Interaction of Soil Texture and Irrigation Level Improves Mesophyll Conductance Estimation

Combining leaf gas exchange with chlorophyll fluorescence, this study quantified the effects of soil water content (<i>SWC</i>) on mesophyll conductance (<i>g</i>_m) and biochemical parameters in 8-year-old pear trees across three soil textures [clay (CS), sandy (SS), loam (LS)], each subjected to three irrigation levels (100%FI, 75%FI, 50%FI). Results showed that <i>SWC</i> differed significantly, with CS > LS > SS, and that the difference in <i>SWC</i> in loam soil was the most obvious among different irrigation levels. The leaf water content (<i>LWC</i>) of SS was higher than that of LS and CS, and SS_50%FI showed 7.53% and 13.30% greater <i>LWC</i> compared to LS_50%FI and CS_50%FI, respectively. Specific leaf area (<i>SLA</i>) peaked at CS_75%FI and SS_100%FI. Soil texture and irrigation level had significant interactive effects on <i>g</i>_m, the product of light absorption coefficient and light energy partitioning ratio (<i>α</i>·<i>β</i>), leaf apparent CO₂ compensation point, dark respiration rate under light, and photosynthetic biochemical parameters. Differences in the values of <i>α</i>·<i>β</i> among the nine treatments were significant and the maximum values in the three soil textures were 0.660 (LS_75%FI), 0.366 (SS_100%FI) and 0.462 (CS_50%FI), respectively. The most sensitive treatment of <i>g</i>_m, responding to photosynthetically active radiation (<i>PAR</i>), was SS_100%FI and the maximal <i>g</i>_m under saturated <i>PAR</i> reached 0.271 molCO₂·m^-2·s^-1, increasing 2.2-fold and 8.8-fold compared to that of SS_75%FI and SS_50%FI, respectively. An underestimation of 26.4% to an overestimation of 30.3% for <i>g</i>_m and an underestimation of 28.8% to an overestimation of 15.5% were observed for biochemical parameters if the empirical value (0.425) of <i>α</i>·<i>β</i> was adopted. Our findings indicated that the maximum leaf <i>g</i>_m could be obtained at 75%FI for loam soil, 100% FI for sandy soil, and 50% FI for clay soil, respectively.