**1. Introduction**

The increase in warming and droughts and the high concentrations of atmospheric CO2 can change the contents and the stoichiometry of nitrogen (N) and phosphorous (P) in plants [1], and they can have an indirect impact on soil and nutrient availability. This increase in high

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CO2 concentrations induces changes in plants, especially in C3 plants, with an increase in the uptake of carbon (C), which may lead to a reduction of transpiration [2], because CO2 absorp‐ tion promotes stomata closure [3], which may limit the ability of plants to assimilate N [4].

The high absorption of CO2 may also lead to a gradual limitation of nutrients that can quickly limit the increase in plant production [5], because it affects the process of acclimatization that involves a series of changes in plant metabolism at different levels of organization (molecular, biochemical, anatomical and morphological) [6].

In recent decades, anthropic activities have altered the P cycle; excessive doses of fertilizers are being used, thus inducing an increase in the input of this nutrient into terrestrial and aquatic ecosystems [7, 8]. Increased application of P may alter the balance between C, N and P in plants, and thus change the C:N:P stoichiometry ratios [8] and reduce the C:P ratio in plant tissues [9, 10]. Another concern is the change in N and P cycles, which can cause several consequences to the environment [11–13].

In this scenario, environmental responses of plants to global changes have a negative charac‐ ter with future losses to food production worldwide. Therefore, it is necessary to recognize the new stoichiometry (C:N:P ratios) that occurs in plants in this new scenario in order to try to identify a plant‐environment interaction that may allow an increase in food production and that will allow greater food security in the future.

The interactions that occur between elements are complex and their effects reflect the mineral composition of plants. An alternative to study the multiple ratios between elements in a plant is to focus on stoichiometric ratios that are considered to be an important biological indicator for elucidating plant responses to various changes and their adaptation to different environ‐ ments [14].

Moreover, the study of plant stoichiometry can influence ecological processes, and thus modulate the structure and function of the ecosystem [15, 16]. It can also effectively indicate changes in C, N and P cycles [17].

The carbon (C):nitrogen (N):phosphorus (P) ratio is one of the most investigated topics in stoichiometry, because N and P limit plant growth and C is the structural basis of plants: they account for 50% of plant dry mass [18].

These elements are strongly linked to the biochemical functioning of plants. P is an important element in the production of ribosomes; it is involved in the synthesis of proteins containing N and C. There are, therefore, fundamental biochemical reasons for using these elements in appropriate proportions [19].

In plants, C:N and C:P ratios represent the ability of photosynthetic fixation of C through N or P accumulation. Also, the N:P ratio can be used as an indicator to study plant nutrient limita‐ tion in adverse habitats [20].

Therefore, the proportions of leaf N and P in plant biomass can be an indicator of vegeta‐ tion composition and nutrient limitation at the community level [21, 22]. An N:P < 14 ratio indicates N limitation, whereas an index >16 suggests P limitation [21]. An ideal N:P ratio is considered to be 10–20, on a mass basis [22].

In view of the above, the present chapter sought to study patterns and values and discuss the stoichiometric changes C:N:P occurring in plants in response to global changes and their implications in the adaptive mechanisms of plants to the environment.
