Chemical constituents of Cinnamomum septentrionale leaf litter and its allelopathic activity on the growth of maize (Zea mays)

Abstract A pot experiment was conducted to study the effect of decomposing Cinnamomum septentrionale leaf litter on the growth of maize. In this study, the morphological traits of maize were significantly inhibited when the leaf litter amount reached or exceeded 40 g per pot; Furthermore, during the early growth stage or with a large amount of litter addition, the pigment contents were inhibited by C. septentrionale leaf litter. Gas chromatography–mass spectrometry was used to determine the volatile substances of leaf litter and 34 compounds were identified, several of which were reported to be phytotoxic. In conclusion, the leaf litter of C. septentrionale showed a strong allelopathic effect on the growth of maize. Thus, it is better to avoid the growing of maize under or near the C. septentrionale plantation unless the leaf litter could be eliminated in time or other effective leaf litter processing methods could be implemented.


Introduction
Agroforestry is a kind of sustainable land management system that can achieve the full and cyclic utilisation of matter and energy (Rizvi et al. 1999). Along with the rapid growth of population and worldwide environmental degradation, this planting pattern has drawn considerable attention for its ecological, social and economic benefits (Paris et al. 2005). Lauraceae, Cinnamomum septentrionale Hand. Mazz, as a member of the Lauraceae family, has been widely introduced for possible planting everywhere in rural areas (around farmland and house in south China) as gardening trees and for agroforestry purposes (Huang et al. 2013); thus, the leaf litter of C. septentrionale is often ploughed into the soil prior to crop planting, then during the decomposing process, it may release plenty of allelochemicals, some of which could be dissolved in soil water, while others may be retained in pores of soil for some time. Among them, most volatile substances are reported to be allelopathic and toxic (Huang et al. 2013), since in related researches, it was found that the volatile oils of Cinnamomum are linalool, 1,8-cineole, camphor and other terpenoids (Lee et al. 2013;Saleem et al. 2015;Sriramavaratharajan et al. 2015), which were reported to be phytotoxic (Santamarina et al. 2015). Furthermore, our related research (Huang et al. 2012) found that the inhibitory effect of C. septentrionale leaf litter on receptor crop Brassica rapa almost disappeared after removing most volatile compounds of leaf litter by distillation, illustrating that the abundant volatile material of C. septentrionale leaf litter was the principal cause of allelopathic inhibition.
Therefore, to elucidate the allelopathic effects of inhibition on some crops which are often planted under or near the trees or plantations of C. Septentrionale, a pot experiment was conducted in this study with the C. Septentrionale leaf litter as allelopathic material and maize as target plant. Furthermore, to identify the potential mechanism of inhibition, chemical constituents of the C. Septentrional leaf litter were analysed using gas chromatography-mass spectroscopy.

Effect of decomposing leaf litter of C. septentrionale on the plant height, basal diameter and biomass of maize seedlings
As shown in Table S1, the height, basal diameter and biomass of maize were dramatically inhibited by the decomposition of C. Septentrionale leaf litter when its amount in soil reached a certain level. As seen in the researches of Liu et al. (2011) andTerzi (2008), the allelochemicals could affect the growth of target plant by disrupting normal cellular physiological activities, substance transport, respiration or cell division.

Effect of decomposing leaf litter of C. septentrionale on pigment content of maize seedlings
In this study, the decline of photosynthetic pigments was evident among all litter treatments, in addition to A1, during the first 40 days of leaf litter decomposition. However, the inhibition effect tended to weaken with the extension of leaf litter decomposition time (at T3). Meanwhile, when the amount of leaf litter reached a certain level, the ratios of Chla/b significantly increased at T2, while the ratios of Chl/Car decreased at T2/T3 (Table S2). Similar results had been found in several studies (Yang et al. 2004;Wu et al. 2012), illustrating that the decomposing leaf litter might suppress the content of photosynthetic pigments, and the target plant could respond positively to this inhibitory effect by altering the proportion of pigments.

Chemical constituents of C. septentrionale leaf litter
Thirty-four volatile compounds were identified in the essential oil extracted from the original C. septentrionale leaf litter, representing 94.84% of the total oil content. The GC/MS analysis revealed that camphor was a major component, with the highest relative content of 30.53%. Other components included geranial, 1,8-cineole, cis-citral and α-pinene with the relative contents of 11.10, 10.85, 8.33 and 4.25%, respectively (Table S3).

Conclusions
In this study, the leaf litter of C.septentrionale showed a strong allelopathic effect on the growth of maize. Similar results were found in Huang et al. (2012) and Yang et al. (2015). Furthermore, since Abrahim et al. (2000) had reported that the volatile oils of camphor, limonene and β-pinene, which were identified in the essential oil extracted from C. septentrionale leaf litter had inhibitory effects on the growth of the maize, we could speculate that it is better not to plant maize under or near the C. septentrionale plantation unless the leaf litter could be eliminated in time or other effective leaf litter processing methods could be implemented.

Supplementary material
Supplementary material relating to this article is available online, alongside Tables S1-S3 and Figures S1 and S2.

Disclosure statement
No potential conflict of interest was reported by the authors.