1400 progeny plants from all generations (five early flowering time-selected generations, three late-flowering time-selected generations and eight generations of unselected controls) were grown in the same environment, under drip irrigation in a randomised block design.
PHOTO: Mike Ashworth
The use of mechanical weed-seed control for wild radish at harvest may see plants select for earlier flowering types
Since the introduction of zero-till, herbicides have been the primary tool for controlling yield-depleting weeds in cropping systems, resulting in widespread and increasing levels of herbicide resistance.
This has led to the use of non-chemical integrated weed management (IWM) strategies, one of which is harvest weed-seed control – which intercepts and destroys weed seeds at harvest before they enter the seedbank.
However, like herbicides, this highly effective technique also applies an intense selection pressure for any growth trait – such as early flowering and early seed shedding – that may enable a weed to evade seed destruction.
Dr Michael Ashworth, formerly of the Australian Herbicide Resistance Initiative (AHRI), has conducted GRDC-funded research into the adaptability of flowering time and growth rates in wild radish (Raphanus raphanistrum).
Dr Ashworth selected for early maturity over five successive generations, and for late maturity over three.
He found wild radish demonstrated a remarkable adaptive capacity to halve its time to flowering after just five generations of selection for early flowering.
“This study demonstrates that if these highly effective seed collection techniques are routinely used, without a break, wild radish will likely evolve to avoid the control measure,” Dr Ashworth says.
For growers, it means that herbicide resistance is not the only obstacle to successful weed management – over-reliance on any single agronomic weed-control practice is likely to result in rapid selection of avoidance traits.
Adaptability of wild radish
Wild radish is a genetically diverse, highly adaptable species, which has been found to thrive in diverse environments and farming systems.
Dr Ashworth’s work shows that when selected for reduced time to flowering, wild radish plants can flower at far lower temperature requirements than normally observed in the field.
Previous studies have indicated that wild radish can flower in as little as 600 growing degree days (°C d).
Growing degree days are a measure of the accumulated daily temperatures during the growing season required for plant development. Plants in higher temperature growing regions tend to develop faster than in cooler regions.
However, following five generations of early flowering time selection in this study, wild radish reduced its thermal requirement before flowering to just 344°C d.
One wild radish plant in the study actually flowered just 22 days after emergence in the middle of winter, demonstrating a thermal requirement of just 281°C d.
Conversely, late flowering time selection over three generations increased thermal time to 1314°C d.
Dr Ashworth says the results of this study clearly demonstrate the diversity and rapid adaptability of wild radish.
The genetic basis for the adaptation has not yet been determined, although similar research in other plant species points to multiple genes being involved.
While this study was done in a glasshouse, the selection treatments mimicked a strong selection force (such as harvest weed-seed control) acting on plant populations that would usually flower at the same time as the crop.
Selections also had several phenotypic consequences for the weed. The early flowering biotypes flowered at a reduced plant size and had less structural integrity, making them more prostrate and with reduced seed production.
This would most likely make the wild radish less competitive within a crop. If this proved to be the case, Dr Ashworth suggests harvest weed-seed collection should be best reserved for tackling other more competitive crop weeds.
In order to manage the growth habit of wild radish, Dr Ashworth reasons that once weed seedbanks have been reduced to manageable levels, the use of harvest weed-seed collection might even be able to be periodically suspended. This would allow a wild radish population to revert its flowering time back to its climatic optimum and become vulnerable once more to control at harvest time.
Dr Mike Ashworth,
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