**4. Discussion**

The precipitations of the BP simulated by the climate model ICHEC\_KNMI are underestimated compared to those observed at theSetif and BBA stations during the BP. Thus, the simulated precipitations are underestimated for all months of the year except June, where P were overestimated. Romeraet al. [40] suggested that the high variability of rainfall and the weak network of observation stations in the Maghreb region made the rainfall simulations in the EURO-CORDEX database full of uncertainties (or bias). The underestimation of Nr in spring and summer at both stations was possibly caused by the climate model overestimation of cloud cover as suggested by [41].

The decline in Ps under Sc1 compared to the BP at both stations is attributed to the shortening of the Mexicali cultivar's GSL. This is induced by the increase in TS, as PS is expected to increase throughout the Mexicali cultivar's GSL. In addition, the increase in spring's precipitations and the lengthening of the Mexicali cultivar's GSL under Sc2 explain why PS is expected to increase in this scenario compared to the BP. The NrS decreases in Sc1 could be due to the shortening of the Mexicali cultivar's GSL and the aerosol pollution projected under these two RCPs scenarios, as suggested by [42]. Meanwhile, the decrease in NrS in Sc2 could be attributed to the atmospheric pollution caused by the presence of aerosols in the atmosphere as the Mexicali cultivar's GSL was prolonged in this last scenario.

The wheat grain yield increase projected under both RCPs scenarios is due to the fertilizing effect of the air enrichment with CO2. This result is more consistent at the BBA site, where the evolution of the grain yield curve perfectly follows that of CO2 concentrations. Thus, the Mexicali cultivar's grain yields projected in Sc2 are higher than those projected in Sc1 at the BBA site because the CO2 concentrations expected under RCP 8.5 are higher than those projected under RCP4.5. However, at the Setif site, the average grain yield simulated in Sc1 is higher than that simulated under Sc2. This could be explained by the decline in TS by −0.3 ◦C in Sc2 because it is possible that the lower temperatures reduce the fertilizing effect of CO2 on durum wheat grain yields. However, under Sc1, the projected grain yields at Setif are better by comparison with those projected at the BBA site. This result can be explained by the combined negative effect on wheat grain yield of very severe water stress and the thermal stress projected under Sc1 at the BBA site (Table 3). According to the Pearson correlation test results, in Sc1 and Sc2, there is a negative and statistically significant correlation between TS and Mexicali cultivar's grain yields at the Setif site. Meanwhile, at the BBA site, the correlation is negative and statistically insignificant. On the contrary, the PS and NrS's correlation is positive and statistically significant with Mexicali cultivar's grain yield simulated by AquaCrop by both future scenarios.

Therefore, the fertilizing effect of CO2 offsets the negative effects of rising TS and decreasing PS and NrS on durum wheat yields projected, under Sc1 and Sc2, at the Setif and BBA experimental sites. These results are compatible with the conclusions of recent studies carried out by ([43–45] respectively, in China, Germany, and Morocco. Likewise,

Pugh et al. [46] reported that rainfed wheat in arid and semi-arid regions located in low latitudes would benefit from the fertilizing effect of CO2, but less in temperate regions located in high latitudes. Moreover, Long et al. [47] reported that 550 ppm high CO2 concentration in experiments in a Free Air CO2 Enrichment (FACE) and in closed chamber experiments, resulted in a wheat yield enhancement of +31 and +13%, respectively, in both these experimental devices. Tubiello et al. [48] suggested that the fertilizing effects of a high CO2 concentration might be overestimated in crop models because their simulation of yield enhancement induced by a high CO2 concentration was much greater than that observed in FACE studies. However, they suggested that the magnitude of these effects is still under debate. In China, the study [49], proved that the fertilizing effect of the CO2 enrichment in the atmosphere slows the negative effects of warm air temperatures and precipitation decline on wheat yield under RCP 4.5 and RCP 8.5, at the beginning of the 21st century in China. Likewise, Xiong et al. [50] found that the CO2 enrichment in the atmosphere enhanced wheat yield by +0.9% by offsetting the negative effect of the drop in solar radiation, but without this fertilizing effect of CO2, the wheat yield would decrease by −9.7%. Under RCP 8.5, in Egypt, the increase in atmospheric CO2 concentrations will act as a growth stimulant. The simulated irrigated wheat yield across Egypt was projected to increase slightly (2.4%) in the 2030s and will decline slowly toward the end of the century (−1.7% by 2050s and−4.0% by 2080s). This result was attributed to the increase in negative impacts of the projected warm temperature [51]. In Jordan, the ESCWA [52] reported that under RCP 4.5 and RCP 8.4, with a fixed CO2 concentration, the rainfed wheat yields simulated with the AquaCrop model will increase by an average of about +33.8 and +48.3% in 2025 and 2045 future periods. Meanwhile, with elevated CO2 concentration, the simulated wheat yield will be enhanced by +53.5 and +81.6%inboth above future periods, respectively, with respect to the baseline yield.

The Pearson correlation test revealed the existence of a negative and statistically significant correlation between Mexicali cultivar's GSL and TS during the BP and under both Sc1 and Sc2.This result is compatible with that of [45] in Morocco and [53] in the entire Mediterranean region. Furthermore, in Palestine and Jordan, the rainfed wheat growth cycle period simulated under RCP 8.5 is projected to shorten by 2030 and 2050 [52].

In addition to these above results, the Pearson correlation test proved that there is a positive and statistically significant correlation between Mexicali cultivar's GSL and PS during the BP and under both Sc1 and Sc2 at the Setif and BBA sites. Thus, the lengthening of the Mexicali cultivar's GSL by 10 days under Sc2 at the Setif site could also be explained by the increase in PS by +65.6 mm. Despite the shortening of Mexicali cultivar's GSL by the effect of the expected increase in TS. The Mexicali cultivar's grain yield is projected to enhance under Sc1.Tao et al. [54] and Liu et al. [55], explained that the shortening of wheat's GSL is due to the vegetative development stage's length reduction, meanwhile the duration of the reproductive stage remained intact. So, this negated the yield losses reported by Zheng et al. [16], who recommended wheat cultivars flower early in order to prevent wheat crops from the risk of yield loss which could be induced by very warm temperatures in late spring throughout the period of grain formation.

The ET0 drop predicted under Sc1 is due to the shortening of the Mexicali cultivar's GSL. This result is consistent with that of [56] on rice in Bangladesh and [45] on wheat in Morocco. However, under Sc2, the lengthening of the Mexicali cultivar's GSL did not avoid the ET0 decrease, which could be explained by the stomatal regulatory effect ofdurum wheat, which made it possible to reduce water losses by evapotranspiration, as it was suggested by [45], under the fertilizing effect of the elevated CO2 concentrations in the atmosphere, projected under Sc2.

WP is the ratio of the amount of durum wheat biomass produced (in kg) to the amount of water lost by evapotranspiration during durum wheat's growing cycle. Thus, according to the AquaCrop model simulations, the WP enhancement under both Sc1 and Sc2 scenarios is due to the increase in Mexicali cultivar's above-ground biomass induced by the fertilizing effect of the enrichment of the atmosphere with CO2. This induced the acceleration of photosynthetic activity and the decrease in water loss by evapotranspiration, thanks to stomatal regulation under Sc2, and the shortening of the Mexicali cultivar's GSL induced by the TS increase under Sc1. According to [52], the AquaCropcrop model, with a fixed CO2 concentration, predicted an enhancement of rainfed wheat's WP by an average of +17.8 and +30% for the 2025 and 2045 future periods, whereas in the case of elevated CO2, an increase of +3 and +56% are projected by both future horizons, respectively, under RCP 4.5 and RCP 8.5.

According to [46], the fertilizing effect of a high CO2 concentration can improve the WP of C3 plants (such as wheat) by stimulating their photosynthetic activity. Ainsworth et al. [57] reported that across a range of FACE experiments, with a variety of plant species, the growth of plants at elevated CO2 concentrations of 475–600 ppm leads to increasing leaf photosynthetic rates by an average of +40%. These last authors explained that CO2 concentrations are essential in regulating the openness of stomata, the pores that allow plants to exchange gasses with the exterior environment. Thus, open stomata allow CO2 to diffuse into leaves for photosynthesis, but also provide a way for water to circulate out of leaves. Plants, therefore, regulate the degree of stomatal opening, a measure called stomatal conductance, which is used as a compromise between the aims of maintaining high rates of photosynthesis and low rates of water loss. So, as CO2 concentrations increase, plants can maintain high photosynthetic rates with relatively low stomatal conductance. Added to that, they also reported that across a multitude of FACE experiments, growth under elevated CO2 concentrations decreases stomatal conductance of water by an average of −22%. However, Taub et al. [58] suggested that generally, the magnitude of the effect of CO2 on crop water use would depend on how it affects other determinants of plant water use, such as plant size, morphology, and leaf temperature.

At both experimental sites, under the climate conditions projected by 2035–2064, under RCP 4.5 and RCP 8.5, the earlier sowings of mid-September and mid-October lead to the best yields because the earlier sowing dates allow the wheat crop to take advantage of the increase in precipitations predicted in the late summer and early fall. That will allow the achievement of the vegetative development stage of the Mexicali cultivar's plant from November until the beginning of February. In addition, this early sowing allows the flowering stage to take place between the period from the end of February until the beginning of April, which allows the Mexicali cultivar's plant to avoid the high temperaturesin May and June. These results are compatible with those of [59], who also predicted the adaptation of wheat to early sowings in 2031–2060 climate conditions, under both RCP 4.5 and RCP 8.5 scenarios in the Mediterranean area. However, late sowing in mid-November and mid-December resulted in poor yields, as they led to the achievement of the durum wheat's flowering and grain-filling stages through the high-temperature period of the mid-April and early June period, which induce durum wheat's grain yield losses by scalding. This result is in concordance with those of [60], who reported that the early maturing cultivar did not show a yield reduction on any sowing dates, thanks to the earliness of the anthesis stage, on which risk of crop exposure to heat stress during the sensitive grain-filling stage is decreased or avoided. In Ethiopia, [61], reported that by the middle and the end of the 21st century, one wheat cultivar is adapted to late sowing date, under low CO2 emissions of RCP 4.5, meanwhile another cultivar is well adapted to early sowing date, under elevated CO2 emissions of RCP 8.5. So, it is important to assess the adaptation of wheat sowing dates under future CC scenarios by simulating different wheat cultivars yields with the crop model in order to select the best-adapted wheat cultivar to the projected CC.
