冬小麦拔节期冻害后高光谱特征

以霜箱模拟冻害, 采用高光谱仪测定盆栽冬小麦叶片叶绿素含量及冠层高光谱反射率曲线, 以期为冻害遥感监测提供借鉴。结果表明, 冻害低温胁迫后, 对照组叶绿素含量比冻害组高约2~5个单位, 差异显著; 短期内高光谱曲线反射率接近, 线形相似, 约20 d后, 受损叶片过早衰败变黄, 高光谱曲线在黄、红波段区域反射率显著增强, “红谷”不明显, 曲线形状呈水平趋势; 光谱 “红边”具有“蓝移”、“红谷”具有“红移”现象。冻害程度的相关系数与绿峰、红边、红谷、光谱吸收指数分别为0.36*、-0.69*、0.42**、0.33**, 与蓝、绿、黄、红宽波段范围反射率面积分别为0.34*、0.43**、0.45**、0.44**, 与红边、红谷归一化植被指数为-0.33*, 与近红外、红波段反射率面积归一化植被指数为-0.39*, 与叶绿素含量为-0.49**。高光谱反射率曲线特征部位值差异显著性检验表明, 可利用近红外、红波段反射率面积构成的NDVI差异进行冻害识别, 其差异分级可划分冻害程度。

Hyperspectral Features of Winter Wheat after Frost Stress at Jointing Stage

The North China Plain is the main production zone of winter wheat (Triticum aestivum L.), where wheat is affected seriously by later spring frostbite. Hyperspectral remote sensing is proved feasible in monitoring crop growth, especially after stresses. But the relationship between hyperspectral feature and crop growth characteristics after frostbite has been rarely studied so far. So we conducted a pot experiment with the artificial frost box to simulate frostbite (-5℃ and -7℃ respectively) in 2006–2007 winter wheat growing season. The wheat seedlings were treated in the box for 4 and 8 hours at jointing stage (treated only once either on April 9 or 15, 2007) respectively. The canopy hyperspectral reflectance curves were measured with ASD FieldSpec, and simultaneously, chlorophyll content (Chl) was tested with SPAD502. The result showed that chlorophyll content in control (CK) was 2–5 units more than that in the treatments after frostbite. In the early stage after frostbite, the reflectance and shape of hyperspectral curves were similar between treatments. After 20 days, frostbitten leaves turned into yellow in advance of CK, hyperspectral reflectance increased distinctly in yellow and red regions, the ‘red well’ (RW) was not distinctive, the shape of curves became plane. The relationship between chlorophyll content and hyperspectral reflectance ρ, logarithm of the reciprocal of ρ log10 (1/ρ)], the first derivative of ρ (dρ/dλ) were analyzed, results indicated that ‘red edge’ (RE) shifted to shorter wavelength and RW to near infrared after frostbite. There were higher correlation between values of hyperspectral curve at special positions and frostbite stress levels, the correlation coefficients of frostbite degree with green peaks (ρg), RE, RW and the spectral absorbance index (SAI) were 0.36*, 0.69** , 0.42**, and 0.33* respectively, with the sum of reflectance in wide bands of blue(SBρ), green (SGρ), yellow (SYρ), and red(SRρ) were 0.34*, 0.43**, 0.45**, and 0.44* respectively, with the normalized difference vegetation index (NDVIa) between RE and RW was -0.33*, with the NDVIb between sum of reflectance at near infrared (SNirρ) and SRρ was -0.39**, with the chlorophyll content was -0.49**. In terms of distinct mean difference analysis, it is feasible to identify the frostbite injury and to estimate the injury degree based on the difference of the NDVIb.