玉米抽穗期雄穗对冠层反射率辐射传输特征的影响

为分析玉米雄穗对冠层可见光、近红外波段辐射传输特征的影响，运用四维轨道塔吊系统获取连续2 a玉米抽穗期的冠层光谱，并在抽穗初期和末期分别进行了3个梯度的剪穗试验，分析玉米抽穗期冠层二向反射率特征以及雄穗干物质含量特征。结果表明：1）比较抽穗期不同时间冠层反射率的模拟值和实测值得出，在抽穗初期实测值高于模拟值，随着生育期的推进，模拟值逐渐高于实测值。但在可见光波段整个抽穗期实测值均高于模拟值，在近红外波段模拟值总体高于实测值。2）分析不同穗梯度冠层二向反射率特征发现，在可见光波段，太阳主平面和垂直太阳主平面方向上，2个散射方向的无穗反射率值在所有观测角度上均最高，1/2穗次之，全穗最低；近红外波段，在太阳主平面方向，3个穗梯度反射率差异不大，但在垂直太阳主平面方向，后向散射方向反射率值总体高于前向散射方向反射率值，且无穗反射率值依然总体高于1/2穗和全穗；在垂直观测条件下得到相同的结果。3）分析PROSAIL模型模拟值和农学参数相关性，得出模拟值与叶绿素含量、叶面积指数在全波段呈显著负相关，无穗实测值和模拟值分别与叶面积指数和叶绿素含量相关性表现一致。4）在玉米整个抽穗期雄穗鲜质量变化差异较大，而干质量变化差异不大。研究可为修正辐射传输模型、提高模拟精度，使之更好地应用于植被理化参数反演提供科学依据。

Effects of maize tassel at the heading stage on radiation transfer characteristics of canopy reflectance

In order to analyze the influence of maize tassel on the radiation transmission characteristics of visible and near infrared band at the heading stage of maize, 4D rail crane system was applied for 2 consecutive years in maize at the heading stage in Huailai Experimental Station in Hebei. The experiment was carried out respectively at the beginning of the heading and the end of the 3 tassel gradients. The maize canopy reflectance characteristics at the heading stage and the characteristics of male tassel dry matter content were analyzed. The results showed that: 1) the measured canopy reflectance rate was higher than the simulated value at the beginning of heading, and the simulated value of the model was higher than the measured value at the late stage of development of the growth period. However, in the visible band, the measured values were higher than the simulated values at the whole heading stage, and in the near infrared band, the simulated values were higher than the measured values. 2) By analyzing the characteristics of bidirectional reflectance of different tassel gradient canopy, we found that in visible light band, reflectance without tassel in the directions of solar main plane and vertical solar main plane was the highest, followed by half tassel and the lowest in whole tassel; in near infrared band, reflectance was not different for the 3 tassels treatments in the direction of solar main plane, however, the reflectance in backward scattering direction was higher than that of forward scattering direction, and the reflectance of tassel-free was still higher than that of half tassel and whole tassel. The same results were obtained under vertical observation conditions. 3) The correlation between the simulated value of PROSAIL model and agricultural parameters showed that the simulated value had a significant negative correlation with chlorophyll content (LCC) and leaf area index (LAI), and the measured and simulated values without tassel were highly consistent with the correlation between LCC and LAI. 4) The total fresh weight of tassel in the whole heading stage of maize varied but the difference in total dry weight was not significant. The water content of tassel decreased from 80% at the beginning to 10% at the end of tassel, indicating water content should be considered for model establishment. The research on the radiation transmission characteristics of tassel at maize heading stage on canopy would be helpful in modifying radiation transmission model and improving the accuracy of simulation.