Evaluation of carbon (¹³C/¹²C) and oxygen (¹⁸O/¹⁶O) isotopic discrimination in wheat under water stress conditions and their relation with crop temperature indexes

A study was carried out between 2008 and 2012 to evaluate the effect of water stress on wheat crop using carbon isotope discrimination (Δ¹³C) and oxygen isotopic ratio (¹⁸O/¹⁶O) and to define crop water parameters through infrared thermometry and porometry, and determine relationship between these parameters and isotopic ratios of C & O at field scale, to calculate water-use efficiency (WUE [g grain yield/m²/mm ET]) for wheat under different water applications. Field experiments were conducted with 4 irrigation treatment (no irrigation; I₁, low: I₂, moderate: I₃ and high irrigation stress: I₄) at Saraykoy Research Station in Ankara-Mürted Basin. Additionally for 2011–2012 growing season, an irrigated treatment on a large field (5 ha) was carried out to evaluate E&T partitioning using the Keeling plot method. In all the above studies, soil moisture was measured with neutron probe. Infrared thermometer and porometer devices were used to measure canopy temperature, stomatal conductance on plant samples. Isotope compositions of carbon and oxygen were determined from soil, plant and atmospheric water vapour. At the end of the study average wheat yields were 3.35 t/ha, 4.54 t/ha, 4.22 t/ha and 4.31 t/ha, respectively for the I₁, I₂, I₃, I₄ treatments. The highest yield was obtained from the full-irrigation treatment while the lowest yield was obtained from the no-irrigation treatment. Average harvest index values were found to be respectively 29%, 31%, 32% 31% and 32%, again according to the treatments. The proportion of transpiration as percent of total ET was between 70.5 to 96% before irrigation while it was approximately 70% after irrigation. A significant positive relationship was found between carbon discrimination (Δ) and yield ratio of both grain and leaf, while a significant but negative correlation between the water-use efficiency (WUE). The positive relationship between the stomatal conductance and yield and the increase of the transpiration from the pores in environments without any irrigation restriction indicate an increase in the plant biomass and yield. This result showed that WUE is more strongly dependent on stomatal conductance. The study showed that irrigation should be recommended to be applied after heading period which increases WUE in Central Anatolia Region of Turkey. Crop water stress index is a useful tool for detecting crop water stress. (author)