**3. Variations in C:N:P ratios in plants**

### **3.1. Growth rate and N:P stoichiometry in plants**

The applicability of growth rate (GR) to plants has been attracting interest because leaves have high concentrations of nutrients (N and P). However, only a few experimental studies have assessed GR for particular plant species.

A study with seedlings of the species *Betula pendula* with P limitation showed a decrease in N:P ratios and high relative growth rates; however, plants with N limitation did not show this pattern [42], probably because of P storage under N limitation.

A study with 14 pine species grown with high levels of nutrients in a greenhouse [43] reported faster plant growth, which was correlated with nutrient concentrations and a decrease in the protein RNA.

Finally, when the researchers compared the seedlings for growth rates among the 14 species under high‐nutrient conditions, they found no correlation with N:P ratios or the protein RNA [43].

The results of Ågren [42] and Matzek and Vitousek [43] suggest that the basic prediction of GR (a negative correlation between N:P and growth rate) may not be useful for plants when nutrients, especially P, are not limiting factors.

Thus, studies have concluded that although the vegetable protein:RNA ratio affects the speed and efficiency of growth; it does not determine, by itself, leaf N:P stoichiometry. Thus, it seems that the advances and understanding of interactions between N:P stoichiometry and growth require both additional studies and development of models that represent the poten‐ tial storage of nutrients, especially P [44].

The correlation between N and P observed in leaves has been recently confirmed in other important plant organs [45]. A study using a high number of species [46] found that, as in leaves, N and P concentrations are correlated in roots, stems and in reproductive tissues.

High concentrations of P and low N:P ratios are linked with growth rate [14]. The effects of high CO2 concentrations on plants and C:P ratios and, in particular, on N:P ratios, still are not clear [47] and deserve further study because N and P are essential in living systems and their relationship is associated with changes in the structure of the ecosystem [14]. In fact, it is important to increase our understanding of changes in the mineralization of N and P in plants and in the soil under high CO2 concentrations because N and P are important in the composi‐ tion of litter and soil decomposition rates [48]. A meta‐analysis showed that added N signifi‐ cantly decreased the C:N ratio of photosynthetic tissues of woody plants (*P* < 0.05, *n* = 25) and herbaceous species (*P* < 0.05, *n* = 6). On average, N reduced the C:N ratios of photosynthetic tissues by 25% (*P* < 0.05, *n* = 31) (**Figure 3**).

In 20 of 36 species, the addition of N increased the N:P ratio of photosynthetic tissues; in 15 species, the N:P ratios were not changed and, in one species, the ratio decreased. The addition of N increased the N:P ratios of the photosynthetic tissues of woody plants (*P* < 0.05, *n* = 10) and herbaceous species (*P* < 0.05, *n* = 12) (**Figure 3**).

Ecological Response to Global Change: Changes in C:N:P Stoichiometry in Environmental... http://dx.doi.org/10.5772/intechopen.69246 153

**Figure 3.** Leaf C:N and N:P ratios of plants growing in the environment and in the N‐addition treatment. Metadata analysis of 31 different experimental results in the case of leaf C:N ratio and of 22 different experimental results in the case of leaf N:P ratio. Soil organic and inorganic C:N ratios under ambient conditions and in the N‐addition treatment. The only studies that were taken into account were those that provided the mean (±S.E.) of leaf C:N and C:P ratios of plants, and soil C:N ratios growing under ambient conditions and in the N‐addition treatment. Meta‐analyses were made by using the MetaWin Package, which is based on the knowledge of control and treatment results (mean ± SD) in each study (considering each species being studied). Different letters indicate statistically different values (*P* < 0.05) [13].

### **3.2. Plant stoichiometry**

**3. Variations in C:N:P ratios in plants**

152 Plant Ecology - Traditional Approaches to Recent Trends

have assessed GR for particular plant species.

nutrients, especially P, are not limiting factors.

tial storage of nutrients, especially P [44].

and in the soil under high CO2

tissues by 25% (*P* < 0.05, *n* = 31) (**Figure 3**).

and herbaceous species (*P* < 0.05, *n* = 12) (**Figure 3**).

protein RNA.

[43].

high CO2

pattern [42], probably because of P storage under N limitation.

**3.1. Growth rate and N:P stoichiometry in plants**

The applicability of growth rate (GR) to plants has been attracting interest because leaves have high concentrations of nutrients (N and P). However, only a few experimental studies

A study with seedlings of the species *Betula pendula* with P limitation showed a decrease in N:P ratios and high relative growth rates; however, plants with N limitation did not show this

A study with 14 pine species grown with high levels of nutrients in a greenhouse [43] reported faster plant growth, which was correlated with nutrient concentrations and a decrease in the

Finally, when the researchers compared the seedlings for growth rates among the 14 species under high‐nutrient conditions, they found no correlation with N:P ratios or the protein RNA

The results of Ågren [42] and Matzek and Vitousek [43] suggest that the basic prediction of GR (a negative correlation between N:P and growth rate) may not be useful for plants when

Thus, studies have concluded that although the vegetable protein:RNA ratio affects the speed and efficiency of growth; it does not determine, by itself, leaf N:P stoichiometry. Thus, it seems that the advances and understanding of interactions between N:P stoichiometry and growth require both additional studies and development of models that represent the poten‐

The correlation between N and P observed in leaves has been recently confirmed in other important plant organs [45]. A study using a high number of species [46] found that, as in leaves, N and P concentrations are correlated in roots, stems and in reproductive tissues.

High concentrations of P and low N:P ratios are linked with growth rate [14]. The effects of

not clear [47] and deserve further study because N and P are essential in living systems and their relationship is associated with changes in the structure of the ecosystem [14]. In fact, it is important to increase our understanding of changes in the mineralization of N and P in plants

tion of litter and soil decomposition rates [48]. A meta‐analysis showed that added N signifi‐ cantly decreased the C:N ratio of photosynthetic tissues of woody plants (*P* < 0.05, *n* = 25) and herbaceous species (*P* < 0.05, *n* = 6). On average, N reduced the C:N ratios of photosynthetic

In 20 of 36 species, the addition of N increased the N:P ratio of photosynthetic tissues; in 15 species, the N:P ratios were not changed and, in one species, the ratio decreased. The addition of N increased the N:P ratios of the photosynthetic tissues of woody plants (*P* < 0.05, *n* = 10)

concentrations on plants and C:P ratios and, in particular, on N:P ratios, still are

concentrations because N and P are important in the composi‐

Plant stoichiometric is a technique that allows investigating energy flow and cycling of mate‐ rials in ecosystems [49], stoichiometric flexibility, physiological adjustment of C:N:P ratios which may improve plant performance in response to environmental changes [50]. Therefore, it is important to investigate the patterns of stoichiometric values and their flexibility within and among plant species [51].

However, this technique may present some disadvantages regarding the variability that occurs in plant C:N:P stoichiometry in several habitats and emerges from two interaction processes: (1) macro‐scale constraints caused by specific geographic environment (climate and soil), and (2) fundamental physiological constraints resulting from growth, development, metabolism, phenology and life history [23].

Moreover, plant size, which changes as a result of seasonal development, may influence the rate indicated by the metabolic scale theory [52, 53], which in turn affects the stoichiometric ratios through metabolic changes [1].

Sampling time is another factor that may compromise the success of this technique, because sampling ranges from months to years, and the effects of organ size within a period of the year of the study are often not kept constant [25, 54].

Thus, a study developed to evaluate the C:N:P stoichiometric flexibility as of the date of sam‐ pling was developed by Zhang et al. [55] in field conditions in Mongolia. Three plant spe‐ cies were selected: *Leymus chinensis* (perennial C3 plant); *Cleistogenes squarrosa* (perennial C4 plant) and *Chenopodium glaucum* (annual C3 plant).

To study the effects of sampling date, 30 individual plants of each species were collected at 15‐day intervals, from 10 July to 25 August 2006, for a total of four sampling dates.

The authors found that the C:N, C:P and N:P ratios in the leaf tissue increased over time com‐ pared with the study species, except for the species *Chenopodium glaucum* (**Figure 4a**–**c**) [55].

For the species *Leymus chinensis*, the C:N, C:P and N:P ratios were the highest among the three species and they increased over time, with the exception of the N:P ratio until the last date of

**Figure 4.** Change in C:N (a), C:P (b), N:P (c) ratios for leaf (left) and C:N (d), C:P (e), N:P (f) root (right) tissues over time for three grassland species in the sand culture study. Error of mean [49].

sampling (**Figure 4c**). However, for *Chenopodium glaucum*, the C:N and C:P ratios increased for the first two sampling dates and then decreased after 10 August 2006 (**Figure 4a** and **b**) [55].

Thus, a study developed to evaluate the C:N:P stoichiometric flexibility as of the date of sam‐ pling was developed by Zhang et al. [55] in field conditions in Mongolia. Three plant spe‐ cies were selected: *Leymus chinensis* (perennial C3 plant); *Cleistogenes squarrosa* (perennial C4

To study the effects of sampling date, 30 individual plants of each species were collected at

The authors found that the C:N, C:P and N:P ratios in the leaf tissue increased over time com‐ pared with the study species, except for the species *Chenopodium glaucum* (**Figure 4a**–**c**) [55]. For the species *Leymus chinensis*, the C:N, C:P and N:P ratios were the highest among the three species and they increased over time, with the exception of the N:P ratio until the last date of

**Figure 4.** Change in C:N (a), C:P (b), N:P (c) ratios for leaf (left) and C:N (d), C:P (e), N:P (f) root (right) tissues over time

for three grassland species in the sand culture study. Error of mean [49].

15‐day intervals, from 10 July to 25 August 2006, for a total of four sampling dates.

plant) and *Chenopodium glaucum* (annual C3 plant).

154 Plant Ecology - Traditional Approaches to Recent Trends

Thus, the study suggests that leaf sampling at different times may influence the stoichiometric ratios of the plant, particularly C:N and C:P ratios of leaves [55].

In general, the C:N and C:P ratios of leaves increased with increasing sampling date within the study periods. This increase was probably driven by the increase in plant size (C content); as plants get older, the C‐enriched material accumulates, which leads to a 'dilution' of N and P contents over time [56, 57].

Thus, over time, C:N and C:P ratios may increase because of reduced nutrient allocation to older leaves and to nutrient dilution as the leaf area and root systems increase over time [55].

C:N:P stoichiometric ratios in plants can also be altered depending on the application of ben‐ eficial elements in agriculture, e.g. silicon (Si).

In this scenario, a study was conducted in a greenhouse in Jaboticabal, São Paulo State, Brazil, in which a rice crop was combined with the application of Si sources (Nano silica and soluble silicon) and concentrations of Si (0, 605, 1210, 103 and 2420 g ha−1 Si, applied on the seeding furrow). They found that Si availability did not affect the C:N:P stoichiometric ratio in the shoot of rice plants, although there were higher stoichiometric C:N:P ratios in the concentra‐ tion of 1210 and 2420 g ha−1 Si, when soluble silicon was used (**Table 1**) [58].

In this study, the stoichiometric ratio found refers to the average, excluding the panicle, and this probably resulted in the absence of more pronounced effects of the treatments applied [58].

There are strong associations between the absorption of Si, N and P, and a study on silicon sources and grass species emphasized that the responses varied according to the sources of silicon in use [59]. In an experiment with *Phragmites australis* [60], it is reported that Si avail‐ ability can have significant effects on stoichiometric C:N:P ratios in different tissues (leaf blades, sheaths and stems).


**Table 1.** Stoichiometry of nutrients affected by sources and doses of silicon applied in the seeding furrow of rice [58].
