Gas exchange characteristics of the hybrid Azadirachta indica × Melia azedarach

Abstract

The hybrid Azadirachta indica × Melia azedarach is a new plant variety obtained through somatic hybridization, showing high concentrations of active insecticidal substances in its seeds. The hybrid could grow normally in regions with an average annual temperature of 15 °C, therefore it will likely become a valuable new tree species in subtropical and warm temperate regions. However, the photosynthetic physiological characteristics of A. indica × M. azedarach remain unknown. The photosynthetic gas exchange of the hybrid at three different ages (one year old, AM1; three years old, AM3; and five years old, AM5) were measured. The specific leaf mass per area (LMA), leaf N, leaf P, and leaf N/P of tree samples were measured, and the photosynthetic N and P use efficiencies (PNUE and PPUE, respectively) were also assessed. The maximum leaf net photosynthetic rate P a (based on area), P m (based on mass), light saturation point (LSP), light compensation point (LCP), stomatal conductance (g s), and transpiration rate (T r) of A. indica × M. azedarach decreased with increasing tree age, whereas the instantaneous water use efficiency (WUE) increased with age. The photosynthetic capacity showed no significant differences between AM3 and AM5, but was significantly higher in AM3 and AM5 when compared with AM1. The P a, P m, apparent quantum yield (AQY), LSP, g s, and T r of A. indica × M. azedarach were significantly lower than that of the parental species M. azedarach, whereas the dark respiration rate (R d) and WUE were significantly higher than that of M. azedarach. The reduction in the maximal photosynthetic rate observed with increased age in the hybrid was primarily related to the increased LMA and the decline in leaf nitrogen (N) and leaf phosphorus (P) concentrations. Additionally, the decline in stomatal conductance (g s) was also an important factor leading to age-dependent reductions in the photosynthetic rate. These findings suggest that tree age significantly affects A. indica × M. azedarach gas exchange during juvenile stages, and the photosynthetic capacity of the hybrid was significantly lower than that of the parental species M. azedarach.