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 (SWC) on mesophyll conductance (gm) 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 SWC differed significantly, with CS > LS > SS, and that the difference in SWC in loam soil was the most obvious among different irrigation levels. The leaf water content (LWC) of SS was higher than that of LS and CS, and SS50%FI showed 7.53% and 13.30% greater LWC compared to LS50%FI and CS50%FI, respectively. Specific leaf area (SLA) peaked at CS75%FI and SS100%FI. Soil texture and irrigation level had significant interactive effects on gm, the product of light absorption coefficient and light energy partitioning ratio (α·β), leaf apparent CO2 compensation point, dark respiration rate under light, and photosynthetic biochemical parameters. Differences in the values of α·β among the nine treatments were significant and the maximum values in the three soil textures were 0.660 (LS75%FI), 0.366 (SS100%FI) and 0.462 (CS50%FI), respectively. The most sensitive treatment of gm, responding to photosynthetically active radiation (PAR), was SS100%FI and the maximal gm under saturated PAR reached 0.271 molCO2·m-2·s-1, increasing 2.2-fold and 8.8-fold compared to that of SS75%FI and SS50%FI, respectively. An underestimation of 26.4% to an overestimation of 30.3% for gm and an underestimation of 28.8% to an overestimation of 15.5% were observed for biochemical parameters if the empirical value (0.425) of α·β was adopted. Our findings indicated that the maximum leaf gm could be obtained at 75%FI for loam soil, 100% FI for sandy soil, and 50% FI for clay soil, respectively.