Date Published: June 28, 2019
Publisher: Public Library of Science
Author(s): Aakansha Chadha, Singarayer K. Florentine, Bhagirath S. Chauhan, Benjamin Long, Mithila Jayasundera, Jose Luis Gonzalez-Andujar.
Global temperatures are predicted to increase by 1.5–5.9°C during this century, and this change is likely to impact average rainfall, with predictions that water deficit will perhaps be the most severe threat to sustainable agriculture. In this respect, invasive weeds, which have traits better adapted to drought stress than crops, add to concerns regarding crop sustainability. Lactuca serriola, an aggressive agronomic weed is thought to be a successful weed because of its ability to maintain high water use efficiency under drought conditions. In this study, experiments were conducted to examine the influence of different soil moisture regimes (100%, 75%, 50% and 25% water holding capacity (WHC)) on growth, photosynthetic capacity, leaf biochemistry and reproduction of this species. Soil moisture significantly affected plant’s height, stem diameter, number of leaves and biomass. The highest plant height (115.14 cm ± 11.64), shoot diameter (9.4 mm ± 0.18), leaf area (1206.5 mm2 ± 73.29), plant fresh weight (83.1 ± 3.98) and dry weight (22.38 ± 1.24) were recorded at 75% soil moisture content. A fundamental adaptation to drought was observed as plants in the 25% WHC treatment had the highest root: shoot ratio. Soluble sugars and total phenolic content were highest in the 25% WHC treatment and significantly different to 100% WHC which was a response to soil moisture stress to ameliorate the damaging effects of reactive oxygen species produced under stress conditions. Results also indicate that L. serriola can survive and produce seeds under water stress as more than 6000 seeds were produced per plant in all WHC treatments. In this study, there was no significant difference in the seed weight, number of seeds produced and their germination ability. This can have a huge impact on agricultural systems as the species can survive both under low and high soil moisture conditions. We therefore suggest that the demonstrated ability of L. serriola to complete its life cycle and produce biomass and seeds under water stressed conditions leads to the introduction of strategies that minimize weed survival while maximizing irrigation efficiency for the crop. A clear understanding of the ecological and biological characteristics of this weed will help land managers take appropriate control measures to mitigate the effect of this species on economic crop productivity.
Plants undergo or display symptoms of extreme water deficiency when the required levels of moisture are unavailable in their habitat soil. This happens when the plants continuously lose water via transpiration or evaporation due to high temperatures and the loss of ground moisture is not refurbished . This extreme dryness, declared as drought, which extends to long periods of time, is prevalent on a global scale. These conditions combine lack of water through rain with high temperatures and radiation, and currently pose the most important environmental threats to plant survival and crop productivity . The prevailing stress conditions are exacerbated by competition from associated weed species, due to photosynthetic decrease, constraint of metabolic processes and interference with nutrient availability [3, 4]. With regard to the outcomes of competition for water in a cropping situation, it depends on the abilities of the crop and weed species to survive under water stress conditions . It has been noted that invasive plant species, by virtue of their traits, are more adaptable to water stress than crop and pasture species . Thus, prevailing arid conditions is an important factor in weed invasion, as it impacts the competitive establishment, physiology, subsequent growth and reproduction of the mixture of plants in a crop , making it an important element of crop production studies.
The above study clearly indicates that Lactuca serriola has the adaptability to survive low soil moisture conditions, as low as 25%, while sustaining an important function like seed production close to optimum levels. Exposure of L. serriola to drought stress resulted in decreased growth rate, and reduced biomass production. A high root to shoot ratio, as well as elevated biomass of the root, allows it to efficiently absorb the required amount of water from the soil and ensure transference to its above-ground parts. An increase in soluble sugars and phenolic content, which are mechanisms to tolerate drought stress was observed as moisture stress increased. Although moisture stress reduced the overall plant biomass, it failed to make any substantial impact on the reproductive abilities of this plant type. While physiological responses were decreased with respect to drought stress, however, the plants were healthy and completed their life cycle even at the low soil moisture level of 25% water holding capacity. Due to its ability to tolerate drought stress, L. serriola is very likely to expand its range under a drying climate. Its ability to sustain growth through morphological adaptation, physiological and biochemical regulation, even during times of water stress, ensures L. serriola with a robust mechanism to continue spreading to new regions.