Rye (Secale cereale L.) : agronomic performance under drought and methods of crop physiology to determine the drought tolerance of winter rye

Winter rye (Secale cereale L.) is predominantly cultivated on light and sandy soils with a low water holding capacity and will therefore be especially affected by drought induced yield losses in Central and Eastern Europe in the future. Drought adaption through breeding is therefore an important task in order to adapt this crop to future climate conditions. In this context, the crop physiology methods canopy temperature depression (CTD = Tair - Tcanopy) and carbon isotope discrimination (delta) were examined for their suitability as selection criterion under drought on a small number of genotypes. Two sets of each 16 genotypes were therefore grown under different drought conditions in rain-out shelters and under well-watered conditions in the years 2011, 2012, and 2013. The CTD was determined several times during the growth period using two infrared (IR) thermometers and an IR camera. delta-analyses were performed on mature flag leaves (delta_leaf) and grains (delta_grains). Furthermore, ash content in mature flag leaves and grains, as well as mineral concentrations in mature flag leaves (Ca, K, Mg, and Si) were examined for their use as surrogates for the expensive and time-consuming delta-analyses. In addition to the evaluation of possible selection criterions, the agronomic performance of rye in the different drought regimes was assessed: Grain-, straw-, and total aboveground biomass yields, the grain yield components spikes m-2, kernels spike-1, and thousand kernel weight (TKW), leaf area index (LAI), and phenological characteristics were examined. \nDrought induced grain yield reductions ranged from 14 to 57%, whereas straw yield was generally lesser affected. The growth period was shortened by up to 12 days under drought conditions compared to optimal water supply. Grain yield was positively associated to straw yield, LAI, spikes m-2, and kernels spike-1 under water deficit. High number of grains per area land seemed to be especially important for high grain yields under drought. Furthermore, the results suggest a strong importance of pre-anthesis reserves for the reallocation of assimilates for grain filling under drought in rye. \nRegarding the suitability of possible selection criterions, CTD was significantly positively related to grain yield under drought. Significant correlations between CTD and grain yield were, however, only observed when the measurements were carried out on days with optimal weather conditions. Optimal conditions turned out to be days with a clear sky, a solar irradiation >700 W m-2, an air temperature of at least 20°C, as well as wind speeds <3 m s-1. Furthermore, the results showed that also rather inexpensive IR instruments are suitable to assess the CTD. Regarding the carbon isotope discrimination, delta_leaf was significantly positively related to grain yield under water deficit, but the correlation was weaker than between CTD and grain yield. Delta_grains was not related to grain yield at all. Ash content and mineral concentrations were significantly related to grain yield under drought, but the correlations were quite inconsistent between the two experimental years. Because of the weak or missing relationship with grain yield, carbon isotope discrimination and its potential surrogates ash content and mineral concentration cannot be recommended for their use as selection criterions under German climate conditions at present. A general limitation of the preset work was, however, the low genetic variability of the genotypes, which may have reduced the significance of the results. The results should therefore be validated with a more diverse set of genotypes. However, especially the CTD seemed to be a promising selection criterion which may help to develop drought tolerant rye genotypes, if this method can be successfully integrated into the breeding process.\n