Herbicides are the foundation of weed control in commercial crop-production systems. However, herbicide-resistant (HR) weed populations are evolving rapidly as a natural response to selection pressure imposed by modern agricultural management activities. Mitigating the evolution of herbicide resistance depends on reducing selection through diversification of weed control techniques, minimizing the spread of resistance genes and genotypes via pollen or propagule dispersal, and eliminating additions of weed seed to the soil seedbank. Effective deployment of such a multifaceted approach will require shifting from the current concept of basing weed management on single-year economic thresholds.

http://ahri.uwa.edu.au/wp-content/uploads/2014/10/573-Norsworthy-et-al-Weed-Sci-2012-HR-BMPs.pdf

Reducing the Risks of Herbicide Resistance: Best Management Practices and Recommendations

https://pubag.nal.usda.gov/download/14449/PDF

Environmental control of dormancy in weed seed banks in soil

Knowledge and prediction of seasonal weed seedling emergence patterns is useful in weed management programs. Seed dormancy is a major factor influencing the timing of seedling emergence, and once dormancy is broken, environmental conditions determine the rate of germination and seedling emergence. Seed dormancy is a population-based phenomenon, because individual seeds are independently sensing their environment and responding physiologically to the signals they perceive. Mathematical models based on characterizing the variation that occurs in germination times among individual seeds in a population can describe and quantify environmental and after-ripening effects on seed dormancy. In particular, the hydrothermal time model can describe and quantify the effects of temperature and water potential on seed germination. This model states that the time to germination of a given seed fraction is inversely proportional to the amount by which a given germination factor (e.g., temperature or water potential) exceeds a threshold level for that factor. The hydrothermal time model provides a robust method for understanding how environmental factors interact to result in the germination phenotype (i.e., germination pattern over time) of a seed population. In addition, other factors that influence seed dormancy and germination act by causing the water potential thresholds of the seed population to shift to higher or lower values. This relatively simple model can describe and quantify the germination behavior of seeds across a wide array of environmental conditions and dormancy states, and can be used as an input to more general models of seed germination and seedling emergence in the field.

https://www.jstor.org/stable/pdfplus/4046371.pdf

Applications of hydrothermal time to quantifying and modeling seed germination and dormancy

Seed persistence is the survival of seeds in the environment once they have reached maturity. Seed persistence allows a species, population or genotype to survive long after the death of parent plants, thus distributing genetic diversity through time. The ability to predict seed persistence accurately is critical to inform long-term weed management and flora rehabilitation programs, as well as to allow a greater understanding of plant community dynamics. Indeed, each of the 420000 seed-bearing plant species has a unique set of seed characteristics that determine its propensity to develop a persistent soil seed bank. The duration of seed persistence varies among species and populations, and depends on the physical and physiological characteristics of seeds and how they are affected by the biotic and abiotic environment. An integrated understanding of the ecophysiological mechanisms of seed persistence is essential if we are to improve our ability to predict how long seeds can survive in soils, both now and under future climatic conditions. In this review we present an holistic overview of the seed, species, climate, soil, and other site factors that contribute mechanistically to seed persistence, incorporating physiological, biochemical and ecological perspectives. We focus on current knowledge of the seed and species traits that influence seed longevity under ex situ controlled storage conditions, and explore how this inherent longevity is moderated by changeable biotic and abiotic conditions in situ, both before and after seeds are dispersed. We argue that the persistence of a given seed population in any environment depends on its resistance to exiting the seed bank via germination or death, and on its exposure to environmental conditions that are conducive to those fates. By synthesising knowledge of how the environment affects seeds to determine when and how they leave the soil seed bank into a resistance–exposure model, we provide a new framework for developing experimental and modelling approaches to predict how long seeds will persist in a range of environments.

The ecophysiology of seed persistence: a mechanistic view of the journey to germination or demise

Temperature ( T ) and water potential ( y ) are two primary environmental regulators of seed germination. Seeds exhibit a base or minimum T for germination ( T b ), an optimum T at which germination is most rapid ( T o ), and a maximum or ceiling T at which germination is prevented ( T c ). Germination at suboptimal T can be characterized on the basis of thermal time, or the T in excess of T b multiplied by the time to a given germination percentage ( t g ). Similarly, germination at reduced y can be characterized on a hydrotime basis, or t g multiplied by the y in excess of a base or threshold y that just prevents germination ( y b ). Within a seed population, the variation in thermal times to germination among different seed fractions ( g ) is based on a normal distribution of y b values among seeds ( y b ( g )). Germination responses across a range of suboptimal T and y can be described by a general hydrothermal time model that combines the T and y components, but this model does not account for the decrease in germination rates and percentages when T exceeds T o . We report here that supra-optimal temperatures shift the ψ b ( g ) distribution of a potato ( Solanum tuberosum L.) seed population to more positive values, explaining why both germination rates and percentages are reduced as T increases above T o . A modified hydrothermal time model incorporating changes in ψ b ( g ) at T > T o describes germination timing and percentage across all T and ψ at which germination can occur and provides physiologically relevant indices of seed behaviour.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2002.00894.x

Rudolfo Sanchez

Papers

Modeling seedling emergence