Extended Abstract 
Introduction: Drought stress is one of the most important factors limiting plant growth worldwide, which causes yield reduction by disrupting physiological processes, including photosynthesis and water relations. The use of arbuscular mycorrhizal fungi along with phosphorus fertilizer can increase plant tolerance to drought by improving water and nutrient absorption, regulating metabolic activities, and strengthening the antioxidant defense system. This study aims to investigate the regulatory effects of mycorrhizal fungi and phosphorus fertilizer on improving gas exchange and their role in increasing the weight of the bitter melon fruit.
Materials and Methods: To investigate the role of mycorrhizal fungi and phosphorus in improving gas exchange and increasing fruit weight of bitter melon (Momordica charantia) under different irrigation stresses, a split factorial experiment was conducted at the research farm of the Faculty of Agriculture, Tarbiat Modares University, Tehran, Iran, in a randomized complete block design with three replications in 2021. The experimental factors included three irrigation regimes (optimal irrigation, moderate, and severe irrigation deficit) in the main plots and a factorial combination of two fungal species Funneliformis mosseae and Rhizophagus intraradices, along with a control (non-inoculation) and phosphorus fertilizer (with and without phosphorus) in the subplots. The evaluated traits included total fruit weight/m2, total chlorophyll and carotenoids concentrations, catalase enzyme activity, electrolyte leakage, anthocyanins concentration, gas exchange parameters (net photosynthesis rate, stomatal conductance, substomatal CO2 concentration, and transpiration rate), and water use efficiency.
Results: The results showed that by reducing irrigation to a severe irrigation deficit, fruit weight, gas exchange parameters, water use efficiency, and total chlorophyll concentration significantly decreased, and electrolyte leakage, carotenoids and anthocyanins concentrations, and catalase activity increased. The results showed that inoculation of two species of mycorrhizal fungi and phosphorus fertilizer in different irrigation regimes increased catalase enzyme activity and reduced electrolyte leakage, protecting the photosynthetic apparatus against oxidative stress caused by irrigation deficiency, which led to increases in chlorophyll concentration and fruit weight. According to the results of the triple interaction of irrigation × phosphorus × mycorrhizae, the highest fruit weight was observed in plants inoculated with Funneliformis mosseae (2858 g m-2) and Rhizophagus intraradices (2863 g m-2) along with phosphorus fertilizer under optimal irrigation conditions. The highest total chlorophyll concentration (0.374 mg g-1 fresh weight) was also observed in plants inoculated with Rhizophagus intraradices under optimal irrigation conditions and phosphorus fertilizer application. The results showed that at all three irrigation levels and in both conditions with and without phosphorus fertilizer, fruit weight and total chlorophyll concentration under inoculation with both mycorrhizal fungi species significantly increased compared to non-inoculation. The highest substomatal CO2 concentration (263.7 μmol CO2 mol-1) was observed in plants inoculated with Rhizophagus intraradices under optimal irrigation conditions and phosphorus fertilizer application, which was not significantly different from those with Funneliformis mosseae at the same irrigation level. The results showed that with the reduction of irrigation to severe deficit irrigation, anthocyanins production increased as a plant strategy to response to water stress. At all irrigation levels, anthocyanins concentration increased in response to both mycorrhizal fungi species. The highest anthocyanin concentration was observed in plants under severe irrigation deficit and inoculated with Funneliformis mosseae in conditions without phosphorus fertilizer (0.34 mg g-1 fresh weight) and inoculated with Rhizophagus intraradices under phosphorus fertilizer application (0.35 mg.g-1 fresh weight). The correlation results showed that fruit weight had a positive and significant relationship with total chlorophyll, gas exchange parameters, and water use efficiency, and it showed a negative correlation with electrolyte leakage. Also, the positive correlation of total chlorophyll concentration with gas exchange and water use efficiency indicates that an enhanced chlorophyll concentration leads to improved physiological status and better productivity of water resources, perhaps by improving light absorption, photosynthesis efficiency, and effective gas exchange.
Conclusions: The results presented, taken together, showed that drought stress led to a decrease in fruit weight by reducing gas exchange and photosynthetic efficiency. However, application of phosphorus fertilizer and inoculation of mycorrhizal fungi, particularly Rhizophagus intraradices, under optimum irrigation conditions protected the photosynthetic apparatus and increased chlorophyll concentration and fruit weight by increasing catalase enzyme activity and reducing electrolyte leakage. Although the effect of the combination of phosphorus and mycorrhizae on gas exchange was limited, but it led to the improvement of total chlorophyll concentration and an increase in fruit weight. Thus, this combination can be suggested as an effective solution to enhance drought resistance and increase the yield of bitter melon.