Changes in Leaf Functional Traits of Osmanthus Fragrans Leaves Parasitized by Cuscuta japonica Choisy
In this study, we selected six plant functional traits which are sensitive to environmental changes and external stress, including chlorophyll content, leaf area, leaf thickness, specific leaf area, leaf dry matter content and leaf tissue density. As shown in Fig. 1, there are significant differences in leaf functional traits of Osmanthus fragrans between healthy leaves and the leaves being parasitic by Cuscuta japonica Choisy. The chlorophyll content index, leaf area and specific leaf area of Osmanthus fragrans were significantly lower than those after parasitism, and with the increase of parasitism intensity, these indexes gradually decreased (CK > T1 > T2 > T3). On the contrary, the leaf thickness, dry matter content and leaf tissue density of Osmanthus fragrans were significantly higher than those after parasitism, and these indexes gradually increased with the increase of parasitism intensity (CK < T1 < T2 < T3). Studies show that chlorophyll is one of the important pigments in plant photosynthesis. As the increase of parasitic intensity, chlorophyll content index decreases gradually. The reasons may be that the poor growth of Cuscuta japonica Choisy due to its foraging of host nutrients, water, and nutrients after parasitization [37, 38, 39]. In addition, the host plant lacks sufficient light due to the overgrowth of the parasitic plant Cuscuta japonica Choisy, which results in large area shading [40, 41].
Spectral characteristics of Osmanthus fragrans before and after parasitizing by Cuscuta japonica Choisy
As shown in Fig. 2, before and after Cuscuta japonica Choisy parasitism, the trend changes of spectral reflectance curves of Osmanthus fragrans leaves generally tend to be consistent. In the visible to the near-infrared band (350 ~ 1800 nm), the spectral reflectance is obviously different, which generally showing before parasitism (average value 0.0676 ~ 1.2633) and after parasitism (average value 0.0430 ~ 1.0061), but after parasitism, the spectral reflectance of Osmanthus fragrans is slightly higher than that of healthy Osmanthus fragrans in the band 350 ~ 750 nm. In addition, in the range of 350 ~ 1800 nm, there are four main reflection peaks and five main absorption valleys in the spectral reflection curve of Osmanthus fragrans leaves to all treatments, and their positions are basically the same. The reflection peaks are respectively located at 560 nm, 1150 nm, 1300 nm and 1650 nm, and the absorption valleys are respectively located in the ranges of 350–560 nm, 600–700 nm, 950–1050 nm, 1150 ~ 1250 nm, and 1400 ~ 1500 nm. According to Table 1, chlorophyll content index of Osmanthus fragrans gradually decreased with the deepening of parasitism. Previous studies have shown that chlorophyll content can better characterize the light reflection curve of plant leaves [31, 42]. Therefore, the spectral reflectance curve of the parasitized Osmanthus fragrans leaves is higher than that of the non-parasitized healthy Osmanthus fragrans leaves, which may be related to the decrease of chlorophyll content.
Figure 2 Spectral characteristics of the healthy Osmanthus fragrans and the Osmanthus fragrans be parasitized with Cuscuta japonica Choisy.
Spectral characteristics of Osmanthus Fragrans leaves under the different parasitic intensity of Cuscuta japonica Choisy
As can be seen from Fig. 3, under different parasitic intensities of Cuscuta japonica Choisy, the leaf surface spectral reflectance curves of Osmanthus fragrans are basically the same, but the spectral reflectance values are significantly different. Spectral reflectance values generally decreasing with the deepening of parasitic intensity, and the reflectance values are CK > T1 > T2 > T3. In the visible light to near-infrared 350 ~ 1800 nm band, the spectral reflectance of Osmanthus fragrans leaves under different parasitic intensities is the most easily distinguished in the range of 750 ~ 1400 nm, which indicated that this band is the sensitive range of host plants' spectral response to parasitic infection, and at the same time, this change characteristic is common under different parasitic conditions. In addition, we can also see that the spectral reflectance curve slope of Osmanthus fragrans leaves has a sharp increasing trend in the range of 700 ~ 780 nm. Studies have shown that the phenomenon of plants increasing suddenly in this waveband belongs to the typical "red edge effect" characteristic of plants. At the same time, the spectral reflectance of the leaves of the host plant (Osmanthus fragrans) with different relative chlorophyll contents has a higher reflection platform in the range of 750 ~ 1400 nm, which is wavy and may be affected by the cell structure of the leaves [43, 44]. Among them, the sample with the lowest reflection coefficient are the sample with the lowest chlorophyll content index (the highest parasitic intensity). The reflectance of the sample with the highest chlorophyll content (without parasitism) is the highest at 1150 nm, which is 0.998. There is a significant valley at 1350 ~ 1800 nm, which may be closely related to light absorption by water [45, 46, 47, 48].
Dynamic changes of spectral characteristic parameters of Osmanthus fragrans in different parasitic stages
From Fig. 4 and Fig. 5, it can be seen that the general trend of the first derivative spectrum of the leaf surface of the host plant under different parasitic intensities from visible light to the near-infrared band (350 ~ 1800 nm) is basically the same, but there are some differences in values. After Osmanthus fragrans was parasitized by Cuscuta japonica Choisy, there was an obvious "blue shift" in the red edge of its leaf surface spectral curve. With the deepening of parasitic intensity, the degree of "blue shift" also increased, indicating that with the increase of parasitic intensity, the influence on the red edge of the leaf surface became more severe. In addition, the slope of the red edge of the host plant decreased obviously after parasitization (CK > T1 > T2 > T3). Many studies show that the red edge slope has a good indication of chlorophyll content [49]. Combined with Table 1, it can be seen that with the deepening of parasitic intensity, chlorophyll is decreasing. Therefore, we suspect that the cause of this phenomenon may be related to the influence of Cuscuta japonica Choisy on the photosynthesis of host plants. Generally speaking, the red valley reflectance of healthy plants is the highest, but with the deepening of parasitic intensity, the spectral red valley reflectance of host plants shows a decreasing trend with T1 > T2 > T3. Under different parasitic conditions, the position of the yellow edge is not affected, and it is all at 570 nm. However, with the deepening of parasitic intensity, the slope of the yellow edge and the reflectivity of the green peak gradually decreases, while the position of the green peak presents shifts to the long wave direction. At this time, the reflectivity of the water stress wave band increases gradually. Studies have shown that the spectral reflectance of vegetation in the range of 1550 ~ 1750 nm is usually closely related to the cell structure and water content of plants, which indicated the water absorption characteristics [50]. Therefore, with the deepening of the invasion degree of Cuscuta japonica Choisy, the cell structure of the leaves suffers certain damage, and the cell fluid of the leaves gradually decreases, thus causing the absorption of light to decrease and the reflection to obviously increase.
Correlation between chlorophyll content and spectral characteristic parameters of host plants with different parasitic degree of Cuscuta japonica Choisy
As shown in Fig. 1, CCI of the host plant (Osmanthus fragrans) gradually decreased with the deepening of the parasitic intensity of Cuscuta japonica Choisy. Previous studies generally believed that chlorophyll was an important parameter to determine the spectral reflectance curve characteristics of plant [51]. When the vegetation is in a healthy growth state and the chlorophyll content is high, the position of the red edge moves towards the long wave direction [51, 52]. However, when vegetation is subjected to external environmental stress, such as drought stress, high temperature stress or pest damage, the red edge position tends to the short-wave direction [53]. Figure 6 and Table 1 showed the correlation between different spectral parameters and CCI and LT. The results of correlation analysis between plant functional traits and spectral parameters show that they show different correlations. It can be seen that there was an extremely significant correlation between spectral parameters and CCI. There was a significant correlation between RGP and LT. Among RRV, RGP, RES, RWSB and CCI were all highly correlated. The correlation between valley reflectance and chlorophyll content reaching the maximum (y=-65913.323x + 9.783, R2 = 0.6888), which indicated that red edge characteristics were very sensitive to parasitic infestation and can be used to characterize changes in chlorophyll content of Osmanthus fragrans under different parasitic degrees. As shown in Fig. 7, we tested the chlorophyll inversion model of red valley reflectance, and found that the prediction accuracy of this model was high and stable (R2 = 0.8811, RMSE = 0.0004).
Table 1
Pearson correlation analysis between chlorophyll content index and spectral feature parameters. * indicates that the correlation reaches a significant level at the level of P < 0.05. and ** indicates a significant correlation between functional traits.
| RRV | RGP | RES | RWSB |
LT | 0.25218 | -0.1787 | -0.28318* | 0.09577 |
LA | -0.01651 | 0.18462 | -0.14292 | 0.0353 |
LDMC | 0.01136 | -0.00523 | 0.03048 | 0.09512 |
SLA | 0.20281 | -0.24112 | -0.06407 | 0.0641 |
LTD | -0.19553 | 0.17124 | 0.13662 | -0.01367 |
CCI | -0.82993** | 0.72953** | 0.65295** | -0.56967** |
Effects of parasitic plants on the correlation of functional traits of Osmanthus fragrans and analysis of leaf economics spectrum
As can be seen from Table 2, there was an interdependent relationship between the functional traits of the leaves. There was a significant positive correlation between LA and SLA. There is a significant negative correlation between SLA and LDMC and LTD. LA was significantly negatively correlated with LDMC and LTD. There was a significant negative correlation between LT and LTD. There was a very significant positive correlation between LDMC and LTD. There was a significant positive correlation between CCI and SLA. At the same time, LT has a negative correlation with SLA and LA, but the correlation has not reached a significant level.
Table 2
Correlation between plant functional traits indicators. * indicates a significant correlation between functional traits at the level of P < 0.05, and ** indicates a significant correlation between functional traits at the level of P < 0.01.
| LT | LA | SLA | LDMC | LTD | CCI |
LT | 1 | | | | | |
LA | -0.1696 | 1 | | | | |
SLA | -0.1502 | 0.3581* | 1 | | | |
LDMC | -0.1293 | -0.4246* | -0.6991** | 1 | | |
LTD | -0.5436** | -0.4218* | -0.5950** | 0.7517** | 1 | |
CCI | 0.2566 | 0.2623 | 0.4993* | -0.2201 | -0.4456* | 1 |
Studies have shown that leaf functional traits can reflect the adaptability of plants to the environment, but compared with a single leaf functional trait, continuous leaf economic spectrum can better reflect the growth strategy and adaptation mechanism of plants [54, 55]. In this study, there was an obvious trade-off relationship between the functional traits of plant leaves, which indicated that when plants are damaged by parasitic plants, host plants show certain ecological trade-off strategies in terms of functional traits for survive. SLA is closely related to the growth and survival strategy of plants, and can represent the ability of plants to adapt the environment and obtain resources [56]. In this study, after being invaded by parasitic plants, the reduction of SLA of the host plants makes the plants more adaptable to resource-poor environment. LDMC represents the of plants to maintain nutrients, while LTD reflects the bearing capacity and defense ability of plant leaves, which is closely related to the turnover growth rate of leaves [56, 57]. In this study, LDMC and LTD increased gradually with the increase of parasitic intensity, and showed a very significant positive correlation. This indicates that the plants can improve the nutrient retention ability of leaves under the adverse environment of parasitic stress, thus making more effective use of limited resources. The increase of LTD is beneficial to enhance the defense ability of plants against biological factors. To sum up, after the host plant was invaded by parasitic plants, its leaf functional traits are generally characterized by large leaf thickness, small leaf area, small specific leaf area, low chlorophyll content index, high dry matter content and high leaf tissue density. Therefore, we suspect that the leaf economics spectrum may also exist in the parasitic environment, and there was a general trend toward “slow investment-return” type in the global leaf economics spectrum (Fig. 8).