Abstract:

In this study， we investigated the effects of bacteria-phosphorus interactions on the anatomical structure and photosynthetic properties of alfalfa （Medicago sativa） leaves， and explored the relationship between alfalfa leaf structure and photosynthetic parameters. A two-factor randomized block design was adopted， with single inoculation of Bacillus mucilaginosus （J₁）， Bacillus megaterium （J₂）， a mixture of both bacteria （J₃）， and no bacteria （J₀）， with phosphorus application （P₂O₅ at 100 mg·kg^-1） （P₁） or without phosphorus application （P₀）. The leaf photosynthetic indexes and anatomical structure indexes were measured， and the relationship between leaf photosynthetic characteristics and anatomical structure was clarified using regression analyses. It was found that under the same bacterial inoculation conditions， the leaf thickness， upper epidermal thickness， lower epidermal thickness， palisade parenchyma thickness （PPT）， spongy parenchyma thickness （SPT）， vessel diameter and diameter of sieve tubes， and the daily mean values of net photosynthetic rate （P_n）， transpiration rate （T_r）， stomatal conductance （G_s） of alfalfa leaves were higher in the P₁ treatment than in the P₀ treatment； the relative chlorophyll content （SPAD value） was significantly higher in the P₁ treatment than in the P₀ treatment （P<0.05）； and the daily mean values of intercellular CO₂ concentration （C_i） were lower in the P₁ treatment than in the P₀ treatment. Under the same phosphorus application conditions， the leaf thickness， upper epidermal thickness， lower epidermal thickness， PPT， SPT， vessel diameter and diameter of sieve tubes， and daily mean values of P_n， T_r， G_s， and SPAD of alfalfa leaves were higher in bacteria-inoculated treatments than in the uninoculated treatment， with maximum values in the J₃ treatment. The daily mean value of C_i was lower in bacteria-inoculated treatments than in the uninoculated treatment. Bacteria and phosphorus had highly significant effects on alfalfa leaf thickness， upper epidermal thickness， lower epidermal thickness， PPT， SPT， and vessel diameter （P<0.01）. Bacteria×phosphorus interactions were significant for alfalfa leaf thickness and palisade tissue thickness （P<0.05）. The daily mean values of P_n were significantly and positively correlated with leaf thickness， vessel diameter， sieve tubes diameter， PPT， and SPT （P<0.05）， and significantly negatively correlated with PPT∶SPT ratio （P<0.05）. In a multivariate evaluation based on the structural and photosynthetic properties of alfalfa leaves， the treatments were ranked， from most to least conducive to leaf development and photosynthesis， as follows： J₃P₁ > J₁P₁ > J₂P₁ > J₃P₀ > J₀P₁ > J₁P₀ > J₂P₀ > J₀P₀. Therefore， a phosphorus application rate of 100 mg·kg^-1 and double inoculation with B. mucilaginosus and B. megaterium was most conducive to the photosynthetic process of alfalfa leaves， with strong positive effects on growth and physiological development.

Key words: alfalfa, phosphate solubilizing bacteria, phosphorus, leaf structure, photosynthetic properties