Arbuscular Mycorrhiza Fungi Reduce Photosystem II Efficiency in Phosphorus-Deficient Maize Without Promoting Growth

Arbuscular mycorrhizal fungi (AMF) are widely reported to restore photosynthetic efficiency under phosphorus deficiency. Whether this holds under combined phosphorus-and-zinc deficiency in early-stage maize (Zea mays), and how AMF modify the relationship between photosynthetic efficiency and growth rate, remains unresolved. Maize variety SUWAN 5819 was grown under five nutrient treatments (+Zn+P, −Zn−P, +Zn−P, −Zn+P, deionized water) with and without AMF in a sand culture under a randomized complete block split-plot design with six replications. Chlorophyll fluorescence, stomatal conductance, and growth-analysis variables were measured. Lindeman–Merenda–Gold (LMG) variance partitioning and piecewise structural equation modelling (SEM) decomposed the relative growth rate (RGR) into net assimilation rate (NAR) and leaf area ratio (LAR) components. AMF reduced the quantum efficiency of photosystem II (ΦPSII) by 29.6–30.8% in phosphorus-deficient treatments. Despite this, AMF had no significant effect on NAR, LAR, or RGR across any treatment. NAR contributed 62.9–99.2% of explained RGR variance across all treatment combinations, consistently exceeding LAR. Under combined −Zn−P deficiency, stomatal conductance was significantly elevated despite reduced ΦPSII and ETR. The piecewise SEM showed that AMF caused a 1.7 times increase in the negative effect of −Zn−P on ΦPSII. This study shows that phosphorus is the main determinant of the quantum efficiency of photosystem II whilst AMF under phosphorus deficiency reduce photochemical efficiency without a compensating growth benefit in early vegetative maize.