Water stress is considered one of the most important obstacles to global agricultural production, and to define the appropriate water regimes for different crops, require adoption of specific technologies and methodologies 30, 31.
In the current study, we detected marked reductions in the yield in peanut crops subjected to water stress at GWR60%; however, the combined applications of Cb + (Znsp or Zngd) are an effective approach for improving the water stress tolerance of the peanut crop which is known to be sensitive to this abiotic stress 32. We hypothesized that under GWR60%, plants are subjected to a critical reduction of the water uptake due the reduction of the root length (Fig. 3 a), and the increases of transpiration rates, thereby inducing higher water loss of the aerial part more quickly than the roots can absorb 33. Moreover, the adoption of the GWR60% irrigation scheme led to increasing soil pH (Fig. 2e) and causing a decline in the solubility and the uptake of the macronutrients at par with micronutrients (Fig. 1 and 2), in particular, that P enhances root growth and architecture 34, 35, 36, that consequently decrease the root efficiency and ultimately reflected in the reduction of peanuts agronomic traits (Fig. 3), and yield (Fig. 4a). Therefore, as a consequence of the foregoing, these impacts have resulted in multiply the adverse influences of water stress.
On the other side, the comparison between untreated treatments with Cb, the results from this study indicated that under GWR60% irrigation scheme, adoption the separate applications of Zngd leads to a better increase in peanut yield and wue. In the same context, under the adoption of GWR100&80%, applying foliar applications of Znsp to plants has had desirable effects in raising the yield and wue. In this regard, we assumed that with the adoption of GWR100&80% led to a kind of imbalance of the activity for nutrient uptake between the most active (plant aerial parts) and roots. At par with, that under the GWR100&80% irrigation schemes, the transpiration rate was higher with a large amount of the opening stomata 37, 38 , thus, applying the foliar applications of Znsp works to increase the absorption molecules of Znsp, which compensates for the lack of the efficiency of the root, improve absorbed nutrients as shown in (Fig 1a, b; Fig 2 c and Fig 3 c, d, f); led to increasing the potential for yield production (Fig 4 a). In this concern, similar results obtained by Man Hong et al. 39 indicated that there was a certain lag in the growth of the root parts of plants than the plants aerial tissue. While in terms of adoption GWR60%, the plants tend to reduce some activities, including reducing the transpiration and closing stomata as a protection methodology from water stress. Under these circumstances the root was the controller of the activity, thus by applying Zngd many benefits including improving nutrients uptake are reflected in the enhancements of crop yield, similar findings are obtained by 40, 41.
Regarding the comparison between treated treatments with Cb, the results showed that under GWR60% irrigation scheme, adoption the separate applications of Cb7.5 leads to a lowest decline in peanut yield and wue. Moreover, with the additions of Zngd application under GWR60%, it was noticed that it has improved the yield at par with wue, however, it had less impact than Znsp. In this concern, we conclude that with the adoption of GWR60%, peanut plants are exposed to a critical reduction of the soil moisture causing to a decline in the uptake of nutrients (Fig. 1 a, b and Fig. 2 a), particularly that P promotes root growth and architecture 35, thus the penetration of the root system within the soil decreased (Fig. 3 a), that ultimately reflected in the reduction of peanut agronomic traits, (Fig. 3 c, d, e, f) and peanut yield (Fig. 4 a). Furthermore, under this circumstance once Cb becomes beside or in the roots, it works on raising the roots susceptibility to water availability and Cb concentrations 42. In addition, the reduction of soil moisture decreases the microorganism activity, leading to a decrease in (N, P, Fe, Mn and Cu) and an increase in soil pH. In this concern, current study also identified antagonistic relations between Cb and (Fe, Mn and Cu) under these conditions, we concluded that partly due the reduction in its availability due pH and the competition between these nutrients on the transport sites in the carriers located in the root which led to reduction in photosynthesis reflected in the obtained yield, these findings are consistent with 27, 28, 29. In addition, Dang et al. 43 indicated that plant growth and metabolism vary according to the concentration and state of Cb in the rhizosphere and soil. On the other side, applying Znsp under GWR60% with Cb7.5 leads to an improvement of the absorption for k, Fe and Cu better than Zngd. We hypothesized; it seems that the absorbed amounts of Cb worked in opening significant part of the stomata. Moreover, under these condition the application of the chelated absorption Znsp molecules through the leaves worked on improving nutrient contents directly and quickly, which compensated the lack of nutrient absorption by the root, which contributed to raising the nutrients transport and in turn, promoted the active symbiosis, stomatal regulation and photosynthesis, these results were supported by previous studies 44, 45, 46, which resulted in an improvement in plant tolerance to abiotic stress and led to decreasing the water losses from plant and improving shoot fresh weight, weight of pods and weight of seeds (Fig. 3 b, d, f), that ultimately reflected in the yield.
On the other hand, we also established that the adoption of the combined applications Cb7.5 + Znsp under GWR80%; increased peanut yield and attained the highest wue values. In this concern, we conclude this happened, in part, due Cb application under GWR80% improved transpiration, stomatal conduction, the nodulation, nitrogen fixation and assimilation in peanut plants under water stress, these findings are in agreement with 26, 47. Also, the combined applications of Cb7.5 + Znsp, work on decreasing the pH value (Fig. 2 e), as a results of the benefits of Cb application on the microorganism activity. Therefore, the findings mention that combination contributes to the improved absorption of (N, P, Mg and Zn) nutrients, where Zn is a main component in many enzymes and proteins and has potential roles in the biochemistry and metabolism 48, water relations 49, 50, enhances membrane stability 51 and stomatal regulation 52, 53, which has led to the improvement of the number of pods weight of pods. In this concern, Banks 54 reported that the foliar application of Zn affected yield and yield agronomic traits of soybean and enhanced number of pods per plant. In addition, these results are in harmony with previous studies indicated that the application of Cb and Zn can enhance the plant biomass, which that associated with the higher enzymatic actions and improved the physio and chemical processes and transpiration rate 55. Therefore, the benefits of applying the combined applications of Cb7.5 + Znsp reflected in attaining the highest yield and wue