Based on the results of the analysis of variance, it was found that both the simple and interaction effects of nitrogen fertilizer and irrigation had a significant impact on all studied traits of chickpea (Table 2). Consequently, we will only discuss the interaction effects of the studied traits in the following sections.
Grain yield
The highest grain yield was obtained in the full irrigation treatment with nitrogen level of 30 kg ha-1 (2645 kg ha
-1) (Fig 1). After this treatment, the highest grain yield was observed in the interacton of full irrigation and nitrogen level of 75 kg ha
-1. Under full irrigation and low nitrogen levels at the beginning of the growing season, chickpea appear to establish faster in the field. This is important before the plant becomes self-sufficient in nitrogen fixation. Moreover, the irrigation in the flowering stage with a nitrogen level of 75 kg ha
-1 (2356 kg ha
-1) and the irrigation treatment in the flowering and podding stage with a nitrogen level of 30 kg ha
-1 (2338 kg ha
-1) were ranked next and did not differ significantly from each other (Fig 1). Considering that the majority of chickpea in Iran, are rainfed and experience drought stress during the flowering stage, it results in a reduction in the length of the flowering period, the number of flowers and grain yield
(Pasandi et al., 2014). Therefore, by employing one or two supplementary irrigation methods during the critical stages of the plant’s growth (flowering and podding) and utilizing 30 kg N ha
-1 as a starter fertilizer, it is possible to achieve high yields that are comparable to those under full irrigation conditions. Moreover, this approach enhances water use efficiency. In this experiment, the lowest grain yield was associated with the irrigation treatment during the flowering stage and nitrogen level, which were 150 kg ha
-1 (1395 and 1481 kg ha
-1 respectively). Consequently, as the irrigation amount is decreased, the plant’s response to nitrogen also diminishes
(Shaban et al., 2011). This occurs because the application of excessive nitrogen fertilizer in arid and semi-arid regions promotes vegetative growth, but leads to drought stress towards the end of the growing season, resulting in a significant decrease in yield. Drought stress can lead to a reduction in nitrogen fixation, nodule respiration and stem dry weight. One of the causes for the decline in yield under semi-arid conditions is the disturbance of the plant’s nutritional balance in such circumstances. In severe drought stress, aging is often accelerated. This effect is particularly more severe in higher nitrogen levels. Consequently, a reciprocal relationship exists between drought stress and nitrogen, where nitrogen consumption under dry conditions negatively impacts grain yield.
Number of pods per plant
In the evaluation of the interaction effect between nitrogen fertilizer and irrigation, the highest number of pods per plant (29 pods per plant) were obtained under full irrigation with an application of 75 kg ha
-1 of nitrogen. Conversely, the lowest number of pods (9 pods per plant) was observed when using irrigation at flowering treatment along with 150 kg ha
-1 of nitrogen (Fig 2). High levels of nitrogen application led to increased vegetative growth during the initial stages of plant growth, resulting in rapid drainage of soil moisture. As a result, the application of supplementary irrigation did not prove to be very effective in increasing the number of pods. However, by using more irrigation water and maintaining balanced levels of nitrogen fertilizer, the plant will develop a larger canopy capable of nourishing larger sinks and allocating an adequate amount of dry matter to them. Consequently, the number of pods in the plant increases. Shortened pollination period due to drought stress can also lead to a reduction in pod numbers. Drought adversely affects flower-producing buds during the reproductive stages, resulting in the shedding of flowers and ultimately leading to a decrease in pod production
(Gusmao et al., 2012). According to
Kashfi et al. (2010), the chickpea plant produced the highest number of pods per plant under full irrigation with the application of 50 kg N ha
-1. Conversely, the lowest number of pods was observed when no nitrogen was applied. The availability of moisture and balanced levels of nitrogen increases the development of the plant canopy, leading to an increase in yield components such as the number of pods per plant (
Halagalimath and Rajkumara, 2018;
Venkatesh et al., 2023).
Seeds per pod
In the investigation of the interaction effect between nitrogen fertilizer and irrigation, the highest number of seeds in the pod was observed under the full irrigation treatment along with 75 kg ha
-1 of nitrogen (2.17 seeds per pod and 2076 seeds m
-2). On the other hand, the lowest number of seeds was associated with the flowering irrigation treatment and 150 kg ha
-1 of nitrogen (0.65 seeds per pod and 193 seeds m
-2) (Fig 3). Providing moisture during the plant growth period, along with a balanced level of nitrogen (75 kg in this experiment), enhances vegetative growth and subsequently increases the number of seeds in the pod. However, if irrigation is carried out during the flowering stage and high levels of nitrogen are applied, it can lead to excessive vegetative growth and make the plants more susceptible to drought towards the end of the season. Consequently, this may result in a decrease in the number of seeds in the pod (
Jagdale and Dalve, 2010). Increasing the amount of irrigation water results in an increase in pod numbers and maturity occurring over a longer period. Additionally, leaves senescence at a slower rate, resulting in an increased number of seeds in the pod. Conversely, a decrease in the amount of irrigation water, coupled with an increase in temperature, leads to premature aging of the plant
(Lobell et al., 2012). Optimal soil moisture influences the duration of the reproductive growth period and the level of photosynthesis, which in turn affects the number of flowers formed on the plant and ultimately leads to the development of fertile pods and increased seed production (
Amiri dehahmadi et al., 2010a).
Biological Yield
The highest biological yield (8900 kg ha
-1) was observed in the full irrigation treatment and 150 kg N ha
-1, while a significant decrease in biological yield was observed with reduced irrigation and nitrogen fertilizer. The irrigation treatment during the flowering stage and the nitrogen level of 30 kg ha
-1 resulted in the lowest biological yield (6600 kg ha
-1) (Fig 4). In chickpea, the availability of moisture and nitrogen increases vegetative dry matter, resulting in an increase in biological yield, as it leads to an increase in the length of the vegetative growth period, effective depth of the canopy, absorption of active photosynthetic radiation and the formation of seeds. Drought stress can lead to a reduction in biological yield due to a decrease in the length of the growth period, crop growth rate reduction, and flower and pod drop. Adequate moisture supply during the flowering stage is crucial for chickpea, as it experiences active vegetative growth during this period. Chickpeas exhibit rapid vegetative growth after flowering, followed by a subsequent decrease during the podding phase. The reduction in plant transpiration under drought stress facilitates a proper balance between transpiration and the maintenance of the critical leaf area index required for photosynthesis. Reducing the leaf area becomes an important adaptive mechanism as it is the plant's initial response to water scarcity (
Amiri dehahmadi et al., 2010b; Chauhan et al., 2019; Parsa et al., 2012).