Early Detection of Plant Physiological Responses to Different Levels of Water Stress Using Reflectance Spectroscopy
Early detection of water stress is critical for precision farming for improving crop productivity and fruit quality. To investigate varying rootstock and irrigation interactions in an open agricultural ecosystem, different irrigation treatments were implemented in a vineyard experimental site either: (i) nonirrigated (NIR); (ii) with full replacement of evapotranspiration (FIR); or (iii) intermediate irrigation (INT, 50% replacement of evapotranspiration). In the summers 2014 and 2015, we
... and 2015, we collected leaf reflectance factor spectra of the vineyard using field spectroscopy along with grapevine physiological parameters. To comprehensively analyze the field-collected hyperspectral data, various band combinations were used to calculate the normalized difference spectral index (NDSI) along with 26 various indices from the literature. Then, the relationship between the indices and plant physiological parameters were examined and the strongest relationships were determined. We found that newly-identified NDSIs always performed better than the indices from the literature, and stomatal conductance (G s ) was the plant physiological parameter that showed the highest correlation with NDSI(R 603 ,R 558 ) calculated using leaf reflectance factor spectra (R 2 = 0.720). Additionally, the best NDSI(R 685 ,R 415 ) for non-photochemical quenching (NPQ) was determined (R 2 = 0.681). G s resulted in being a proxy of water stress. Therefore, the partial least squares regression (PLSR) method was utilized to develop a predictive model for G s . Our results showed that the PLSR model was inferior to the NDSI in G s estimation (R 2 = 0.680). The variable importance in the projection (VIP) was then employed to investigate the most important wavelengths that were most effective in determining G s . The VIP analysis confirmed that the yellow band improves the prediction ability of hyperspectral reflectance factor data in G s estimation. The findings of this study demonstrate the potential of hyperspectral spectroscopy data in motoring plant stress response. Remote Sens. 2017, 9, 745 2 of 23 decreasing crop productivity and yield  . Therefore, studies focusing on early detection and warning of plant water stress are of considerable significance. Grapevine (Vitis spp.) is considered the most commercially-important berry crop in the world . North American Vitis species play a vital role in the global grape industry by imparting important pest resistance through their use as rootstocks (the below-ground part makes up the lower stem and roots), their contributions to hybrid scions (the above-ground part of the plant; the scion produces the stem, leaves, flowers and berries) or through planting as an ungrafted plant. Today, grape growing is emerging as a more important part of rural agriculture in the Midwestern United States. For instance, in Missouri, the economic impact of grapes and wine has grown to be worth $1.6 billion with a 16% annual growth rate  . However, abiotic and biotic stresses are restrictive factors impacting cultivation of even the grafted V. vinifera ssp. Vinifera and hybrid scions derived from crosses between V. vinifera ssp. vinifera and one of the native North American Vitis species are dominant cultivated grapevines in the Midwest. Moreover, fruit quality and yield will be significantly affected by prolonged drought, which is projected in the Midwestern United States  . The severity and duration of a plant's dehydration determines the impact of water stress on the photosynthetic performance of the plant and whether or not a plant can recover from stress damage when irrigated. In the short term, water stress causes stomata closure, which in turn not only leads to leaf temperature increase by reducing the transpiration rate, but also creates a reduction in CO 2 concentration [6, 7] . If there is an imbalance between the absorbed light energy and the energy requirement for carbon fixation, promoted by stomata closure, this may cause over-excitations and subsequent the photodamage to photosystem II (PSII) reaction centers  . As a result, the maximum quantum efficiency of PSII will decline in response to the onset of water stress  . When plant stress severity or duration exceeds a critical threshold, plants manifest biochemical and morphological symptoms as adaption strategies. Some adaptation strategies include reduction in chlorophyll content, leaf area, premature leaf senescence and stunted growth [10, 11] . In general, if the stressor is removed before the damage is visible to the naked eye, plants revive and develop a new physiological standard  . Compared to traditional field measurements, remote sensing can provide timely and reliable information about the current plant physiology in a cost-effective and timely manner  .