Allelopathic Effects of Sphaeranthus suaveolens on Seed Germination and Seedling Growth of Phaseolus vulgaris and Oryza sativa

Weeds with allelopathic eﬀect have been reported to cause signiﬁcant damage in agriculture particularly in smallholder farming systems. This study assessed the allelopathic eﬀects of diﬀerent concentrations of crude extract of a noxious weed Sphaeranthus suaveolens on seed germination and seedling growth of Phaseolus vulgaris and Oryza sativa by examining germination, seedling height, and total chlorophyll content after seven and fourteen days of crude extract treatment, respectively. Results showed that seed germination and seedling growth were signiﬁcantly ( p < 0 . 001 ) decreased with increase in concentration of crude extract, signifying concentration dependency. Highest concentration (100%) of S. suaveolens crude extracts resulted in 90% and 100% inhibition of P. vulgaris and O. sativa seed germination, respectively. Chlorophyll content, fresh weight, and root and shoot length of both P. vulgaris and O. sativa were also signiﬁcantly ( p < 0 . 001) aﬀected by highest concentration (100%) of S. suaveolens crude extracts. Results from this study suggest that the extract of S. suaveolens contains water-soluble allelochemicals which signiﬁcantly reduce growth and productivity of P. vulgaris and O. sativa .


Introduction
Weed invasion is becoming a major challenge in agricultural sector worldwide particularly in smallholder farming systems [1,2]. Weeds have been reported to significantly affect crop production by competing for light, water, nutrients, and space thereby threatening the economic growth and food security of smallholder farmers [3][4][5][6]. Additionally, these unwanted plants have been observed to host insect pests and diseases [7][8][9] as well as disrupting and interfering with natural interactions by displacing native species, distracting pollinators, and other insects that are beneficial in the smallholder farming systems [10][11][12].
Most weeds have been alleged to possess allelopathic effects which play an important role in their invasion success [13][14][15]. Allelopathy is a phenomenon, whereby one plant influences the growth of biological systems, including microorganisms, by the release of chemical compounds into the environment [16][17][18]. e allelopathic effects are the result of plant's secondary metabolites known as allelochemicals, which are usually byproducts of the principal metabolic pathways in plants [19][20][21]. ese allelochemicals can be found in the leaves, stem, flowers, fruits, and roots [22].
Plants with allelopathic properties have been observed to significantly affect the growth and development of other neighboring plants by inhibiting seed germination, causing soil infertility and nutrient imbalance as well as limiting the microbial population in the soil [23][24][25]. Due to these effects, allelopathy has become a research hotspot for making comprehensive analysis about the mechanism of weed invasions and possibilities of utilizing these naturally occurring phytochemicals in managing weeds and insect pests in agricultural ecosystems [26,27].
Common bean (Phaseolus vulgaris) and rice (Oryza sativa) are among important food and income generating crops globally [28]. e two crops have been reported as the principal source of protein and main calorie supply to a significant portion of the households in Africa and globally at large [29][30][31]. Despite the importance of the two crops in the agricultural sector and livelihood of most smallholder farmers, yields are generally low with the average revolving around 990 kg/ha for P. vulgaris and 2400 kg/ha for O. sativa [32]. e potential yields under favorable conditions are estimated to be around 1500-3000 kg/ha for P. vulgaris and 2500-4000 kg/ha for O. sativa [33]. Among the reasons behind this low yields are heavy infestation from weeds, insect pest attacks, and poor crop management skills such as late weeding [7,34,35].
Sphaeranthus suaveolens is a spreading weed from the family Asteraceae, commonly found in swampy and cultivated farmlands [36,37]. A heavy infestation of this weed results in adverse effects on the growth of adjacent plants [38]. It has also been observed that S. suaveolens has an ability to suppress crops in a wide range over a short period of time [39]. Additionally, significant portion of common bean and rice smallholder farmers reported major yield loss due to S. suaveolens infestations [7]. Despite of these tragic losses, the allelopathic effects of S. suaveolens to P. vulgaris and O. sativa have not yet been studied or reported. Understanding these effects could considerably improve the S. suaveolens management in farmlands and significantly reduce its effects on crop productivity. e present study was carried out to evaluate the allelopathic effects of S. suaveolens using different aqueous extract concentrations on germination and seedling growth of P. vulgaris and O. sativa crops under laboratory and screen house conditions.

Seed Preparation and Treatment. Seeds of P. vulgaris and
O. sativa were collected from Selian Agricultural Research Institute (SARI) in Arusha, Tanzania, in June of 2019. Before the experiment, the seeds were air dried and stored in plastic bags. Seed viability of both plants was determined by the germination test [40], in which all the 20 seeds (100%) for each crop (10 P. vulgaris and 10 O. sativa seeds) that were selected randomly from a seed stock and planted in a Petri dish lined with cotton wool in early September 2019 germinated. Seeds were later washed using tap water and sterilized with 5 % NaOCl for 2 min and then rinsed with distilled water before planting.

Crude Extract Preparation.
Freshly matured plants of S. suaveolens were collected from Arumeru and Moshi rural districts, Tanzania, between June and July 2019. e plants were shade dried under room temperature for 14 days, grounded into powder using a grinder, and stored in plastic containers before the experiments. Extracts were prepared according to Ngondya et al. [41] with few modifications as follows: 100 g of S. suaveolens powder was soaked separately in 1 liter of distilled water and left for 72 h. Afterwards, crude extracts were filtered using Whatman filter paper no. 1 to obtain a final volume of 1 liter each. Both crude extracts (ml) were diluted with distilled water (ml) in the ratio of 25 : 75, 50 : 50, 75 : 25, and 100 : 0 (extract : distilled water) to obtain different concentrations of 25%, 50%, 75%, and 100%. e diluted extracts were kept in the refrigerator at 4°C.

Laboratory Experiment.
e effects of S. suaveolens crude extracts on the seed germination, seedling height, and leaf chlorophyll content of P. vulgaris and O. sativa were studied using a completely randomized design (CRD) from October to November 2019. Ten seeds of each crop (P. vulgaris and O. sativa) were placed in each of the five Petri dishes (each with the surface area of 70.8 cm 2 ) lined with cotton wool. Each Petri dish was moistened once a day with 10 ml of different concentration treatments, i.e., 0% (for control) and 25%, 50%, 75%, and 100% (for S. suaveolens crude extracts). Each treatment was replicated three times. Seeds were observed every day under the 12 h dark and 12 h light conditions. Number of germinated seeds was recorded and counted for 7 days for P. vulgaris and 14 days for O. sativa. Seedlings were harvested and fresh weight, seedling height, and leaf total chlorophyll content were determined for each germinated seedling. e entire experiment was repeated three times.

Screen House Experiment.
e effects of crude extracts of S. suaveolens on the seed germination, seedling height, leaf total chlorophyll content, and fresh and dry weight of P. vulgaris and O. sativa crops were studied using a completely randomized design in a screen house from October to November 2019. Six seeds for each crop (P. vulgaris and O. sativa) were placed each in five pots with the surface area of 763.8 cm 2 . e pots were then moistened on daily basis with 100 ml of different concentration treatments (25%, 50%, 75%, and 100%) of S. suaveolens crude extracts and distilled water for the control. Each treatment was replicated three times. Seeds were observed every day and the number of germinated seeds were recorded and counted for 7 days for P. vulgaris and 14 days for O. sativa. ereafter, seedlings were harvested and fresh weight, seedling height, and leaf total chlorophyll content were determined for each germinated seedling. Similar to the laboratory experiment, this experiment was also repeated three times.

Chlorophyll Content
where A 663 and A 645 are absorbance readings at 663 and 645 nm, respectively.

Data Analysis.
Data on allelopathic effects of S. suaveolens on seed germination and seedling growth (shoot length, root length, fresh weight of shoot, and fresh weight of root and chlorophyll content) of P. vulgaris and O. sativa were compared using one-way ANOVA. e normality and homogeneity of variance were verified using Shapiro-Wilk test and Levene's test, respectively. Fishers LSD test was used to compare the significance differences between the group means. e statistical software used for all tests was Origin (version 2018b) at a significance level of 5%.

Seed Germination.
Generally, higher concentrations (75% and 100%) of S. suaveolens in both laboratory and screen house experiments were effective in suppressing both P. vulgaris and O. sativa seeds germination. e germination of P. vulgaris and O. sativa seeds was delayed at higher concentrations (75% and 100%) compared with the negative control (0%) and lower concentrations (25%) of the S. suaveolens crude extract. e mean percentage germination under 0% concentration (negative control) was 100% for P. vulgaris and 90% for O. sativa in the laboratory, as compared with 100% for both P. vulgaris and O. sativa in the screen house experiment. Additionally, under higher concentration (100%), the mean percentage germination for P. vulgaris was 10% and 0% in laboratory and screen house experiments, respectively, while for O. sativa, it was 0% in both experiments (Table 1). In general, the seed germination for both P. vulgaris and O. sativa decreased significantly (p < 0.001) with the increase in the concentration of S. suaveolens crude extract (Table 1).

Discussion
is study revealed that the crude extract of S. suaveolens significantly reduced seed germination of P. vulgaris and O. sativa.
is suggests that S. suaveolens possess watersoluble allelochemicals which showed inhibitory effects on the two tested crops. Moreover, at higher concentrations (75% and 100%), the S. suaveolens extracts showed maximum inhibition in the germination of P. vulgaris and O. sativa.
ese results are in agreement with the study conducted by [42,43] on the allelopathic effects of various weeds on seed germination of rice and beans where germination was reduced to 20% and 6%, respectively. e Advances in Agriculture reduced seed germination in P. vulgaris and O. sativa might be caused by the allelopathic stress of different extract concentrations resulting from different abnormalities in metabolic activities and cell division due to the effect of allelochemicals [44]. is is reported to affect the productivity of P. vulgaris and O. sativa in different farming systems, thereby lowering yields. e findings in this study also indicate that root and shoot lengths of P. vulgaris and O. sativa were significantly reduced by the S. suaveolens crude extracts. However, the      effects were concentration dependent and differed between the two tested crops (P. vulgaris and O. sativa). e roots and shoots of O. sativa were found to be more sensitive to the applied allelopathic stress than P. vulgaris, whereby at high concentration (100%) of crude S. suaveolens extract, root and shoot length was reduced considerably as compared with those of P. vulgaris at lower concentrations and in control treatments.
ese results corroborate with findings from Lodha [42] who revealed that extracts of different plant parts of S. indicus weed had strong inhibitory effects and reduced seed germination by 80% and root length and stem length by 94.4% and 83.3%, respectively, of O. sativa. Root and shoot lengths are very important parameters which determine plants' growth and health due to their importance in nutrients uptakes and physical support of the plant. e reduced root and shoot lengths observed in this study may in one way or the other negatively affect crop production particularly in smallholder farming systems. e association between shorter roots and failure of plants to compete and search for water and minerals from the ground has been well reported by Sofi et al. [45], Subudhi et al. [46], and Yamane et al. [48]. On the other hand, shorter shoots have been associated with plants' inability to withstand environmental stresses such as drought [48]. Also, shorter shoots hinder plants' ability to compete for space, light, and air which are important parameters during photosynthesis and their shortage may result into poor plant growth [49]. Additionally, Laizer et al. [7] and Lodha [42] reported lower P. vulgaris and O. sativa yields, respectively, in farms that were invaded with S. suaveolens. e low yields may have been attributed to the allelopathic effects of S. suaveolens which negatively affect root and shoot lengths.
Furthermore, results from this study show that, fresh weights of shoots and roots for P. vulgaris and O. sativa were significantly affected by the higher concentrations of S. suaveolens in both laboratory and screen house experiments. e seedling fresh weight is an important factor for a plant to withstand physical stresses from the environment [49]. erefore, affecting the fresh weight of the P. vulgaris and O. sativa may affect their ability to withstand harsh    [52], and Siyar et al. [44]. Reduced chlorophyll content may negatively affect plant's ability to perform photosynthetic functions, hence lowers the chance to survive or compete with other neighboring plants [53].

Conclusion
e findings from this study are among the first to demonstrate effects of S. suaveolens crude extracts to seed germination and growth of P. vulgaris and O. sativa. e results further show that O. sativa is more sensitive to the applied allelopathic stress than P. vulgaris. is might be due to its genomic characteristics which influence tolerance levels to chemical and other environmental stresses. At higher concentrations (75% and 100%), the S. suaveolens crude extract exerted deleterious effect on seed germination and seedling growth for both P. vulgaris and O. sativa compared with lower concentrations (25%) and the control (0%). ese effects might be caused by the presence of the water-soluble allelochemicals in S. suaveolens crude extracts which are largely unknown and need to be isolated, identified, and characterized for profound understanding and further investigations on their applications in agriculture and other fields.
Data Availability e germination and seedling growth data used to support the findings of this study are available from the corresponding author upon request.