# Hyperspectral Imaging to Evaluate the Effect of Irrigation Water Salinity in Lettuce
## 1 Introduction  
Soil and irrigation salinity are key factors in the growth of most vegetables and can cause many problems in agriculture, particularly in arid and semi-arid regions [1; 2]. Industrial development and population growth have increased the contamination and salinization of surface and underground water and thus of agricultural soils. Saline soils cover a significant area in the southeast of Spain, a major region of horticultural crops, and in many other areas in the world. Lettuce (_Lactuca sativa_ L.) is one of the most important crops in this region, the major producer area in the European Union. This production is primarily employed for the fresh consumption of a ready-to-eat product. [PERSON] et al. [3]reported some peaks value of 6.4 decisiemens per meter (dS/m) electrical conductivity (EC), which corresponded with 50 mM NaCl. This high electrical conductivity observed in the soils in the southeastern area of Spain is mainly the result of poor quality irrigation water used in agriculture. Lettuce is moderately sensitive to salinity. When the EC is greater than 1.3 dS/m (nearly 10 mM of NaCl), growth is affected, and its yield decreases by 13% for each unit of EC above that level [4]. Salinity primarily affects crop growth in two ways: (a) by increasing the osmotic potential of the soil, making water less available to plants; and (b) by creating excessive concentrations of specific elements in the plant [5]. Herbaceous crops affected by salinity at moderate levels do not show visible damage in their leaves and appear to be normal, although their growth and yield decrease. They may have deep green and more succulent leaves with more density and thickness [6]. However, when excess salinity induces imbalances in concentrations of certain mineral elements, such as Cl, Na, B or Ca, necrosis, chlorosis and tip burn can appear in the leaves [7; 8; 9]. Marginal necrosis is a typical foliar symptom in plants suffering from salinity stress [10; 11]. In lettuce, tip burn is a physiological disorder displayed as necrosis in the margins of young developing leaves and is commonly observed under saline conditions [9; 12]. Hyperspectral imaging has been widely employed to detect salinity levels in soils and canopies through remote sensing, with the development of numerous indexes for estimating saline concentrations in relation to reflectance at different wavelengths [13; 14], giving information about spatial distribution with spatial resolutions higher than meters. This remote sensing technique has been applied to many different crops and plants, including sugarcane [14], cotton, corn, cogon grass, reeds, saltcedar, suaeda, and aeluppus [15]. In addition, there are numerous remote sensing and at laboratory level studies that have observed the effects of water stress [16; 17; 18; 19] and nutrient deficiencies [20; 21; 22]. However, there are not many studies at laboratory level that have been devoted to study changes in the optical behavior of leaf tissues due to alterations in their structure and/or composition when saline concentration increases. This work applies proximal hyperspectral imaging techniques as a non-destructive procedure for early identification of the influence of saline stress on one by one, newly harvested \"baby\" lettuces, exploring simultaneously the presence of any distribution of the effects on the surface of the leaves. The current work constitutes a proof of concept to the development of a rapid, non-destructive and cheap procedure for the analysis of the symptoms of saline stress in the crops. This procedure could be of application in fields such as precision agriculture, robotic monitoring or physiological and plant breeding research instrumentation. The first part of this paper explains the materials and methods employed to obtain the hyperspectral images and other analytical measurements; it also outlines the data and processed hyperspectral images used to obtain and evaluate the models and to identify the effects of different saline concentrations on the leaves.