Assessment of crop yield losses for Triticum aestivum in Punjab and Haryana using in-situ measurements, relay seeding experiments and the DO3SE model

Measurements of leaf-level stomatal conductance (g_sto) are central to the ozone (O₃) risk assessment as the allow to recorded environmental response functions that describe how a wheat cultivar responds to different environmental stressors. The calculation of Triticum aestivum yield loss based on the absorbed O₃ phytotoxic dose over a threshold of y (POD_y) has been introduced as a new way to conduct ozone  risk assessment. 

In this study we present environmental response functions of two triticum aestivum cultivars grown as irrigated winter wheat in the state of Punjab, in the North West Indo Gangetic Plain based on measurements conducted during winter 2016-17, 2017-18, and 2018-19. The cultivars PBW550 and HD2967 were directly obtained from breeders and were sown on November 15^th in 2016-17 and 2017-18, and in a relay seeding experiment on November 1^st, 15^th and December 1^st in 2018-19.

We use meteorological observations and ozone measurements obtained at the Central Atmospheric Chemistry facility of IISER Mohali in Punjab, India between November 2016 and April 2019 to derive environmental response function for these two cultivars and estimate triticum aestivum relative yield losses and crop production losses due to ozone. We demonstrate that environmental response functions are not only useful to assess the impact of ozone on plant growth but that they can also be used to assess the impact of heat stress and climate change on yields. We show that modifying the phenology function used in the DO3SE assessment such that it incorporates the impact of heat stress experienced between anthesis to maturity permits a more accurate assessment of the impact of ozone on the wheat yield. We also demonstrate that a thermal time calculation method that is consistent with the temperature response function used in the DO₃SE improves the quality of the assessment.

We evaluate the impact of both heat stress and ozone exposure during different growth stages on several yield parameters including the number of active tillers, 1000-grain weight, flower sterility, number of shrivelled grains.

Late sowing typically not only results into high thermal stress during sensitive growth stages, but also in higher ozone exposure. We find that PBW550 is more sensitive to stress during the grain filling stage than HD2967. However, for both cultivars moderate heat and ozone stress can be associated with superior rather than reduced yields in the real world. We explain this yield-loss yield paradox with the help of meteorological observations.