Abstract:

Cuticular waxes are hydrophobic compounds covering plant surfaces and play vital roles in protecting plants from various biotic and abiotic stresses. However, less is known as related to the cuticular wax on leaves or stems of summer legume green manures. In the current study, stems and leaves from three summer legume green manure crops grown in the fields were sampled, including Crotalaria juncea, Sesbania cannabina, and Phaseolus calcaratus Roxbwere. Using gas chromatography-mass spectrometer (GC-MS), we identified most of the chemical compounds on these plant species. In total, eight classes of wax compounds were identified, including fatty acids, primary alcohols, aldehydes, alkanes, alkyl esters, diols, terpenoids and sterols. Among these compounds, fatty acids, primary alcohols, aldehydes and alkanes with serial homologs could be observed in stems and leaves of all tested plant species excepting fatty acids in leaves of C. juncea, with their weight proportions accounting for more than 60% in total wax. This suggested that alkane forming and alcohol forming pathways were two major biosynthesis pathways in these plants. In stems of S. cannabina, two compounds were identified as diols with the functional hydroxyl group located at 1,18 and 1,16 positions. Using gas chromatography with flam ionized detector (GC/FID), we quantified the total wax and wax compounds. Total wax coverage differed among three plant species and between two organs. The total stem wax coverage was the highest in C. juncea (16.33 μg cm ^-2), followed by S. cannabina (6.45 μg cm ^-2) and P. calcaratus (0.72 μg cm ^-2). The total wax coverage on stems of C. juncea was significantly higher than that on leaves, whereas no significant difference in total wax coverage between stems and leaves was observed in S. cannabina and P. calcaratus. The predominant wax class differed in plant species and organs. For C. juncea, alkanes were the predominant composition in stems accounting for 57.38% of total wax, whereas primary alcohols were the predominant composition in leaves accounting for 50.12%. For S. cannabina, both stems and leaves were dominated by primary alcohols, accounting for 30.12% and 71.21% of total wax, respectively. For P. calcaratus, both stems and leaves were dominated by alkanes, accounting for 40.79% and 39.27% of total wax, respectively. We further analyzed the chain length distributions of the wax classes in stems and leaves. Generally, fatty acids, primary alcohols and aldehydes were consisted of serials of even carbon number homologs, whereas alkanes were consisted of both even- and odd-carbon number homologs with odd number predominance over even number. The predominant compound in each wax class also differed between plant species and organs. The dominant fatty acids on stems and leaves in C. juncea, S. cannabina, and P. calcaratus were C₃₀, C₃₀ and C₂₆, and C₂₈, respectively; the dominant primary alcohols were C₂₈, C₃₀, and C₂₈ and C₃₂ respectively; the dominant aldehydes were C₃₂ and C₃₀, C₃₀, and C₃₀ and C₂₈, respectively; while the dominant alkanes were C₃₁, C₂₉, and C₃₁, respectively. The terpenoids identified in current study were β-amyrin, α-amyrin and lupenol. The variations of predominant wax class and wax compound between plant species and organs implied that genes involved in wax biosynthesis might also be different. These results provide basic knowledges in studying the molecular mechanisms of cuticular waxes in legume green manure crops fronting biotic and abiotic stresses.

Key words: Cuticular wax, green manures, Crotalaria juncea, Phaseolus calcaratus, Sesbania cannabina