Changes in leaf functional traits of the endangered plant Disanthus cercidifolius var. longipes with plant growth and development

Background: Disanthus cercidifolius var. longipes H. T. Chang is a rare and endangered plant distributed only in the high mountains of southeastern China. In order to reveal the variation in leaf functional traits and plant investment strategies with the change of growth and developmental stages of this species, the leaf functional traits and the trait syndrome including leaf thickness (LT), leaf area (LA), leaf water content (LWC), specific leaf area (SLA), leaf total nitrogen content (LNC), and leaf total phosphorus content (LPC) of plants at different growth and developmental stages were investigated. Results: The leaf functional traits of the plants significantly differed at different developmental stages. LT and LA of the plants increased during growth and development. LT and LA of the adult plants were 36.65% and 84.23% higher than those of the seedlings, respectively. In contrast, SLA, LWC, LNC, and LPC decreased, and in adult plants they were 48.91%, 6.63%, 8.49%, and 34.66% lower, respectively, than in seedlings. Principal component analysis showed that as the plants developed, the trait syndrome changed toward increasing LT and LA and decreasing LWC, SLA, LNC, and LPC. Conclusions: The characteristics of leaf functional traits and trait syndromes changed across different stages of growth and development. The investment strategy changed from fast return to slow return as the plant grew and developed.

relationship between plant functional traits and plant growth and development may effectively reveal the mechanism of the survival of species (Liu et al., 2015). The leaf is an important organ for photosynthesis, and synthesized organic compounds are the basis for growth and development of plants. The leaf is the most sensitive organ perceiving the change in external resource conditions, especially the changes in light (He et al., 2013).
The variation in functional traits reflects, to some extent, the variation in the ability of the plant to acquire light (Zhang et al., 2008). Leaf functional traits mainly include morphological traits, structural traits, and physiological traits. Morphological and structural traits are closely related to physiological traits such as the photosynthetic rate and hydraulic conductivity, which reflect the functional relationship between plants and environmental factors such as light, water, and soil fertility (Sun et al., 2017). At present, the study of leaf functional traits mainly focuses on the relationship among functional traits of adult plants (Yang et al., 2012) or the variation of leaf functional traits in different species and along different gradients of environments (Ding et al., 2011). Those studies revealed the diversity of species and the mechanisms of species responding to environmental changes. Nevertheless, few studies have explored the relationship between plant leaf traits and plant growth and development by investigating the dynamic change in leaf functional traits at different developmental stages (Hu et al., 2014). The endangerment of a species is caused by the weakened or even blocked processes of growth and development due to some environmental stresses (Chen et al., 2005).
Studying the variation of leaf functional traits in endangered species at different growth stages is helpful for revealing the variation regularity of the trait syndrome during plant growth and development (Gen et al., 2019), which may provide a reference for the protection of endangered species.
Disanthus cercidifolius var. longipes H. T. Chang is a perennial deciduous shrub of the genus Disanthus in the family Hamamelidaceae. The plants are 2-6 m tall, grow in broadleaved evergreen forests and coniferous and broad-leaved mixed forests in areas with an altitude of 600-1100 m, and are tolerant to wet and shade condition. The leaves emerge in early April and fall in late October (Wei, 2015). The species has a narrow distribution region and is found only in some areas of Jiangxi, Zhejiang, and Hunan provinces in China.
It is a second-class protection tree species in China . At present, research on the ecology of D. cercidifolius var. longipes mainly involves flora characteristics and the genetic diversity of the populations (Wei, 2015;Gao et al., 2013;Liao 2010). Thus far, there is no report on functional trait variation with development.
Based on a field investigation, we selected four D. cercidifolius var. longipes communities in Jiangxi province and measured the leaf functional traits at different developmental stages to reveal the variation regularity of the leaf functional traits and trait syndrome during growth and development. The regulatory effect of the development of individuals on the intraspecific economic spectrum of this species was studied, and the status of growth and development of each community was analyzed. Our results may provide a reference for the management of this endangered species.

Characteristics of leaf functional traits at different developmental stages
One-way ANOVA showed that the developmental stage had significant effects on the functional traits of the leaves (LT: F = 88.44, P < 0.01; LA: F = 36.42, P < 0.01; SLA: F = 176.19, P < 0.01; LWC: F = 6.62, P < 0.05; LNC: F = 9.68, P < 0.01; LPC: F = 18.65, P < 0.01) ( Fig. 1). The order of LT and LA at different developmental stages was seedling < sapling < adult plant, which indicated that LT and LA increased with the increase in the developmental stage, and the LT and LA of adult plants were 36 Table 2 Pearson correlation coefficients between leaf functional traits and developmental stages (n = 155) Moreover, all of the leaf functional traits were significantly correlated with the two principal components (P < 0.05). The PCA ordination of leaf functional traits of the plants at different developmental stages was computed (Fig. 2).

Determination Of Leaf Functional Traits
The leaf thickness (LT), leaf area (LA), leaf water content (LWC), specific leaf area (SLA), leaf total nitrogen content (LNC) and leaf total phosphorus content (LPC) were determined.
The collected leaves were placed in water and kept in the dark at 5 °C for 12 h. Then, the water on the leaf surface was quickly removed with filter paper, and an electric balance with an accuracy of 0.01 g was used to weigh the saturated fresh weight of leaves. An electric vernier caliper with an accuracy of 0.01 mm was used to measure LT. Three spots on a leaf were selected for measuring, which were evenly spaced along the main vein, and each was about 0.25 cm from the main vein; the average value measured from the 3 spots