Shade is widespread in agricultural production and affects lignin biosynthesis and lodging resistance of crops. We explored the effects of shade intensity on lignin biosynthesis and lodging resistance at the physiological and molecular levels in two soybean cultivars (Nandou12 and E93) with different shade tolerance under four progressively severe shade treatments, S0–S3 (S0: no shade, S1: slight shade, S2: moderate shade, S3: heavy shade). Our results showed no significant difference in breaking strength of the two cultivars under S1 and S0 treatments, with no prominent decrease in the lodging resistance index. The activity of lignin biosynthesis rate-limiting enzymes phenylalanine ammonia-lyase (PAL), peroxidase and cinnamyl alcohol dehydrogenase (CAD), which were considerably related to the two lodging resistance indexes above, was not significantly decreased by slight shade, while 4-coumaric acid ligase (4CL) activity was increased. Most genes involved in lignin biosynthesis were not significantly down-regulated by slight shade (S1) compared to S0, while p-coumarate 3-hydroxylase (C3H
), 4-coumaric acid ligase (4CL
) and laccase (LAC
) genes were upregulated. Under heavy shade (S3), enzyme activity and gene expression associated with lignin synthesis in both soybean cultivars were strongly inhibited; moreover, stem mechanical strength and lodging resistance were remarkably decreased compared with those under S0. These physiological and molecular changes suggested that applicable shade levels do not significantly affect the mechanical strength and lodging resistance of soybean stem. Exploiting the lodging resistance potential of existing soybean cultivars was an effective and efficient way to address yield reduction caused by lodging in intercropped soybeans.
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