Seifert Birgit, Zude Manuela, Spinelli Lorenzo, Torricelli Alessandro
Leibniz Institute for Agricultural Engineering Potsdam-Bornim, 14469, Potsdam, Germany.
Physiol Plant. 2015 Feb;153(2):327-36. doi: 10.1111/ppl.12232. Epub 2014 Jun 18.
Analyzing the optical properties of fruits represents a powerful approach for non-destructive observations of fruit development. With classical spectroscopy in the visible and near-infrared wavelength ranges, the apparent attenuation of light results from its absorption or scattering. In horticultural applications, frequently, the normalized difference vegetation index (NDVI) is employed to reduce the effects of varying scattering properties on the apparent signal. However, this simple approach appears to be limited. In the laboratory, with time-resolved reflectance spectroscopy, the absorption coefficient, μa , and the reduced scattering coefficient, μs ', can be analyzed separately. In this study, these differentiated optical properties were recorded (540-940 nm), probing fruit tissue from the skin up to 2 cm depth in apple (Malus × domestica 'Elstar') and plum (Prunus domestica 'Tophit plus') harvested four times (65-145 days after full bloom). The μa spectra showed typical peak at 670 nm of the chlorophyll absorption. The μs ' at 670 nm in apple changed by 14.7% (18.2-15.5 cm(-1) ), while in plum differences of 41.5% (8.5-5.0 cm(-1) ) were found. The scattering power, the relative change of μs ', was zero in apple, but enhanced in plum over the fruit development period. This mirrors more isotropic and constant structures in apple compared with plum. For horticultural applications, the larger variability in scattering properties of plum explains the discrepancy between commercially assessed NDVI values or similar indices and the absolute μa values in plum (R < 0.05), while the NDVI approach appeared reasonable in apple (R ≥ 0.80).
分析水果的光学特性是对水果发育进行无损观测的有力方法。在可见光和近红外波长范围内使用经典光谱学，光的表观衰减是由其吸收或散射造成的。在园艺应用中，通常采用归一化植被指数（NDVI）来减少散射特性变化对表观信号的影响。然而，这种简单方法似乎存在局限性。在实验室中，利用时间分辨反射光谱法，可以分别分析吸收系数μa和约化散射系数μs'。在本研究中，记录了这些不同的光学特性（540 - 940 nm），对苹果（Malus × domestica 'Elstar'）和李子（Prunus domestica 'Tophit plus'）从表皮到2厘米深度的果实组织进行探测，这两种水果在盛花后65 - 145天内分四次采收。μa光谱在叶绿素吸收的670 nm处显示出典型峰值。苹果在670 nm处的μs'变化了14.7%（18.2 - 15.5 cm⁻¹），而李子的差异为41.5%（8.5 - 5.0 cm⁻¹）。散射能力，即μs'的相对变化，在苹果中为零，但在李子的果实发育过程中有所增强。这反映出与李子相比，苹果的结构更加各向同性且恒定。对于园艺应用，李子散射特性的较大变异性解释了商业评估的NDVI值或类似指数与李子中绝对μa值之间的差异（R < 0.05），而NDVI方法在苹果中似乎是合理的（R ≥ 0.80）。
