Early physiological and biochemical responses of soyabean to neighbouring weeds under resource‐independent competition

Abstract

Summary The mechanisms by which weeds compete with crop plants are poorly understood. To gain insight into these mechanisms, we characterised early physiological responses of soyabean to neighbouring weeds using a biological weedy system that generated a consistent far‐red‐enriched light environment and excluded direct resource competition. Neighbouring weeds decreased superoxide dismutase activity in unifoliate leaves. This coincided with increased hydrogen peroxide (H2O2) and oxidized ascorbate levels, while the steady‐state level of superoxide, catalase activity and lipid peroxidation remained unchanged. These responses suggested increased leaf production of singlet oxygen (1O2), which was demonstrated by detection of increased Singlet Oxygen Sensor Green (SOSG) fluorescence within 3 h after staining of unifoliate leaves. This finding was further supported by increased ratios of the photosensitiser protochlorophyllide to both chlorophyllide a, and total chlorophyll in the dark as well as enhanced sensitivity to cell death by a 1O2‐generating compound in the light. These responses coincided with dramatic changes in photosynthesis, carbon partitioning and biomass allocation with a persistent decline in leaf sucrose level and biomass production at later growth stages. This study provides direct experimental evidence that under resource‐independent competition, far‐red‐enriched light reflected by neighbouring weeds can alter the balance between ROS production and detoxification and thereby generate an oxidative stress signal in soyabean leaves.

Duration of Weed Presence Influences the Recovery of Photosynthetic Efficiency and Yield in Common Bean (Phaseolus vulgaris L.)

“…An alternative explanation is that low R/Fr reduces photosynthetic efficiency through imbalanced excitation of two photosystems and generation of reactive oxygen species (Chow et al, 1990). Reactive oxygen species are known to impair CO 2 assimilation (Foyer and Shigeoka, 2011) and exposure to low R/Fr light during early growth stages of soybean (McKenzie-Gopsill et al, 2019) and Arabidopsis (Page et al, 2017) increases singlet oxygen ( 1 O 2 ) generation and reduces A n . The concurrent declines in A n , PSII and J as well as the restriction on V c,max could be evidence of oxidative stress due to an imbalance in energy distribution between the photosystems from reflected Fr light.…”

“…Other studies reported the negative impact of Fr light on the synthesis, activity and capacity of Calvin cycle enzymes (Graham et al, 1968;Schopfer, 1977;Fox et al, 2015). Reductions in photosynthesis and changes in the activity of Calvin cycle enzymes as a result of proximity to neighboring plants or phytochrome depletion are also implicated in reduced biomass and yield in Arabidopsis, maize, and soybean (Kasperbauer, 1987;Iqbal and Wright, 1999;Reynolds et al, 2007;Kharshiing and Sinha, 2016;Yang et al, 2016;McKenzie-Gopsill et al, 2019). The impacts of Fr light on photosynthetic responses, however, are not consistent across plant species.…”

“…Despite the demonstration of long-term impact of weed presence during the CPWC on season-long crop growth, development and yield potential, the underlying mechanisms have remained largely unknown. A solid body of evidence indicates that phytochrome depletion via Fr light reflected from neighboring weeds and induction of the SAR impact a suite of photosynthetic processes (Eskins and Duysen, 1984;Barreiro et al, 1992;Boccalandro et al, 2009;Liu et al, 2012;Hu et al, 2013;Fox et al, 2015;Yang et al, 2016;McKenzie-Gopsill et al, 2019). Other studies reported the negative impact of Fr light on the synthesis, activity and capacity of Calvin cycle enzymes (Graham et al, 1968;Schopfer, 1977;Fox et al, 2015).…”

Varying Weed Densities Alter the Corn Transcriptome, Highlighting a Core Set of Weed‐Induced Genes and Processes with Potential for Manipulating Weed Tolerance

“…Indeed, under field conditions genes associated with phytochrome responses in corn were impacted in response to weeds (Horvath et al, 2018). There is considerable evidence to support the hypothesis that far-red-enriched light caused by the presence of nearby weeds results in enhanced oxidative stress through the generation of singlet oxygen in the chloroplasts (McKenzie-Gopsill et al, 2019). Previous studies in corn grown in the presence of weeds also supported this hypothesis, in that the genes involved in Photosystem I protection were consistently upregulated by weed presence (Horvath et al, 2018).…”

“…Indeed, increased ratios of far‐red light perceived by crops growing in the presence of weeds induce oxidative stress responses that damage the plant and inhibit photosynthetic processes (Ballaré and Pierik 2017). Other studies have indicated that enrichment of far‐red light from nearby weeds increases the concentrations of singlet oxygen (McKenzie‐Gopsill et al, 2019). These responses occur even when there is no direct contact or shading between the crops and the weeds (Liu et al, 2009).…”