Limitations in Grain Yield and Carbon Partitioning Differs in Soybean (Glycine max (L.) Merr.) Cultivars with Contrasting Photosynthetic Phosphorus-Use Efficiency

The aim of the work was to elucidate how different P doses alter photosynthesis and to identify their effects on carbon partitioning and grain production in two soybean cultivars with contrasting photosynthetic phosphorus-use efficiency (PPUE). Two cultivars, UFVS80B10 and TMG7063, were subjected to increasing doses of phosphorus (0.01 mM, 0.1 mM, 0.5 mM, and 1 mM) in Hoagland nutrient solutions. The cultivar with a higher PPUE (TMG7063), presented 24%, 25%, 41%, 46%, and 31% of net photosynthetic rate, stomatal conductance, maximum carboxylation rate of rubisco, electron transport, and triose phosphate use, respectively, than the UFVS80B10 cultivar. However, in both cultivars, there was a reduction in those variables due to the lower P content in the tissues of plants subjected to P deficit. The P deficit caused damage to photosystem II (PSII) only when the plants were close to the R6 stage. The cultivars were efficient in dissipating nonphotochemical energy during P deficit (0.01 mM), but with higher photochemical efficiency in the cultivar TMG7063 at the reproductive stage in higher P doses. The higher absorption of P in the cultivar with low PPUE reflected higher starch and sugar contents in the leaves, stems, and roots, causing a greater biomass gain but with grain yield of 45% lower than the cultivar with the highest PPUE. A higher PPUE in soybean is associated with the maintenance in carbohydrate partitioning under low P, maximizing grain production and presenting photoprotective mechanisms mainly in the grain filling stage.