Efforts to maximise wheat yields are being compromised by phytotoxic levels of ground-level ozone which persist in many wheat-growing areas of the world. This research investigated inter-related questions regarding ozone’s effect on wheat growth, grain yield and quality, and effects on senescence and interactions with nitrogen, particularly during the critical reproductive growth stages. Throughout the research the key issue of wheat’s ozone sensitivity and the potential for breeding more ozone tolerant lines was explored.

During summer plant trials in 2018, 2019, and 2021 several UK spring wheat cultivars and synthetic wheat lines, along with wheat’s closest wild relatives, were grown in 6 L pots of soil, and exposed to a realistic range of ozone concentrations, from 30 ppb to 110 ppb, in unheated solardomes in North Wales (UK) over a prolonged (10.5 – 11.5 week) period, under normal nitrogen fertilisation regimes.

In Trial 1 it was found that, of the three closest wild relative genome donors which created hexaploid wheat (Triticum aestivum L., AABBDD), T. urartu (AA) and T. dicoccoides (AABB) were more ozone sensitive and a potential genetic source of wheat’s ozone sensitivity, whilst Aegilops tauschii (DD) was ozone tolerant, adding to evidence of its useful abiotic stress tolerance properties. In Trial 1 and 2, whilst one line of primary Synthetic Hexaploid Wheat (SHW) was found to be ozone tolerant, another was ozone sensitive, but an F2 line derived from SHW and Paragon had both ozone tolerance and larger grain size. Of the cultivars grown across all three plant trials (cv. Maris Dove (1971), cv. Paragon (1999), and cv. Skyfall (2014)), the more recent the cultivar the more ozone sensitive it appeared to be.

In Trial 2, X-ray microcomputed tomography (µCT) digital imaging enabled the 3D visualisation of ozone-affected wheat spikes for the first time, revealing that reductions in grain number were occurring across the middle of the spike, whilst reductions in grain volume were being driven more by reduced width and depth than length.

In Trial 3, ozone triggered earlier visible senescence in all four leaf cohorts (4th, 3rd, 2nd, and flag) of cv. Skyfall, preceded by reduced leaf chlorophyll, particularly in the lower, older leaves, and especially during anthesis/post-anthesis growth stages, with implications for floret fertility and grain fill. Ozone reduced Nitrogen Remobilisation Efficiency between anthesis and harvest, and increased levels of residual nitrogen found at harvest in ‘source’ plant parts. This increase in residual shoot nitrogen was found in cultivars in all three plant trials despite large variations in each trial’s grain yield (‘sink’). Measurements of soil nitrate also indicated that ozone can sometimes increase the potential for nitrate leaching from agri-ecosystems. A 15N trace experiment, with an additional 20 kg ha-1 of nitrogen fertiliser applied at anthesis, revealed that ozone did not affect the uptake of post-anthesis nitrogen, although this extra nitrogen did appear to ameliorate the effect of ozone on other parameters.

Findings from all three plant trials can contribute towards the breeding of ozone tolerant wheat. This research has added to evidence suggesting the more recent the release date of the elite cultivars, the more ozone sensitive they have become, but also identified potential sources of tolerance within one of the main genome donors (DD) and a line of synthetic wheat (BC1). The data relating to the impact of ozone on senescence, leaf chlorophyll, reduced nitrogen remobilisation and increased residual foliar nitrogen, can contribute towards improvements in modelling. The ameliorating effect of the additional nitrogen fertilisation at anthesis highlights the need to ensure that rates of nitrogen fertilisation, and the timing of applications, are taken fully into account in ozone research and modelling.