Enhancing Callus Proliferation and Somatic Embryogenesis in <em>Litchi chinensis</em> cv. “Feizixiao” with Phenylacetic Acid and Its Derivatives

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. Although the effects of various growth regulators on the in vitro regeneration have been studied, more types of auxins need to be investigated for their influence on somatic embryogenesis and plantlet regeneration of litchi. In this study, the effects of phenylacetic acid (PAA) and its derivatives—4-chlorophenylacetic acid (CPA) and 4-iodophenylacetic acid (IPA)—were assessed in relation to callus proliferation, somatic embryogenesis (SE), and plantlet regeneration in ‘Feizixiao’ litchi. The inclusion of CPA or IPA in callus proliferation medium significantly enhanced the proliferation rate, with 20 mg·L−1 CPA the most effective treatment. Embryonic calli following CPA and IPA treatments exhibited a deeper yellow color, smaller cell clusters, and thicker cytoplasm compared to the control and PAA-treated calli. The highest number of somatic embryos per gram of fresh embryonic callus weight (gFW−1) (1131 embryos·gFW−1) was observed when 40 mg·L−1 PAA was added to the proliferation medium. The addition of 10 mg·L−1 IPA to the proliferation medium yielded the highest plantlet regeneration rate (50 plantlets·gFW−1). The optimal supplementation of the somatic embryo induction medium was 5 mg·L−1 PAA, which resulted in 460 somatic embryos·gFW−1 and 86 regenerated plantlets·gFW−1. These findings indicate that PAA and its derivatives are effective PGRs in the in vitro regeneration of litchi, and provide a valuable protocol for the propagation of elite cultivars.