Effects of planting density on the growth and photosynthetic characteristics of Alternanthera philoxeroides under different nutrient conditions

Density and nutrient level are important factors that might affect the growth of invasive plants. To reveal the effects of plant density on the performance of invasive plant Alternanthera philoxeroides under different nutrient conditions, a greenhouse experiment was conducted in which A. philoxeroides was planted at three densities (low, medium and high) under three nutrient levels (low, medium and high). The results showed that both planting density and nutrient levels had significant effects on the growth of the plant. The biomass of individual plant and all plants in one pot under medium nutrient level were the highest while the photosynthetic rate and total chlorophyll content were the highest at the high nutrient level. Under different nutrient levels, the photosynthetic rate was the highest at medium planting density. The biomass of single plant decreased with the increase of population density, while the total biomass in the whole pot increased with the increase of density. These characteristics might contribute to the invasion of A. philoxeroides and help the plant to form monodominant community.


57
The competition among plants is also an important factor affecting the spatial distribution, Determination of morphological indexes of A. philoxeroides 155 We measured the length of stolon and recorded the number of internode before harvest.

156
The internode length of A. philoxeroides under different treatments was calculated. Statistical analysis 165 The date of different variables, such as biomass, photosynthetic rate, SLA and total 166 chlorophyll content was analyzed by two-way ANOVA with SPSS 21.0 software (  (1) 184 and medium density (2) was slightly greater than the difference between medium density (2) and 185 high density (4) ( Table 3). At every nutrient levels, there was a significant difference in the 186 biomass of single plant among all three planting densities (Table 3). In low nutrient level (A), the 187 difference between medium planting density (2) and high planting density (4) is slightly larger 188 than that between low planting density (1) and medium planting density (2) ( Table 3). Under 189 medium nutrient (B) and high nutrient (C), the gap between low density (1) and medium planting 190 density (2) is slightly larger than medium planting density (2) and high planting density (4) ( Table   191 3).

192
At the three nutrient levels, the total leaf biomass and the total biomass of a pot all 193 increased significantly with the increase of planting density (Table 3). To total leaf biomass of A.  (Table 3). For total biomass of a pot, at low nutrient level (A), the difference between 198 medium planting density (2) and high planting density (3) is significantly smaller than that 199 between low planting density (1) and medium planting density (2), besides there is no significant 200 difference between low planting density (1) and medium planting density (2) at medium (B) and 201 high (C) nutrient levels (Table 3).

202
According to the two-way ANOVA analysis ( According to the two-way ANOVA analysis (Table 2), there is no interaction between effects of 216 nutrient level and plant density on SLA of A. philoxeroides.

217
The SLA of A. philoxeroides increased significantly with the increase of nutrient level (Fig 1).

218
Different planting density had certain effects on the SLA of A. philoxeroides, which was the 219 highest under medium planting density (2) and the lowest under high planting density (4), and 220 both have significant differences (Fig 1).

225
Effects of density on photosynthetic rate and total chlorophyll 226 content of A. philoxeroides under different nutrient levels. 227 We found that nutrient levels and planting densities on the photosynthetic rate of A.

228
philoxeroides has a obvious interaction, and for the total chlorophyll content, they had no 229 significant interaction to it (Table 2).

230
The analysis of photosynthetic rate and total chlorophyll content of A. philoxeroides under 231 different treatments showed that the change trend of the two indexes are basically the same, 232 and both of them increased with the increasing nutrient level (Fig 2). The photosynthetic rate of 233 medium planting density (2) of the three nutrient levels were the highest and had significant 234 differences, and they were the lowest under the treatment of high density (3) (Fig 2-A). The total 235 chlorophyll content of the medium nutrient (B) and high nutrient (C) had obvious differences (Fig   236  2-B). The total chlorophyll content of A. philoxeroides reached the maximum under high nutrient 14 237 level (C), and for the planting density, when the planting density was medium (2), the total 238 chlorophyll content of A. philoxeroides reached the maximum (Fig 2-B).  (Table 4).

257
The light compensation point (LCP) and light saturation point (LSP) of A. philoxeroides under 258 different nutrient treatments were the biggest when planting density was medium (2), and when 259 the planting density was high level (4), the value of LCP and LSP were the smallest (Table 4). At 260 the same planting density, both LCP and LSP increased with increasing nutrient levels, among 261 them, there was a larger gap between medium nutrient (B) and high nutrient (C) ( Table 4). Under 15 262 the same nutrient level, at the difference of the value of LCP and LSP between medium planting 263 density (2) and high planting density (4) was larger than that between medium planting density 264 (2) and low planting density (1) ( Table 4). 265