Moderate salinity facilitates the tillering in S. alterniflora
As one of the most common abiotic stresses, soil salinization has been proved to cause an inhibition on plant tillering or branching of glycophyte even subjected to low salinity, such as rice (Kibria et al. , 2017), wheat (Ruan et al. , 2008) and lettuces (Aroca et al. , 2013). However, a proper soil salinity is required for halophyte growth. In the present study, we found that both of the S. alternifloraseedlings grown in the field and greenhouse exhibited more tillering under the moderate salinity condition (13-18‰ or 15‰) than in low or high salinity conditions (Figure 2, Figure 3, Figure 4). Similar with our results, study conducted by Wang et al . (2006) showed thatS. alterniflora produced more tillers under 15‰ salt treatment. Besides, Xiao et al . (2011) also found that the tiller number of S. alterniflora were much higher in medium salinity. Importantly, the common garden experiment performed by Zhanget al . (2012) found that the shoot density of S. alterniflora , which in some extent represented the tiller number ofS. alterniflora , was much higher in mesohaline site (13‰-18‰ salinity) and polyhaline site (21‰-26‰ salinity) than that in oligohaline site (0‰-6‰ salinity) and euhaline site (29‰-32‰ salinity). Ruan et al (2012) reported that the removal of tiller profoundly enhanced the salt tolerance in salt-sensitive wheat. This can be explained by the reduction of tiller number in high salinity condition. In addition, the plant height of S. alterniflora was gradually decreased with the increase of salt treatments (Figure 2b; 4c), which agreed with other studies (Wang et al. , 2006; Xiao et al. , 2011; Zhang et al. , 2012). Plant height is an important character for crops to determinate the yield. In rice, an opposite additive effect on plant height and tiller number was dissected by using the quantitative trait loci method (Yang et al ., 2006). This negative relationship between plant height and tiller number may due to the trade-off of limited nutrient supply between tiller generation and stem elongation (Cui et al ., 2004). In the present study, less tiller but much taller of S. alterniflora seedlings under control salinity condition may be due to the trade-off between tiller outgrowth and plant growth. According to the above results and analyses, it can be concluded that a proper moderate salinity was favorable for tillering development in S. alterniflora .
Moreover, the asexual propagation by vegetative tillering in S. alterniflora was regarded as one of the reasons for its fast spread in salt marsh area (Proffitt et al ., 2003; Taylor et al ., 2004). Takahashi et al . (2019) reported that the lateral growth (tiller production) of S. alterniflora colonies, rather than seedling recruitment, was an important factor to control the expansion of S. alterniflora . Ning et al . (2020) found that the suitable salinity in soil with a higher propagule pressure conditions provided a firm foundation for S. alterniflora seedling establishment, which was the potential implications for controllingS. alterniflora invasions. Both of the results from previous studies and our observation showed that S. alterniflora produced more tillers in moderate salinity condition, suggesting the higher invasive of S. alterniflora under such condition. These may partly explain why S. alterniflora were more invasive in the mesohaline area along the coastal intertidal wetland in China.