Effects of asymmetric warming in daytime and nighttime on canopy spectral reflectance of post-anthesis winter wheat

Abstract: Global warming is one of the most important environmental issues today. Temperature data over the past ten decades show that the warming rate of the global land surface during the night is larger than that during the day. Temperature affects the growth condition of winter wheat (Triticum aestivum L.), and alters its canopy spectral reflectance. To our best knowledge, there are still rare researches on the asymmetric effects of daytime and nighttime warming on winter wheat canopy spectral reflectance. So this paper explored the asymmetric effects of daytime and nighttime warming on winter wheat canopy reflectance through experiments in an open field warming system. The experiment field is located at the agro-meteorological research station, Nanjing University of Information Science and Technology, Nanjing, China. Three treatments were carried out: asymmetry warming (daytime warming 1℃, nighttime warming 3℃, T2), symmetric warming (daytime warming 2℃, nighttime warming 2℃, T1), the control (CK), and two varieties of wheat Yangmai 15 and Xumai 31 were used. The ceramic heating lamps were used to heat the experimental plot. In order to ensure the warming effect, the work status of ceramic heating lamps was controlled by single-chip microcomputer. The ceramic heating lamps were hanged 40-50 cm above the wheat canopy to avoid harming wheat plants. The CK treatment also hanged the same ceramic heating lamps, but no power supply. Canopy reflectance was observed in the three treatments and the first derivative spectra were calculated. The canopy spectral reflectance of two varieties of winter wheat was measured by FieldSpec Pro FR spectrometer (ASD, USA). The results showed that winter wheat canopy reflectance of T2 significantly decreased in near infrared bands as compared to CK. Moreover, the canopy reflectance decreased more significantly in anthesis stage than in other growth stages. In anthesis stage, the reflectance of Yangmai 15 were 0.70, 0.61, 0.55 under T1, T2 and CK treatment in near infrared bands, while the reflectance of Xumai 31 were 0.65, 0.60, 0.52, respectively. The decrease pattern of T1 was similar to T2, but with smaller magnitude. The first derivative spectra showed that the red edge position of Yangmai 15 was 738 nm at the anthesis stage, and then shifted to 731 nm, 730 nm at 20 days after anthesis under T1, T2 treatment, respectively. Xumai 31 had the same rules. The decrease of red edge peak value in T1 and T2 was significant, especially in anthesis stage. Corresponding to the change of canopy spectral reflectance, Chlorophyll content of leaf and leaf area index of two varieties of winter wheat notably decreased in T1 treatment, especially in T2 treatment. Therefore, the changes of chlorophyll content of leaf and leaf area index, also the cell structure, resulted in the above variation in canopy spectral reflectance.