spp.) is one of the most essential food crops in the world [1
]. Recently, wheat production has not met consumer demand, which results in hunger and price instability worldwide. In 2050, the demand for wheat is expected to increase by 60% with a predicted world population of 9 billion [4
]. To meet the needs of a dramatically increasing population, annual wheat production must increase from the current level (below 1%) to at least 1.6% [3
]. Therefore, all agricultural policies must be directed toward enhancing wheat production by developing a global strategic agenda for wheat research, encouraging efficient investment in wheat research, and developing genotypes tolerant to both biotic and abiotic stress conditions—knowing that the available water reserves worldwide are limited and declining continuously [5
Currently, wheat production in Jordan is not sufficient to cope with the needs of the Jordanian population and Syrian refuges. Achieving food security in Jordan depends on maintaining a reserve of this strategic crop. In Jordan, 24.7 thousand hectares were used to grow wheat with an average yield of 1.22 ton ha−1
during the period 2010 to 2014 [9
]. The amounts of harvesting yield (25.1 thousand tons) were less than 3% of local consumption (916.4 thousand tons) during that period [9
]. This will lead to increased wheat imports, which puts more pressure on Jordan’s national budget. Therefore, the main priority of wheat research programs in Jordan is to enhance food security by increasing and stabilizing national wheat production. In Jordan, wheat was planted under rain-fed conditions with fluctuating precipitation within the season and between successive seasons, which led to poor crop cultivation success. Moreover, scientists predicted that Jordan, as a part of the Mediterranean region, will be subject to climate change, as indicated by increases in temperature and rainfall variability that would in turn increase drought and limit wheat yield production [10
]. Annual precipitation around the Mediterranean region is likely to decrease by 4–27%, which appears to be a typical response across global coupled climate models [11
]. Such models have also predicted an increase of 3–5 °C in temperature [11
] and around a 20% loss in soil moisture [12
], supported by the fact that wheat production suffered from yield variability from year to year and from one location to another, caused by drought stress. Drought may occur pre-anthesis and negatively affect crop establishment and floret fertility [13
] or post-anthesis and adversely affect grain filling and grain development [15
]. Therefore, improvement of wheat productivity under drought conditions became the main objective of the current breeding programs in arid and semi-arid regions.
Many studies indicated that water deficit at any developmental stage results in significant reductions in wheat yield and yield components. Such yield reduction will be larger if water deficit occurred during the tillering (GS20) and heading stages (GS60) [16
]. However, other studies indicated that the sensitive stages are the milk-ripe (GS40) and booting stages (GS70) [18
]. Grain yield has two components: number of grains m−2
and grains spike−1
) and grain weight. Calderini et al. [20
] demonstrated that wheat yield could be increased by selecting varieties with higher grains spike−1
. Moreover, it has been reported that grains spike−1
is typically the yield component that is most sensitive to heat and drought stresses due to severe competition for nutrients during stem elongation [19
]. In addition, post-anthesis drought caused earlier aging and a shortened grain filling duration [8
Previous findings confirmed that the contribution of the morpho-physiological parameters in the adaptation of wheat depends on the intensity of the drought stress [27
]. CTD and chlorophyll content measurements help breeders in investigating wheat yield stability since they are correlated with a number of adaptive physiological traits [28
]. CTD is positively correlated with stomatal conductance [30
], transpiration rate [5
], water usage [30
], leaf area index [32
], root traits [33
], and grain yield [34
]. Chlorophyll content is positively correlated with photo inhibition, high membrane thermo-stability [35
], and water usage [36
]. Moreover, chlorophyll content is considered a reliable indicator of WUE and adaptation to drought stress in wheat [36
]. Therefore, the development of wheat cultivars that are able to use available water more efficiently and tolerate drought is a major goal for increasing wheat productivity under water-limited environments and countries should authorize policies that allocate water to adapt with climate change [10
Supplemental irrigation (SI) could allow earlier wheat planting and thus avoidance of terminal heat stress during the grain filling period. Therefore, breeders should select varieties with higher WUE. This system proved to be essential in Jordan, where the variation in rainfall and distribution from year to year results in great fluctuations in production [14
Farmers should avoid drought during the most sensitive stages of growth, mainly the flowering and the grain filling stages, when drought causes poor growth and consequently poor yield. Therefore, SI, if applied in the right amount and at the right time, could make a substantial improvement in the yield potential of wheat [41
]. Recently, many studies have shown that proper SI can increase crop yield via improving soil water conditions and the crop WUE. For example, Semcheddine and Hafsi (2014) [42
] tested 10 durum wheat cultivars (Triticum durum
Desf.) under rain-fed and three irrigation treatments in semi-arid conditions of Eastern Algeria. They reported that irrigation treatments significantly affected grain yield, yield components, and chlorophyll content.
Alternatively, wheat yield can be stabilized by improving crop management to enhance water availability and WUE. Thus, adoption of SI in the rain-fed condition in Jordan should be assessed to cope with the expected water limitation implied by the global climate change and dramatic increase in population. The objectives of the current study were to: investigate the physiological basis controlling yield responses of three spring wheat cultivars in dry areas of Jordan under different water regimes using CTD and SPAD, establish stability levels under fluctuated precipitations and high temperatures as related to the climate change in Jordan by assessing the responses of yield and yield components under different water regimes, and investigate the feasibility of implementing supplemental irrigation of wheat crop in Jordan as a national strategy to cope with global climate change.
2. Material and Methods
2.1. Plant Materials and Cultivar Information
Two durum wheat (Triticum durum
Desf.) cultivars (Cham1 and Acsad65) and one bread wheat (Triticum aestivum
L.) cultivar (Ammon) were selected for this experiment (Table 1
) based on their reputed differences in yield performance under irrigated and non-irrigated conditions. Cham1 and Acsad65 were widely used by farmers for its stability, especially in good seasons, whereas Ammon, is a newly released cultivar and still under adoption stage by farmers. These cultivars were not previously assessed using SPAD and CTD. Cham1 is a spring durum wheat variety, erect, low tillering, short, characterized by medium heading, strong pigmentation of coleoptiles and can be distinguished from the other closely related Cham varieties by its very thick parenchyma wall of straw and the sloping shoulder shape of lower glumes. Acsad65 is a spring durum wheat variety, semi-erect, medium tillering, short, suitable for rain-fed agriculture, characterized by early heading and medium maturity, non-pigmented coleoptiles, elevated shoulder of glumes and ovoid grain shape, while Ammon is a spring bread wheat variety, erect, high tillering, tall, medium to late heading, suitable for environmental areas where rainfall is more than 350 mm [43
2.2. Site Description
Wheat seeds were sown under field condition at Maru Agricultural Station located in Irbid governorate at 32°33′ N latitude, 35°51′ E longitude, and 589 m above sea level. The experiments were performed from December to June 2013–2014 and 2014–2015 growing seasons. Maru has atypical Mediterranean climate conditions with hot and dry summer and an average annual precipitation of about 380 mm. The soil of experimental field was classified as silty clay with pH of 7.8 (Table 2
2.3. Cultural Practices and Experimental Design
Seeds were sown by hand. The planting rate was adjusted for a density of 300 plant m−2 according to the standard practices of the National Center for Agriculture and Extension (NCARE), Jordan. Planting dates were 23 November for 2013–2014 and 6 December for 2014–2015 growing seasons respectively.
Four water supply treatments were applied as a main factor consisting of rain-feds control (T0) and three levels of supplementary irrigations T1, T2 and T3 based on field capacity (FC). The three wheat varieties were split over irrigation treatments as a sub factor in a split plot design with 4 replications. The experiment consisted of 48 sub plots of 20 m2 area (4 replications × 4 water treatments × 3 cultivars). Water was applied using a dripper discharge four litters hour−1 when the soil water content dropped below 50% of the total available water in the upper 90 cm of the soil depth. A flow meter was used to measure the amount of applied irrigation water. Irrigation was applied at 50% (T1), 75% (T2) and 100% (T3) while the rain-fed treatment (T0) was maintained under no irrigation throughout the growing season.
2.4. Soil Water Monitoring
Irrigation decisions were made based on weekly readings of the Time Domain Reflectometer (TDR) (BICO-BT, IMKO 2010, Munich, Germany), which monitors soil moisture content at 15, 30, 45, 60, and 90 cm soil depth in one replicate. In both years, three access tubes were installed in the central sub plot of each treatment to measure soil water content in 15 cm increments starting from 15 cm soil depth down to 90 cm depth. Field capacity was calculated based on soil depth and root development stage. Actually, root development stage itself reflects a certain period in shoot development. For example, the field capacity was calculated based on soil depth; 15 cm was corresponding toGS1 to GS25, 30 cm soil depth corresponds to GS25 to GS30, 45 cm soil depth corresponds to GS30 to GS59, and 60 cm soil depth corresponds to GS60 to GS90. In this experiment, nitrogen deficiency was avoided by applying 50 kg ha−1 of di-ammonium phosphate at sowing and 50 kg ha−1 Urea (NH2)2CO (45%N) were applied at the tillering stage based on the NCARE-Jordan standard recommendation.
2.5. Crop Phenology
Regular observations were made of phonology in terms of thermal time to heading (from emergence to the day when 50% of plants had spikes) and thermal time to physiological maturity (from emergence to the day when all green lamina had senesced, and there was less than 10% of stem area remaining green). Using a base temperature of development and growth for wheat crop is equal to 5 °C [45
]. The dates of most important growth stages (flag leaf emergence (GS39), anthesis (GS69) and medium of milk stage (GS75))were observed when 50% of the plants reached a given developmental stage based on the decimal codes of the growth stages (GS) as revised by Tottman and Broad (1987) [47
]. Complete canopy senescence was taken as the date when all green lamina had senesced, and there was less than 10% of the stem area remaining green. Weather data were daily recorded from the automated weather station at Maru experimental station, less than 0.5 km from the experimental site. Rainfall (mm) and maximum and minimum air temperatures were recorded from sowing to harvest. Accumulated thermal time was calculated from maximum and minimum daily air temperatures using a base temperature of 5 °C.
2.6. Canopy Temperature Depression, Chlorophyll Content, Yield Analysis, and Water Use Efficiency
Canopy temperature was measured per plant based on the decimal code of growth stages (GS) as revised by Tottman and Broad (1987) [47
] for all sub-plots at three growth stages (GS39, GS61, and GS75). A hand-held infrared thermometer (Mikron M90 series, Mikron Infrared Instrument Co., Inc., Oakland, NJ, USA) was used to monitor the canopy temperature. The instrument was held to view the crop at an angle of 30° from the horizontal at right angles to the rows at a distance of 2.0 m from the sample row and at 50 cm above the canopy. Readings were made between 13:00 h and 15:00 h on sunny days. Each canopy temperature reading was the average of three regarding recorded from different points in each plot. Air temperature and humidity were measured at the same time as canopy temperature to calculate the canopy temperature depression.
Total chlorophyll content (TCC) was determined non-destructively using a portable chlorophyll meter; SPAD 502 Chlorophyll Meter (Spectrum Technologies Inc., Plainfield, IL, USA). Five plants from each irrigation × genotype combination in four replications were investigated and measured at three stages (GS39, GS61, and GS75) as a leaf chlorophyll index. Readings were taken by clamping the SPAD sensor over the leaf lamina on the first fully expanded flag leaf about halfway between the tip and the base of the leaf. Measurements were not taken over the midrib. A close linear correlation between SPAD values and extractable chlorophyll content has been observed for a wide range of species [29
At harvest, shoots from 1 m2 sample area were hand-harvested and counted in two categories: (i) fertile shoots (with spikes) and (ii) infertile shoots (without spikes). For fertile shoots, the spikes were cut off at collar, and the fresh weight of the straw was recorded. The dry weight of the components was recorded after drying for 48 h at 80 °C. The spikes were counted and then threshed. The grain and chaff were collected, dried at 80 °C for 48 h and weighed. A sub-sample of dried grain was taken, cleaned by removing all broken grains by hand, weighed after drying at 80 °C for 48 h and the thousand grain weight was calculated. The total dry weight of infertile shoots was recorded. Number of grains spike−1 was determined as an average of 10 randomly selected ten spikes plot−1. Harvest index was calculated by dividing total grain weight over total plant weight multiplied by 100.
Gravimetric soil moisture content was measured every two weeks by taking soil samples from different soil depths (0–15, 15–30, 30–45, 45–60 and 60–90 cm) and multiplying the value obtained by the average measured bulk density (1.25 gm cm−3) for the experimental site to evaluate the volumetric soil water content. The bulk density was measured by using the core method for different soil depths by digging a soil profile adjacent to the experimental site. The recorded range of bulk density was 1.2 to 1.3 gm cm−3.
The change in the volumetric soil moisture content at planting and harvesting was relatively small so long as the crop root was not accessed below a depth of 60 cm. The WUE was calculated by taken into consideration the rainfall (R) during the growing season (from date of planting until date of maturity) and irrigation quantity (Irr) and the difference in volumetric soil moisture storage (+/−DS) in the soil from the date of planting until the date of harvesting (Equation (1)), whereas deep percolation was eliminated from Equation (1) because the amount of rainfall within the two growing seasons (168.2 and 359.6 mm; respectively) is lower than that necessary to cause deep percolation (Dp) and the change in the volumetric soil moisture content in the lower soil depth (60–90 cm) was constant at pre-planting dates compared with the harvesting date. Most of the rainfall was stored in the root zone of the crop (0–60 cm) because the crop was planted in a soil with a high water holding capacity and more than 60% clay (Table 2
). Water consumption (WC) was calculated in mm using the following equation = R +(+/−DS) + Irr +Dp (Equation (1)). WUE (WUE) (kg m−3
) for grain yield was then calculated by dividing the yield (kg ha−1
) by the total water consumption (m3
2.7. Statistical Analysis
All data were statistically analyzed using Statistical Analysis System STATISTICA 7.0 (Stat Soft Inc., Tulsa, OK, USA). Standard analysis of variance procedures for a split plot design in randomized blocks were used to calculate treatment means, standard errors, and significant differences between treatments. Probability of significance was used to indicate significance among treatments and interactions according to Steel and Torrie [49
]. Means were compared using Fisher’s protected LSD test at p
≤ 0.05. The correlation coefficients of agronomic and physiological characteristics were also analyzed.
In both growing seasons, irrigation regime is the principal limiting factor for grain yield. Results indicated that normal distribution of monthly precipitation, especially during the spring months, may positively affect grain yield of wheat rather than total seasonal rainfall. Analyses of variance for grain yield and yield components demonstrated that these traits were highly affected by SI. In fact, the effects of irrigation regimes were pronounced for the majority of the traits studied. On the other hand, grain yield was higher in the cooler than in the warmer season as a consequence of more grains m−2
, heavier grains, and a longer life cycle. Rain-fed conditions caused reductions in grain yield, estimated at 48% and 9% for the first and second seasons, respectively, in comparison with irrigation treatments. In the first season (dry), wheat had the greatest yield with SI at 100% FC, after which yield decreased significantly, with SI at 75% and 50% FC. In the second season (wet), the results showed that yield was not affected by the amount of water added. Effects acquired in this study showed that the degree of yield decrease may depend on weather conditions during the growing season. These different yield responses to irrigation for the three spring wheat cultivars in the first season were probably related to little rainfall (especially in spring) coupled with poor distribution. Indeed, Loss and Siddique [52
], Passioura [89
] and Del Pozo et al. [90
] showed that the best yield was produced when SI matches plant growth and water demand, while Rasmussen et al. reported that yield was commonly greater if rainfall was partially distributed during the spring months, predominantly in May and June. To avoid a possible decline in yield with small rainfall amounts in spring months, SI supply during this stage of the crop growth was found to regulate and stabilize yield under the Mediterranean conditions of northern Jordan. Acsad65 yielded less than Ammon under the same climatic conditions and cultural practices, which was particularly significant in the production of grains m−2
. The reason for the lower yield in Acsad65 may be that this cultivar is considered a drought-tolerant cultivar, and therefore its reaction to water was not obvious due to the confusing effects of genotype and the physiological mechanisms of tolerance of this cultivar. For the same water supply levels, Ammon had significantly higher WUE values than Acsad65. SI significantly influenced WUE in the first season and slightly in the second season. Also, wheat cultivars responded differently to the irrigation regime; Ammon appeared to be more sensitive than Acsad65.
In general, it has been pointed out that wheat cultivars exposed to drought stress are able to flower earlier and, hence, bring the grain filling period forward, helping the plants to avoid the adverse effects of the drought and harsh climate [8
]. This study suggested that the use of CTD and SPAD as appropriate selection criteria for breeding programs in Jordan is extremely useful since both parameters showed highly positive correlation with grain yield and significant variations existed with respect to the cultivars and water regimes applied. Indeed, the current study suggested that improved cultivars and wise water management can play an important role inthe stability of wheat production, meeting the needs of an increasing population, and coping with climate change in Jordan and in the whole world.