Effects of BR on the growth of SRPs
The BR content significantly (p < 0.001) affected the growth of roots and shoots of SRPs without rooting. Statistical results showed that the length of roots and shoots of SRPs under the treatment of 1 µg/L BR was significantly higher (p < 0.05) than that of the 10 µg/L BR and control group (Fig. 3 A). In addition, the needles were light green and the root system consisted of only one primary root, with no lateral roots in the control group. When the rooting medium was supplemented with 1 µg/L BR, the needles were dark green and a well-developed root system emerged (more lateral roots, greater root volume, and more root tips). However, when the BR concentration was increased to 10 µg/L, the SRPs showed a similar differentiation to the control group (Figs. 3 B, C, D). These results clearly indicated that 1 µg/L BR promoted the growth of P. thunbergii SRPs via root system differentiation. Further, they implied that the BR promoting the growth of P. thunbergii SRPs might related to photosynthetic regulation.