Some plants can “escape” drought stress by speeding up their life cycles, minimising water loss and allocating more resources to roots when water is available. Over half of the world’s worst weeds are annuals and occur in arid or semi-arid regions. Understanding how some weeds can tolerate or escape water stress can help with predicting weed distributions in the future.
Drs Shana Welles and Jennifer Funk from Chapman University and UC Davis studied the invasive California wild radish (hybrid between Raphanus raphanistrum and R. sativus) populations across an aridity gradient in California. The scientists found that invasive wild radishes from arid regions flower earlier and grow differently than wild populations in the southern regions and cultivated radishes. Whilst there were no clear differences in the wild populations along the latitudinal gradient, this is the first study to document drought escape and tolerance by the California wild radish. The researchers have recently worked on the evolution of the invasive Salsola ryanii plant which can form tumbleweeds and the phenotypic plasticity of the California sagebrush, Artemisia californica.
Two wild radishes (Raphanus raphanistrum and R. sativus) were introduced in the 19th century in California. The two species hybridised which led to the invasive California wild radish which has replaced all the native R. raphanistrum.
The scientists collected seeds from six wild populations in California and also purchased seeds from five cultivated radish varieties. The seeds were weighed and the plants were grown under two watering regiments (“high” and “low”) outside Chapman University for two years. Researchers measured plant growth, gas exchange, photosynthetic rate, water use efficiency, leaf mass per area and after harvesting some of the radishes after the first year, they measured leaf nitrogen (N) concentration and dry weight of above- and belowground plant parts. In the second year, two wild populations were grown again under lower soil nutrient conditions.
Welles and Funk found that the cultivated radish varieties had delayed flowering, more biomass belowground and lower leaf mass per area than wild populations. Wild California radish populations from arid regions flowered earlier than northern ones but overall, the phenotypic plasticity did not differ along the latitudinal gradient.
Aridity negatively correlated with days to maturity and plants that flowered earlier had high resource acquisition, leaf N concentration and growth rate. Overall, water stress lowered photosynthetic rate, smaller leaf mass per area (LMA) but higher leaf N concentration.
“[W]hile LMA increases along aridity gradients at a global scale, annuals exhibiting a drought escape strategy might benefit more from maximizing rates of carbon assimilation and growth in order to flower and seed before the onset of the summer drought”, Welles and Funk wrote.
“Thus, creating thinner leaves (low LMA) to maximize carbon assimilation is advantageous, which is consistent with our observations in this study.”
“Studies, such as this one, that examine the extent to which invasive species adapt to changing climate are urgently needed to enhance distribution models for invasive species.”