Predicting weed seed persistence: towards a technique for rapid and reliable assessment

Field: Agricultural and Biological Sciences (Life Sciences)

Management of weed species is often complicated by the unknown status of their soil seed banks. Field trials for evaluating seed persistence, whilst accurate in their context, are timeconsuming and expensive. Weed scientists and land managers alike would benefit from a more rapid method for estimating weed seed longevity. This study aimed to develop a laboratory-based method for predicting weed seed persistence in the field. The accelerated ageing test, which ages seeds at high relative humidity (RH) and temperature, is used at the Royal Botanic Gardens, Kew, UK, to predict the rate of death of seeds kept in cold dry storage for conservation purposes. In this study, a field seed burial experiment, laboratory experiments, and longevity data provided by other research organisations were used to thoroughly assess whether results of the accelerated ageing test correlate with field persistence of seeds. Initially, the seed persistence of three weed species in the field was compared to their longevity in the accelerated ageing test. A 21-month field trial was established to evaluate the persistence of Avena sterilis L. ssp. ludoviciana (Dur.) Nyman (wild oats), Gomphocarpus physocarpus E.Mey. (balloon cotton bush) and Ligustrum lucidum W.T.Aiton (broadleaf privet) seeds in three contrasting soil types introduced to the same location. The viability and vigour of filled seeds retrieved from the field trial were monitored, as were the physical and chemical characteristics of the soils. Seed persistence and vigour of each species varied according to soil type in the trial. However, species had the same order of life expectancy (shortest-lived to longest-lived) within each soil type. Importantly, field persistence of species supported the order of seed longevity for species in the accelerated ageing test. Moreover, a comparison of larger externally available data sets verified the observed correlation, indicating that the accelerated ageing test could be an informative predictor of seed persistence in the field. The correlation between seed persistence in the field and longevity in the accelerated ageing test, although positive, did not explain all of the variation in the field persistence data. Moisture fluctuations experienced by seeds in the field but not in the laboratory were hypothesised to influence seed persistence, and further laboratory studies were conducted to elucidate the effects of wetting and drying cycles on seed longevity. When seeds of G. physocarpus were hydrated to 94% RH or higher then re-dried, their longevity was extended, suggesting that rain may extend the persistence of seeds of some species. In other laboratory tests, the physiological basis for the correlation between field and accelerated ageing results was investigated. As seeds age, deteriorative processes such as lipid peroxidation and free radical accumulation cause membranes to become more permeable and the redox status of antioxidants to change. Membrane integrity and antioxidant capacity differed between A. sterilis seeds that were aged in the field and those aged in the accelerated ageing test. The disparities were attributed to differences in the moisture content of seeds between the two environments. Thus, despite the correlation between the longevity of seeds under natural and artificial ageing, the physiological basis for the relationship was dubious. To better simulate field ageing conditions on seeds in the laboratory, a new laboratory test was developed. In the soil-specific accelerated ageing test (SSAAT), more factors from natural environments, including soils, were introduced to the accelerated ageing regime. When RH and temperature were controlled, no differences in seed longevity were observed between soil types. These results suggest that temperature and seed moisture content are the two major factors affecting seed longevity in moist unsaturated soils; soils interact with climate to alter these parameters and affect seed ageing in situ. Thus the SSAAT conferred no advantages over the standard accelerated ageing test protocol. Based on its positive correlation with field persistence and the lack of improved predictions arising from SSAAT, the standard accelerated ageing test was re-embraced as an informative predictor of weed seed persistence. Comparison of the seed longevity of 13 Queensland weed species in the accelerated ageing test with field persistence data enabled three persistence categories to be defined. Results of the 3-month accelerated ageing test may now be used to predict the maximum likely field persistence of a seed lot within the categories of ;l> 3 years. In summary, this study identified a laboratory-based test for estimating seed persistence in the field, and improved our understanding of the physiology of natural seed ageing. In the future, these advances should enable better-informed weed management decisions, as the appropriate duration of weed monitoring programmes can now be readily estimated.