeds were buried in the soil, both in flooded and non flooded conditions, at 3 different depths: 0, 3 and 6 inches. The difference between the 2 locations was the flooding condition treatment that in Stuttgart was applied flooding the field in the fall, while in Kibler the flooding condition was realized using bucket filled of water. At the excavation time, germinated and deteriorated seeds were removed and counted and the remaining seeds were incubated in Petri dishes at 30°C for 10 days. After this period, the non germinated seeds were undergone to the tetrazolium test to assess the viability. The results of the experiment showed the lowest germination at the shallower depth and the highest viability at deeper depths. Seed viability did not differ significantly between flooded and non flooded conditions, with the exception of the Petri dishes germination in which the seeds in flooded conditions showed a higher germination capacity. This result suggests that in flooded situations, the dormancy of red rice seeds was broken but the seeds were not able to germinate till the establishment of the favorable conditions. A blackhull awned population (LIN 1) showed the greatest dormancy, while among the other populations not significant difference were found in term of viability. In Kibler 2 populations, LON 5 and LON 6, showed the lowest viability in every conditions. The cultivated rice displayed the lower viability (60%) compared to red rice populations. In the experiment conducted in Kibler, no field germination was shown and the germination in Petri dishes was also lower compared to Stuttgart for all the population. The lower germination can be explained with the better dormancy preservation before the burial, the 2 weeks delayed of the burial compared to Stuttgart, and the different flooding conditions applied in this experiment. This first findings suggest a probable low degree of red rice seed longevity because of the higher germination and deterioration percentage after about one month of burial. According to previous studies, fall soil tillage need to be delayed till the next spring to permit the germination of red rice seeds in the early fall and in the spring, that facilitate the depletion of the soil seed bank. Moreover, red rice control can be more effective if the soil tillage are shallow to avoid the weed seed burial deep in the soil.