Nursery-plant grading relies on morphological traits, but leaf metabolic variation associated with nursery-plant grade remains unclear. In this study, untargeted HPLC-QTOF-MS metabolomics was applied to mature and young leaves of Grade I, II, and III apple nursery plants to explore grade-associated metabolic variation and its relationship with growth performance. After quality filtering, 198 positive-ion features were retained from mature leaves, while 259 positive-ion and 8 negative-ion features were retained from young leaves. Multivariate analysis showed clear grade-associated separation in both leaf types, with stronger discrimination in young leaves. A combination of orthogonal partial least squares (OPLS) modeling and trend analysis identified 20 and 28 differential features in mature and young leaves, respectively. Cross-model prioritization further highlighted key metabolites putatively annotated as methyl nicotinate and 1-palmitoyl-sn-glycero-3-phosphocholine in mature leaves, and methyl nicotinate, nicotinamide riboside, cis-jasmone, and methyl (9Z,14Z)-12,13,16-trihydroxyoctadeca-9,14-dienoate in young leaves. These key metabolites were potentially associated with NAD precursor metabolism, membrane lipid remodeling, and oxylipin-related signaling. Correlation analysis showed that they were associated with both initial grading traits and post-transplant growth performance. These metabolites represent candidate molecular correlates of grade-associated physiological variation and growth performance in apple nursery plants.

Processing quality in chestnut (Castanea spp.) is a complex trait jointly determined by fruit development, postharvest metabolic changes, and responses to processing. However, its genetic basis and regulatory networks remain poorly understood. This review provides an integrated framework linking product-specific processing requirements with their biochemical basis, candidate genes, and molecular breeding strategies. Starch composition and fine structure primarily determine cooked texture, storage hardening, and digestibility; starch degradation and sugar metabolism affect sweetness and thermally induced flavor formation; and phenolic substrates, together with oxidative enzymes, determine browning potential and color stability. We review the biochemical basis underlying these traits and summarize candidate genes and regulatory pathways involved in starch synthesis and structural modification, starch-to-sugar conversion, enzymatic browning, flavor formation, and the accumulation of nutritional and bioactive compounds. Nevertheless, stable quantitative trait loci, favorable haplotypes, and causal genes associated with chestnut processing quality remain insufficiently validated. Future research should develop product-oriented, standardized phenotyping systems and integrate multi-environment genetic analyses, multi-omics network dissection, marker-assisted selection, genomic selection, and gene editing to elucidate the genetic mechanisms underlying chestnut processing quality and enable the precision breeding of processing-specific cultivars.