This study focused on the physiological regulation and mechanism of exogenous 5-aminolevulinic acid (5-ALA) in the late growth of P. heterophylla. In the middle of May, different concentrations of 5-ALA (0, 10, 20, 50 mg·L⁻¹) were sprayed on the leaves. The effects of 5-ALA on tuberous root growth, antioxidant enzyme system, gas exchange, photosynthetic pigment contents and photosynthetic characteristics were measured from 23 May to 13 June. A concentration of 20 mg·L⁻¹ of 5-ALA led to a significant increase in the yield of fresh root and biomass allocation at 38.12% and 25.07%, respectively, in comparation with the control (0 mg·L⁻¹). The moderate concentration of 5-ALA statistically stimulated antioxidation activities. 5-ALA treatment enhanced photosynthetic activity and reduced photodamage. Compared to the control, there were increases in the chlorophyll fluorescence parameters of P. heterophylla under 5-ALA treatment. Moreover, 20 mg·L⁻¹ of 5-ALA significantly changed the kinetic parameters of fluorescence. It enhanced the light absorption and distribution efficiency of PSII and the activities of leaves, resulting in alleviating photoinhibition by the excess excitation energy. The correlation indicated that there was a significant positive correlation between the yield of tuberous roots and biomass allocation, P_n and catalase (CAT), and a negative correlation between the yield of tuberous roots and malondialdehyde (MDA). The appropriate 5-ALA concentration in the late growth stage of P. heterophylla effectively enhanced the net photosynthetic capacity, mainly resulting from the enhancement of PSII photochemical activity to promote the increases in excitation energy absorption, capture and electron transfer efficiency of the leaves. Finally, 5-ALA treatment can increase the photochemical activity of PSII in the whole leaf and ultimately delay the senescence of P. heterophylla.

Taxus mairei is an important source for industrial extraction of taxol in China. However, the standard and steps of extraction are currently not uniform, which seriously affects the taxol yield. In the present study, the influence of four factors (methanol concentration, solid-liquid ratio, ultrasonic extraction temperature, and ultrasonic extraction time) on the taxol yield was successively explored in T. mairei. A response surface methodology (RSM) was used to optimize the extraction process based on the single-factor experiments above. The optimal conditions were as follows: methanol concentration was 90%, solid-liquid ratio was 1:15 (g/mL), ultrasonic extraction temperature was 40 °C and ultrasonic extraction time was 60 min. Moreover, the twigs and needles from T. mairei with different tree ages were treated by the optimum extraction process, which further revealed temporal and spatial distribution of taxol in the reproducible tissues. Interestingly, the taxol content was relatively higher in needles of T. ‘Jinxishan’ (a cultivar from T. mairei with yellow aril, FY), but was less in FY twigs. The accumulation of taxol in twigs and leaves of females (with red aril, FR) was significantly higher than that of males (M); however, the content showed a decreasing trend with the increasing tree ages. Therefore, it is suitable to increase the proportion of female trees especially the FY leaves as raw materials for the industrial production of taxol from T. mairei, and the tree ages should be better controlled at 3–7 years.