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This paper discussed the response of spectral characteristics on high temperature at grain filling stage of different spring maize varieties by adopting two spectrometer (SPAD-502 Chlorophyll Meter and Sunscan Plant Canopy Analyzer), and analyzed the impact of high temperature on the photosynthetic properties of spring maize in North China Plain. The test was conductedfrom the year 2011 to 2012 in Wuqiao County, Hebei Province. This test chose three different varieties, i. e. Tianyu 198 (TY198), Xingyu 998 (XY998) and Tianrun 606 (TR606), then two sowing date (April 15th and April 25th) was set. We analyzed chlorophyll relative content (SPAD), leaf area index (LAI) and photosynthetically active radiation (PAR) at grain filling stage. The results showed that the days of daily maximum temperature above 33 °C and the mean day temperature at grain filling stage in spring maize sowing on April 15th increased 3.5 d and 0.8 °C, respectively, compared to that sowing on April 25th, moreover the sunshine hours, rainfall, diurnal temperature and length of growing period were similar. Compared with XY998 and TR606, TY198's stress tolerance indices (STI) increased by 2.9% and 11.0%, respectively. According to STI from high to low order, TY198, XY998 and TR606 respectively as heat resistant type, moderate heat resistant type and thermo-labile type variety. TY198, compared with XY998 and TR606 sowing on April 15th, yield increased by 4.1% and 13.7%, SPAD increased by 12.5% and 19.6%, LAI increased by 5.3% and 5.6%, PAR increased by 4.0% and 14.0%. Sowing on April 15th, yield increased by 1.3% and 2.8%, SPAD increased by 3.5% and 6.0%, LAI increased by 1.7% and 4.1%, PAR increased by -4.4% and 0.9%. Three varieties had significant yield differences in the environment of high temperature stress, heat resistant type have significant (p < 0.05) advantage in the aspect of yield, SPAD and LAI. The production of TY198, XY998 and TR606 sowing on April 15th compared to that sowing on April 25th decreased by 3.2%, 5.9% and 12.6%, and SPAD decreased by 8.6%, 12.4% and 15.7%, LAI decreased by 11.7%, 17.6% and 19.8%, PAR decreased by 3.4%, 11.3% and 14.5%; STI had a significant negatively correlated with SPAD fall range (r = -0.883, p < 0.05) and LAI fall range (r = -0.853, P < 0.05), and highly significantly negatively correlated with PAR fall range (r = -0.923, p < 0.01); while SPAD fall range and PAR fall range showed a significant positive correlation (r = 0.872, p < 0.05); LAI fall range and PAR fall range were significantly positive correlation (r = 0.943, p < 0.05). In conclusion, heat tolerant type varieties of spring maize under high temperature stress at gain filling stage could maintain a relatively high content of chlorophyll at the individual level, a relatively high leaf area at the group level, and then keep a higher luminous energy interception and utilization, and weakened inhibition magnitude of high temperature on photosynthetic capacity, reduced the yield fall range, then achieved high and stable yield. The heat tolerance in varieties could be one of the main indicators for identification and evaluation the response to high temperature by spectral characteristics (SPAD, LAI and PAR). Thus it provides a basis by using spectral characteristics to study heat tolerance on maize.

In order to explore a non-destructive monitoring technique, the use of digital photo pixels canopy cover (CC) diagnosis and prediction on maize growth and its nitrogen nutrition status. This study through maize canopy digital photo images on relationship between color index in the photo and the leaf area index (LAI), shoot dry matter weight (DM), leaf nitrogen content percentage (N%). The test conducted in the Chinese Academy of Agricultural Science from 2012 to 2013, based on Maize canopy Visual Image Analysis System developed by Visual Basic Version 6.0, analyzed the correlation of CC, color indices, LAI, DM, N% on maize varieties (Zhongdan909, ZD 909) under three nitrogen levels treatments, furthermore the indicators significantly correlated were fitted with modeling, The results showed that CC had a highly significant correlation with LAI (r = 0.93, p < 0.01), DM (r = 0. 94, p < 0.01), N% (r = 0.82, p < 0.01). Estimating the model of LAI, DM and N% by CC were all power function, and the equation respectively were y = 3.281 2x(0.763 9), y = 283.658 1x(0.553 6) and y = 3.064 5x(0.932 9); using independent data from modeling for model validation indicated that R2, RMSE and RE based on 1 : 1 line relationship between measured values and simulated values in the model of CC estimating LAI were 0.996, 0.035 and 1.46%; R2, RMSE and RE in the model of CC estimating DM were 0.978, 5.408 g and 2.43%; R2, RMSE and RE in the model of CC estimating N% were 0.990, 0.054 and 2.62%. In summary, the model can comparatively accurately estimate the LAI, DM and N% by CC under different nitrogen levels at maize grain filling stage, indicating that it is feasible to apply digital camera on real-time undamaged rapid monitoring and prediction for maize growth conditions and its nitrogen nutrition status. This research finding is to be verified in the field experiment, and further analyze the applicability throughout the growing period in other maize varieties and different planting density.