Influence of Phenylacetic Acid and Its Derivatives on Callus Proliferation and Somatic Embryogenesis in Litchi chinensis

The efficiency of in vitro regeneration in litchi (Litchi chinensis Sonn.) is highly influenced by the type and concentration of plant growth regulators (PGRs), particularly auxins. This study evaluated the effects of phenylacetic acid (PAA) and its derivatives—4-chlorophenylacetic acid (CPA) and 4-iodophenylacetic acid (IPA)—on callus proliferation, somatic embryogenesis, and plantlet regeneration in ‘Feizixiao’ litchi, as well as to establish an efficient regeneration protocol. The inclusion of CPA or IPA in callus proliferation medium significantly enhanced the proliferation rate, with 20 mg·L−1 CPA being the most effective. The highest number of somatic embryos per gram of fresh embryonic callus weight (gFW−1) (1131 embryos·gFW−1) was observed with 40 mg·L−1 PAA in proliferation medium. The addition of 10 mg·L−1 IPA to the proliferation medium yielded the highest plantlet regeneration rate (50 plantlets·gFW−1). Supplementing the somatic embryo induction medium with 5 mg·L−1 PAA resulted in 460 somatic embryos·gFW−1 and 86 regenerated plantlets·gFW−1. These findings indicate that PAA and its derivatives are effective PGRs for the in vitro regeneration of litchi, providing a valuable protocol for the propagation of elite cultivars.