Wheat Physiology and Yield Unaltered By Climatic Pressures.

Scientific and agronomic communities continuously focus on increasing crop yields. These efforts are usually economically driven, but as higher incidence of drought and pests threaten yield stability, the preeminent driver becomes food security. The future of global food supply is linked with research investigating crop responses to climate change. This project focuses on yield trends and physiological performance of rainfed California winter wheat in a 23-year field experiment. Isotopic analysis of archived wheat samples was used to determine changes in stomatal conductance and other physiological outcomes.

It was hypothesized that physiological and agronomic performance metrics would closely follow fluctuations in precipitation and rising atmospheric CO₂ levels; however, the analyses indicate that wheat yields have not been affected by seasonal precipitation or atmospheric [CO₂]. These findings contrast with previous experimental results that show biomass burgeoning with elevated [CO₂]. The discrepancy may be attributed to the near-immediate increase in [CO₂] in controlled experiments, as opposed to a gradual shift under field conditions, implicating less of a yield impact from elevated [CO₂] than initially reported.

Over the observed time period, physiological performance and yield are responding more strongly to agronomic decisions than to climatic pressures. For example, the difference between isotopic signatures of atmospheric and plant carbon pools (Δ¹³C) depends on fertilizer inputs, suggesting that soil treatments are affecting plants’ water-use efficiency. At any given yield, wheat receiving nitrogen from crop residues is more enriched in carbon-13 than wheat receiving nitrogen as mineral fertilizer. This result indicates the former plants transpired less and wheat receiving organic nitrogen had about 3% higher water-use efficiency.

The current study did not find significant relationships between yield and seasonal precipitation or atmospheric [CO₂], indicating climatic conditions throughout the study never reached a threshold for declining physiological performance. Analysis of precipitation values, carbon isotope data, and grain isotope signatures all support the assertion that the conditions experienced over the 23-year experiment did not encompass severe drought stress.