Ozone flux-effect relationship for early and late sown Indian wheat cultivars: Growth, biomass, and yield

Highlights

• Sensitivity of early and late sown Indian wheat cultivars to O₃ were evaluated.

• O₃ flux-effect relationships were investigated for AGB and yield in wheat plants.

• Higher accumulation of POD₆SPEC was found in early sown than late sown cultivar.

• Higher longevity, g_max and lower critical level made early sown cultivars more sensitive.

• The grain yield loss under EO₃ was 30.5 % in early and 23 % in late sown cultivars.

Abstract

Precise ozone (O₃) flux models remain helpful to identify the sensitivity of crops and their cultivars under high O₃ levels by assessing biomass and yield losses under natural climate and future O₃ levels. In this study, we have evaluated the effects of O₃ on growth, biomass, and yield of four Indian wheat cultivars by exposure-linked as well as through flux-effect relationship-based analysis using two years of empirical data. Early and late sown cultivars were exposed to ambient O₃ (AO₃) and elevated O₃ i.e., ambient+20 ppb (EO₃). The POD₆SPEC (species-specific phytotoxic O₃ dose above a threshold of 6 nmol m⁻² PLA s^-1) on flag leaves for each cultivar and treatment was derived by using DO₃SE (Deposition of Ozone for Stomatal Exchange) model. Above-ground biomass was reduced more in early sown cultivars (26.1 %) than late sown (21.3 %) because of the larger negative impacts on plant growth under EO₃ treatment. Loss of grain yield under EO₃ exposure was high in early sown cultivars (30.5 %) compared to late sown (23 %) due to high stomatal O₃ uptake during the O₃ exposure period. Based on model outputs, early sown cultivars showed higher accumulated POD₆SPEC (1.6–3.75 mmol O₃ m⁻²) compared to late sown cultivar (0.65 to 1.97 mmol m⁻²). A negative linear relationship was obtained in above-ground biomass and grain yield concerning POD₆SPEC. The relationship showed that 0.284 mmol O₃ m⁻² accumulated POD₆SPEC is responsible for a 5% reduction of grain yield in early sown cultivars, while 0.393 mmol O₃ m^-2 accumulation is required for a 5% reduction in yield of late sown cultivars. The results also suggest that O₃ flux-effect relationship could be a valuable tool for assessing and predicting the risk of O₃ on wheat cultivars in India and for crop productivity models for predicting climate change impacts.

Introduction

Ground surface ozone (O₃) is the most damaging air pollutant to horticultural and agricultural crop plants (Mills et al., 2007; Osborne et al., 2019). During the preindustrial time, the background O₃ levels were 10−15 ppb. Owing to anthropogenic activity, the emissions of O₃ precursors (NOx, CO, and VOCs) have been increased. The ambient O₃ level has reached above the threshold of critical level over many regions of the world (Hartmann et al., 2013; Solberg et al., 2005). If current rising trends of O₃ continue, the annual global concentration of O₃ will reach up to 68 ppb by 2050 (Meehl et al., 2007). Air pollution and GHG emissions have risen dramatically in India over the past three decades due to tremendous growth in industrialization, urbanization, and transport. In recent decades, high O₃ concentration has been detected in several regions of India (Lal et al., 2017; Sharma et al., 2019; Singh and Agrawal, 2017; Yadav et al., 2020).

Wheat is a staple crop in India and has been identified as O₃ sensitive species (Feng et al., 2009; Singh et al., 2018). The deleterious effects of high O₃ on wheat are described as visible leaf injury, premature leaf senescence, and reduced leaf area, plant height, and above-ground biomass, which collectively lead to a reduction in the economic yield (Liu et al., 2015; Mishra et al., 2013; Ojanpera et al., 1992; Singh et al., 2018). Long-term assessments on crop loss due to O₃ have been documented since the 1980s in the USA under National Crop Loss Assessment Network (NCLAN), and in Europe under European Open-Top Chambers Programme (EOTCP) (Fuhrer, 1994; Heagle, 1989; Heck et al., 1984). Pleijel et al. (2014) have reported a 9% reduction in above-ground biomass and 14 % yield loss in wheat due to ambient O₃ concentration over the Europe. However, globally O₃ induced reduction in grain yield of wheat was established at 9.9 % in the northern hemisphere and 6.2 % in the southern hemisphere (Mills et al., 2018). The yield losses of 8.2–22.3 % in wheat were estimated for India because of high ambient O₃ (Tang et al., 2013). India's breadbasket, the Indo-Gangetic Plain (IGP), is one of the most air-polluted regions in the country and estimated to suffer nine million tons of wheat loss annually due to high surface O₃ (Lal et al., 2017).

Furthermore, the concentrations of O₃ above the critical levels are known for significant impairment in plant growth and economic yield (Ainsworth et al., 2012; Feng et al., 2009; Singh et al., 2018). To derive the critical level of O₃, cumulative exposure metrics such as AOT40 and cumulative stomatal flux-based metrics such as PODy have been used in many studies (Emberson et al., 2018; Mills et al., 2018). The stomatal O₃ flux methods are quite useful for assessing O₃ risk and provide a robust dose-response relationship between O₃ uptake and yield of wheat cultivars (CLRTAP, 2017; Harmens et al., 2018; Osborne et al., 2019; Pleijel et al., 2014; Wu et al., 2016). But, this approach is more dependent on input data as compared to the exposure-based approach. In the past, various exposure-based field studies have been conducted in India to assess the yield losses of major crops under ambient and future O₃ levels (Mishra et al., 2013; Mukherjee et al., 2020; Singh et al., 2018). In exposure-based metrics, we only consider the O₃ concentration in the air nearby the plants to measure its impact on plants. However, many important factors such as stomatal conductance, soil moisture, weather condition, vegetation characteristics, phenology of plant are not considered, which play vital roles in stomatal O₃ uptake (Feng et al., 2012; Mills et al., 2007).

As the sensitivity of a species to O₃ mainly depends on the amount of O₃ uptake and their intrinsic defense response (Dumont et al., 2013; Yadav et al., 2019), the flux-based concept is suggested to be the most preferred metrics over the exposure-based metrics (Anav et al., 2016; Pleijel et al., 2004). Over the last 20 years, many significant developments have been made in stomatal flux modelling to improve the O₃ risk assessment (CLRTAP, 2017; Emberson et al., 2018, 2000; Grunhage et al., 2012; Pleijel et al., 2007). The previous studies have found a strong correlation of phytotoxic O₃ dose above a flux-threshold of y (PODy) with grain yield and biomass of wheat (Grunhage et al., 2012; Harmens et al., 2018; Pleijel et al., 2014). Moreover, a significant negative flux-effect relationship has also been found, which helped in identifying the level of sensitivity of wheat cultivars under elevated O₃ (Grunhage et al., 2012; Harmens et al., 2018; Pleijel et al., 2014).

In the present study, Deposition of Ozone and Stomatal Exchange (DO₃SE) model was used for assessing the O₃ risk and to identify the species-specific O₃ sensitivity in Indian wheat cultivars. The DO₃SE model estimates stomatal O₃ flux of plant with a function of the O₃ concentration at and the transfer of O₃ across the boundary layer of leaf. Although several O₃ flux-based studies have been attempted in Europe, Southeast Asia and China (Danh et al., 2016; Feng et al., 2012; Osborne et al., 2019), but this will be the first O₃ flux-model based study for India to estimate stomatal O₃ uptake of wheat cultivars using multiplicative algorithms. The study will be important for screening O₃ tolerant Indian wheat cultivars to get the maximum yield against the future level of O₃. For this purpose, early and late sown wheat cultivars were selected based on their differences in sowing time, life span, and popularity in the IGP region of India. On availability of field after rice harvest, it is a common practice to sow wheat either in early winter at the start of November using early sown cultivars (life span 5 months) or up to mid of December with late sown cultivars (life span 4 months). The aims of the present study are: (i) to determine the effects of elevated O₃ on growth, biomass, and yield of early and late sown wheat cultivars; (ii) to parameterise the stomatal flux DO₃SE model for Indian wheat cultivars to calculate the species-specific accumulated stomatal O₃ flux (POD₆SPEC); and (iii) to drive O₃ flux-biomass and flux-yield relationships in early and late sown cultivars from the two years experimental data for O₃ risk assessment.