4.1.1. Yield
In the experiment, water and fertilizer had no significant effect on the SFW of cucumber, mainly because the fruit size was basically the same when cucumber was harvested according to artificial experience. Therefore, the SFW of cucumber fruit was not related to different water and fertilizer application rates.
The interaction of I * N had significant effects on cucumber yield, which was consistent with previous studies(Shahrokhnia and Sepaskhah, 2016). I * Mg had a significant effect on cucumber yield, because reasonable irrigation and magnesium application could promote the growth and development of plant roots, and this condition is conducive to the full absorption of water and nutrients by plant roots, which increase the sink-source ratio and the photosynthetic production capacity of source leaves, thereby increasing production(Luo et al., 2020). As an enzyme-assisted factor and heterologous regulator, Mg can regulate the Calvin cycle by activating various enzymes, thereby increasing the dry matter accumulation of plants(Shaul, 2002). Therefore, I * Mg can promote the synthesis and transformation of carbohydrates in plants to a certain extent, thereby promoting plant dry matter accumulation and affecting crop yield(Verbruggen and Hermans, 2013).
In this experiment, under the low irrigation level (I1), the yield of T1 was the lowest, and T3 achieved the highest yield with a slight difference from T9 (the largest yield of all treatments), indicating that supplementation with nitrogen and magnesium could promote cucumber growth. Especially in low water input, N * Mg could compensate for the effect of water shortage on cucumber growth to a certain extent and promote the formation of cucumber yield, and this finding is consistent with the research results of Peng et al(Peng et al., 2020). This finding was obtained, because magnesium affects nitrogen transport in plants. Mendes et al(Mendes and Marenco, 2015) found that magnesium can improve plant photosynthesis, promote crop nitrogen uptake, and increase plant dry matter accumulation, thereby increasing crop yield. Therefore, the reasonable application of nitrogen and magnesium fertilizer can significantly alleviate the inhibition of drought stress on plant yield.
4.1.2. Water and fertilizer utilization
Reasonable fertilization can improve crop water use efficiency, and irrigation can improve fertilizer use efficiency(Ye et al., 2022). I * Mg had a significant indigenous effect on plant NPP. This finding was obtained, because I * Mg can promote the photosynthetic capacity of plants, promote the nitrogen metabolism pathway in roots and the synthesis of related enzymes and hormones, and enhance the ability of plants to absorb nitrogen fertilizer, thereby improving plant NPP(Bagheri et al., 2021). I*N had a significant interaction on MPP, and I*N could change the osmotic pressure of cells and increase the synthesis of enzymes in the photosynthetic system(Zamora-Re et al., 2020). The appropriate amount of I * N can improve the net photosynthetic rate and the accumulation of photosynthetic assimilates, thereby promoting the absorption of magnesium by plants(Chen et al., 2022).
A synergistic effect was observed between magnesium and nitrogen, and the increase in magnesium and nitrogen can promote the utilization of water and nutrients by plants, which was consistent with our results. Mg can promote nitrogen uptake by crops(Shi et al., 2021). Magnesium deficiency affects nitrogen transport in maize, because magnesium stress increases nitrate reductase activity and promotes the transformation of inorganic nitrogen in plants, resulting in a decrease in NPP under low-magnesium conditions(Yin et al., 2009). In addition, low magnesium can lead to poor root development, thereby reducing the total nitrogen uptake of roots and affecting plant nitrogen accumulation. The appropriate proportion of nitrogen and magnesium fertilizer is conducive to the full absorption of water and nutrients by plant roots and promotes the crop utilization of deep soil moisture(Li et al., 2021). Therefore, N * Mg has significant effects on WPP, NPP, and MPP.
4.1.3. Quality indicators
In the present study, magnesium had a significant effect on RS and FAA (Table 5), which was consistent with previous studies(Cole et al., 2016). This finding was obtained, because the rational application of magnesium can increase plant chlorophyll content, thereby affecting photosynthesis and promoting carbohydrate, fat, and protein synthesis(Li et al., 2020). Reasonable water and nitrogen application could significantly decrease the NIT content in cucumber fruit and maintain the contents of VC, TSS, FAA, and RS at a high level, which could improve the cucumber quality to a certain extent. This property can be attributed to glutamine synthetase, which is the main enzyme of ammonia assimilation in higher plants(Wang et al., 2008), and its activity affects the metabolism of amino acids and proteins in plants. Water and nitrogen interaction had significant effects on glutamine synthetase activity. At the same time, the application of nitrogen fertilizer in soil can improve the activity of N reductase and glutamine synthetase and promote the transformation to N ammonium. Therefore, the contents of RS and TSS in fruits will increase(Plett et al., 2020).
I * Mg has a very significant effect on plant quality(Guo et al., 2016), and the same results were obtained in the present study. This finding was obtained, because the increase in I can promote the development of plant roots and thus affect the absorption of magnesium fertilizer by plants. I * Mg can promote the synthesis of chlorophyll, promote the synthesis and metabolism of protein and nitrate reductase in plants, promote the transportation of carbohydrates in plants, and facilitate the accumulation of antioxidant active substances and carbohydrates in fruits, thereby improving crop quality(Thalooth et al., 2006). The contents of amino acids and soluble solids in plants gradually increase with the increase in nitrogen and magnesium fertilizers(Potarzycki, 2008). In the present study, the quality indexes of cucumber under T3 treatment were higher than those under other treatments mainly because of the synergistic effect between magnesium and nitrate nitrogen(Qian et al., 2013). The increase in magnesium application rate will promote the absorption of nitrogen in leaves and improve the quality of plants. In addition, under the condition of irrigation level I1, except for VC, the contents of all fruit quality indexes can be ranked as T3 > T1, indicating that under water stress, more N and Mg fertilizers were helpful to improve fruit quality, which was consistent with the results of previous studies.
4.1.4. Comprehensive evaluation
Considering that the optimal water and fertilizer conditions for different indicators were not the same, an evaluation system needs to be established to develop a reasonable application strategy. Cucumber production has many indicators, and the selection of indicators will affect the evaluation results. Considering that a large number of indicators would disperse the evaluation weight, correlation analysis was conducted to determine the important indexes that affect cucumber yield, quality, and water and fertilizer utilization rate for more reasonable and scientific results. Moreover, AHP analysis and entropy method were used to determine the weight of each index, and game theory was used to calculate the optimal combination coefficient to determine the final weight, which effectively reduced the deviation of single weighting method. GRA model has computational efficiency and is flexible and convenient for analyzing data with a small sample size(Tan et al., 2021). Therefore, combined weighting coupling GRA was adopted to obtain the best treatment on the evaluation of multiple indicators, and then determine the optimal amount of irrigation, nitrogen, and magnesium to promote the comprehensive benefit of greenhouse cucumber.
For the application rate of water and fertilizer, the results of this experiment were inconsistent with previous studies, mainly because the interaction of three factors for magnesium, irrigation, and nitrogen were investigated, and the efficiency index was considered on the basis of conventional yield and quality. Under low-irrigation conditions, increasing the input of magnesium fertilizer and nitrogen fertilizer improved the fruit quality and had no significant effect on cucumber yield reduction. Under the same irrigation conditions, increasing magnesium fertilizer can alleviate the problem of insufficient nitrogen fertilizer input, improve nitrogen use efficiency, and improve cucumber quality.