**6. Relation between abscisic acid and proline**

The ABA hormone is synthesized in the plastids and is linked to the stomatal mechanism [58] and quickly responds to water deficiency [59]. The ABA can be produced in the roots and/or shoots, but this hormone is usually synthesized under water deficiency in the roots and translocated to leaves in order to improve stomatal control. The ABA signalization pathway depends on the Ca+ influx into the cytosol [60], activating the K+ , Cl<sup>−</sup> , and malate−2 efflux channels to external medium, through plasmatic membranes and concomitantly blocking the K+ entrance to cytosol. Therefore, the cytosol solute flux in the direction of the cell wall results in a decrease of turgescence pressure in the guard cells, and, consequently, the stomata are closed [61].

The progressive increase in ABA concentration in the plants of the stress treatment is related to the stomatal mechanism because this hormone, under these conditions, provokes stomatal closing [62], consequently reducing the water losses during the gas exchanges in essential physiological processes such as transpiration and photosynthesis [59].

Based in study carried out by Costa et al. [5] on impact of water deficit and rehydration on nitrogen compounds and ABA in *V. unguiculata* leaves (Figure 4), the research detected that leaf relative water content influences ABA concentration present in the leaf.

**Figure 4.** (a) Abscisic acid concentration and (b) proline in *Vigna unguiculata* plants cv. Vita 7 subjected to 4 days of water restriction and 2 days of rehydration. Means followed by the same letter are not significantly different by Tu‐ key's test at 5% of probability. The bars represent the mean standard error, and the arrow indicates the rehydration point [5].

Therefore, the relative water content acts as a signal, and the ABA works during the signal transduction due to the easy and fast movement of this compound into plant tissue, and as a response, the stomatal closing occurs in *V. unguiculata* plants subjected to water deficit.

The fast decrease in the ABA concentration after rehydration indicates the efficiency of the signalization pathway, transduction, and consequent response of this compound. The results reported in this study on ABA are corroborated by Hsu et al. [63] evaluating the consequences of water stress in *Oryza sativa* L. and the effects of heavy metal stress in *Cicer arietinum* L.[64].

The results obtained by Costa et al. [5] prove that ABA mediates actively and significantly the proline accumulation and consequent osmotic adjustment in *V. unguiculata* leaves induced to water deficit and rehydration (Figure 5). A recent study indicated that *V. unguiculata* plants considered resistant to water deficit presented proline accumulation [65] and, consequently, are more adapted to environments with low water supplement, when compared to sensitive plants. The rehydration reduced the proline levels, suggesting that this nitrogen compound participates actively in the osmotic adjustment in this species. The proline accumulation during water deficit presented in this study is similar with results reported by Sarker et al. [66], investigating *Triticum aestivum*, and Smita and Nayyar [67], evaluating *C. arietinum*.

**Figure 5.** Relationship between abscisic acid concentration and proline in *Vigna unguiculata* plants cv. Vita 7 subjected to 4 days of water restriction and 2 days of rehydration. The bars represent the mean standard error, and the asterisks (\*\*) indicate the significance at 0.01 probability level [5].
