Enhancement of SRPs by culture conditions: sucrose, liquid medium, and transplantation stroma
The needles of the SRPs were sparse and yellow-green under a 10 g/L sucrose treatment, showing an obvious nutrient deficiency phenomenon. When the sucrose concentration was increased to 20 and 30 g/L, the needles were darker in color and the plant height was significantly higher than under the 10 g/L treatment. However, when the sucrose concentration was raised to 40 g/L, the growth of the SRP was inhibited. Furthermore, the plant height of the SRP was higher in the 1/2 WPM and GD than the DCR and LP liquid medium. Similarly, the plant height of the SRP in perlite and vermiculite = 1:1 medium treatment was higher than that of only the perlite and vermiculite. In general, the 20 and 30 g/L sucrose, liquid medium GD and 1/2 WPM and culture stroma (perlite: vermiculite = 1:1) were observed to significantly enhance the SRP growth (Fig. 2 A).
With further study, the sucrose concentration (shoot, p< 0.001; root, p = 0.002), liquid medium (shoot,p < 0.001 and root, p < 0.001) and culture stroma (shoot, p = 0.03; roots, p = 0.09) significantly affected the SRPs growth. Under the sucrose treatments, when the concentration was doubled from 10 to 20 g/L, the shoot lengths of the SRPs were increased from 1.00 to 2.05 cm, and the root lengths were increased from 2.04 to 3.23 cm. Simultaneously, the lengths of both the shoots and roots reached their peak. However, when the sucrose concentration was raised to 30 g/L, the shoots and root lengths of the SRPs began to decrease. When the sucrose concentration was further increased to 40 g/L, the growth of the SRPs was significantly inhibited (Fig. 2 B). Under the liquid medium treatments, the longest shoots (1.44 cm) and roots (4.47 cm) of SRPs were obtained with the 1/2 WPM medium, followed by the GD liquid medium treatment (shoots = 1.24, roots = 3.89 cm). However, among all the tested liquid media, the shortest shoots (0.54 cm) and roots (0.75 cm) of the SRPs were obtained with the DCR medium (Fig. 2 C), which was obviously not suitable for the growth ofP. thunbergii SRPs.
For the transplantation stroma treatments, the minimum shoot (0.72 cm) and root (1.48 cm) values were obtained under the perlite treatment, followed by the vermiculite treatment (shoot = 0.77 cm, root = 1.90 cm). However, the lengths of the SRP shoots and roots cultured in a stroma mixture of perlite and vermiculite (v/v = 1:1) were significantly higher than when they were cultured in perlite and vermiculite alone. The results revealed that the shoot and root lengths were significantly (p < 0.05) increased to 1.26 cm and 3.89 cm, when the vermiculite and perlite were mixed at a 1:1 ratio as the transplantation stroma (Fig. 2 D). This suggested that the 20 g/L sucrose, 1/2 WPM liquid medium, and transplantation stroma of vermiculite and perlite mixed at a 1:1 ratio were the most suitable for the growth of post-germination nematode-resistant P. thunbergiiplantlets.