**1. Introduction**

The water supplement inadequate in soil is considered one of the limiting factors to the productive potential in several species [1–2]. Water deficit is an abiotic factor that affects the agricultural production with high frequency and intensity, influencing morphological, physiological, and biochemical aspects [3–4].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

As in other crops, performance in leguminous is affected by water deficiency, which can cause lower growth and development, with progressive reduction in leaf dry matter [5], moreover, to promote the abortion of flowers during drought periods and to affect the yield significantly [6], with consequent repercussion on production parameters, such as number of grains and pods per plant.

The deficit water is characterized by water losses that exceed the absorption rate and by this way acts directly in the plant–water relations [7–8], depending on intense and exposure period, in addition to promote changes in the cell and molecular pathways [9], whereby accumulation of organic solutes with the carbohydrates and proline [10], differential gene expression of DNA [11], and quantity variation in the photosynthetic pigments, mainly chlorophylls and carote‐ noids [12], in which the stomata enclosed interfere in photosynthetic rates occur [13].

The osmotic adjustment is considered one of the important mechanisms developed by the plants to tolerate the water deficiency [14], which promotes the protection of the plant cell structures with membranes and chloroplasts [15], as well as avoid the cell toxicity provoked by the free radicals and maximize the water retention in cell inside [16]; besides it has the advantage of using carbohydrates as energy source under severe stress [6].

Drought is directly related to the overproduction of reactive oxygen species (ROS) [17], such as hydrogen peroxide (H2O2) and superoxide (O2 − ) [18], which are highly toxic compounds. ROS promote the oxidation of membranes and damage essential organelles such as chloro‐ plasts [19] and mitochondria [20], which result in cell damage or death [21,22].

Ascorbate (ASC) and glutathione (GSH) have essential functions in antioxidant metabolism [23,24] because ASC is used as a substrate [25–27]. In addition, GSH produces ASC and glutathione disulfide (GSSG), which is used to regenerate GSH via glutathione reductase (GR) [28,29].

The soybean is considered a species sensitive to several abiotic stresses [30], when compared with other tropical legumes, such as *Vigna unguiculata* and *Phaseolus vulgaris* [31,32], as well as other species such as *Gossypium hirsutum* and *Sorghum bicolor* [33,34], in which the sensitivity at water deficit can be emphasized, mainly during the growth and development period, which might cause strong reduction in the yield [35]. However, *V. unguiculata* (L.) Walp. is a species tolerant to drought due to rusticity, and it presents large protein content in grain. This crop is frequently found in agricultural areas in Brazil that are under the influence of abiotic stresses. These areas present small rain index and high temperature. In addition, the soil is susceptible to salinity or to fertility loss [36].
