Biostimulatory Response of Easily Extractable Glomalin-Related Soil Protein on Soil Fertility Mediated Changes in Fruit Quality of Citrus

Arbuscular mycorrhizal fungi secrete glomalin deposited into the soil as glomalin-related soil protein (GRSP), which possess multiple utility to benefit both soil as well as plant health. The present study aimed to assess the effects of the foliar application of an easily extractable GRSP (EE-GRSP) on the fruit quality, soil nutrients, and soil structural changes in three important citrus varieties (Satsuma mandarin Oita 4, Newhall navel orange, and Cocktail grapefruit). The exogenous EE-GRSP significantly elevated root mycorrhizal fungal colonization and soil hyphal length in Newhall and Oita 4 varieties, but without any such response in Cocktail grapefruit variety. The foliar spray of the EE-GRSP improved different external (e.g., pericarp, sarcocarp, and single fruit weight) and internal (e.g., soluble solids, titratable acids, and sugar contents) qualities of fruits to varying magnitudes, depending on citrus variety, with a more prominent effect on Cocktail grapefruit. EE-GRSP-treated fruits of Newhall and Oita 4 were more suitable for processing than non-treated control because of a low fruit hardness. However, no significant effect of the EE-GRSP was observed on the internal quality parameters of Newhall. EE-GRSP-treated citrus trees represented higher soil available nutrients over control, to some extent, especially on Oita 4. The foliar application of the EE-GRSP also increased various GRSP fractions to varying proportions and improved the distribution of water-stable aggregates in the size fraction of 0.25–2 mm, thereby increasing the mean weight diameter, particularly in Newhall and Cocktail grapefruit varieties. These observations provided clues about the stimulatory role of the EE-GRSP through soil structure and nutrient pool-mediated improvements in fruit quality.