Plant Photosynthesis-Irradiance Curve Responses to Pollution Show Non-Competitive Inhibited Michaelis Kinetics

Photosynthesis-irradiance (PI) curves are extensively used in field and laboratory research to evaluate the photon-use efficiency of plants. However, most existing models for PI curves focus on the relationship between the photosynthetic rate (Pn) and photosynthetically active radiation (PAR), and do not take account of the influence of environmental factors on the curve. In the present study, we used a new non-competitive inhibited Michaelis-Menten model (NIMM) to predict the co-variation of Pn, PAR, and the relative pollution index (I). We then evaluated the model with published data and our own experimental data. The results indicate that the Pn of plants decreased with increasing I in the environment and, as predicted, were all fitted well by the NIMM model. Therefore, our model provides a robust basis to evaluate and understand the influence of environmental pollution on plant photosynthesis.

Many models have been used to assess the relationship between the photosynthetic rate (Pn) and photosynthetically active radiation (PAR), including the exponential function (EF, [8,10]), hyperbolic tangent function (HTF, [1]), nonrectangular hyperbola model (NHM, [11, v ¼ in these equations, v, V m , [S], and K m are the same as mentioned above; [I] is the content of the inhibitor; and K i is the inhibition constant. As mentioned above, photosynthesis is a process of enzymatic reactions, and photons play the role of a resource [21], the PAR in photosynthesis is similar to the [S] in an enzymatic reaction. The RHM (Eq 5) was derived from the Michaelis-Menten equation [11,14,18,19], where α is the photochemical efficiency of photosynthesis at low light, P m is the maximum photosynthetic rate, PAR is the photosynthetically active radiation, and Rd is the dark respiration rate. Ye [14] presented a new model (Eq 6) modified from the RHM (Eq 5) for predicting the relationship between Pn and PAR, Where α is the photochemical efficiency of photosynthesis at low light, i.e., the initial slope of the PI curve; β is a correction factor for the decreasing trend of Pn when PAR exceed light saturation point due to photoinhibition, and the β is similar to the convexity [9,11] or the sharpness of the knee [20] of the PI curve, γ is a conversion factor for the α (i.e., the initial slope of the PI curve) and the P m (i.e., the maximum photosynthetic rate), and the γ is proportional to the radio of α and P m (i.e., g / a P m ); α, β, and γare coefficients that are independent of irradiance [14]; PAR is the photosynthetically active radiation, and Rd is the dark respiration rate. Here, we assumed that, 1) the Pn of plants decreased with increasing concentrations of a pollutant; and 2) the effect of the pollutant on the PI curve is non-competitive inhibited, and we presented our new non-competitive inhibited Michaelis-Menten model (NIMM) as: Where α, β and γare the same as mentioned above; Pn denotes the net photosynthetic rate; K i denotes an inhibition constant; I is the relative pollution index and Where C i is the actual concentration of pollutant i in water or soil; and C imax is the maximum concentration of pollutant i in water or soil.

Experimental design
Establishing a single pollutant model is the first step in the research of effects of pollution on plants.
Here, we chose one pollutant to a plant research model. We tested effects of a variety of common pollutants to corresponding representative plants as shown in Table 1. Phenolic pollution is often the chemical hazards and accidents that take place in the chemical industry. And the soil heavy metal pollution result from rapid industrialization and urbanization during industrial and agricultural development and population growth. So, we tested the pollutants including phenol and some common metal pollutants, e.g., Cu 2+ , Pb 2+ , Cd 2+ , and Al 3+ . The Bordeaux mixture (a mixture of coppersulfate and lime) or animal manure use in agriculture results in the potential risk of soil copper pollution. The lead and cadmium pollution also result from automobile exhaust. The soil acidity increasing leads to aluminum pollution.

Pot-culture experiment and PI curves measurement
The pot-culture experiments were carried out in a greenhouse at the Fuqing Branch of Fujian Normal University from June to September in 2013. T. pratense L. and W. trilobata, two types of ornamental groundcover that often appear on roadsides and plantations, were planted in flowerpots filled with % 1.8 kg soil. Each treatment had 15 replicates. The properties of the soil were pH: 6.4, total nitrogen: 24.2 mg kg -1 , total phosphorus: 1.15 g kg -1 , available phosphorus: 9.03 mg kg -1 , total potassium: 68 mg kg -1 , and clay particles: 21.7%. T. pratense seeds were germinated for 48 h in the dark (on wet filter paper at 25°C) and sown into a flowerpot (diameter: 200mm, height: 200mm) filled with phenol treated soil. Before being filled into pot, air-dried soil was treated with 0 (as control), 100, 200, or 300 mg kg -1 of phenol. W. trilobata were collected from the roadsides and cut, and the apex meristem with two leaves (% 100-mm length, two internodes) were planted in a flowerpot (diameter: 200 mm, height: 200 mm). Three apex meristems were planted in every flowerpotwith CuSO 4 Á5H 2 O added soil. Air-dried soil was added with 0 (as control), 500, 1000, or 2000 mg kg -1 of CuSO 4 Á5H 2 O, and then was filled into the flower port.
We selected a sunny day (three months after planting) to measure the PI curves using a CIRAS-2 Portable Photosynthesis System (PP Systems, USA) with an LED radiation source.  (Table 1) to further evaluate our NIMM. All data were collected from pot-culture experiments.

Data collection and detailed data descriptions
The pot-culture experiments of Z. mays seedling [22] were conducted in silica culture. And the seedlings consisting of one bud and two leaves were treated with three Hoagland solution (including equal amount of Pb 2+ and EDTA at different concentrations: 0, 0.25 or 0.5 Data collected from literature [24] mmolÁL -1 ). After 15 days of treatment, the PI curves were measured with a Ciras-2 portable photosynthesis system (PP systems, UK). For more detailed information, please see S1 Table. The one-year old C.sinensis Osbeck [23] was grafted onto Citrus aurantium L. before Cu stress treatment. The pot-culture experiments of C.sinensis Osbeck were conducted in a 10-L pot filled with 8 L of Alva nutrient solution (pH 6.5). The Alva nutrient solution was aerated 3 times with each time for 2 h in every day, and it was renewed every 10 days; And the C.sinensis Osbeck were treated with five Alva nutrient solution (containing Cu 2+ concentration at0, 0.1, 5, 20 or 40 μmolÁL -1 ). After 60 days of treatment, the PI curves were measured with a CID-301 PS (CID Bio-Science, Inc., USA). For more detailed information, please see S2 Table. The other pot-culture experiments of Z. mays [24] were conducted in paddy soil. The properties of the paddy soil were pH: 6.42, organic matter: 1.63%, total Cd: 0.32 mgÁkg -1 , total nitrogen: 0.09%, available phosphorus: 0.05%, available potassium: 0.04%. The paddy soil was airdried and sieved through a 2-mm sieve, mixed with different amount of CdCl 2 Á2.5H 2 O, and then the post-treated paddy soil was added into each pot up to three kg with one gram of compound fertilizer (including N 15%, P 15%, K 15%). Finally, the germinated Z. mays were planted; So far, the germinated Z. mays were treated with six paddy soil (including Cd concentration at: 0.32, 1, 5, 15, 50 or 100 mgÁkg -1 ). After 20 days of treatment, the PI curves were measured with Li-6400 (Li-Cor Inc., USA). For more detailed information, please see S3 Table. The P. asiatica [25] seed was sterilized with 0.1% HgCl 2 for 10 min, following by washing and soaking in distilled water for 8 h, and then the seed was sowed in sterilized silica culture. The two-leaves old plantswere transplanted into a 20 cm × 23cm flowerpot with three kg medium (peat soil: sand = 3:1). On the six-leaves old plant, the Al stress was performed. 10 mL of AlCl 3 solution (pH 4.0) with different concentration at 0, 100, 500, 800 or 2000 mgÁL -1 were respectively poured into the flowerpot to simulate different leaching of Al 3+ in soil every day. After 20 days of treatment, the PI curves were respectively measured with a Ciras-2 portable photosynthesis system (PP systems, UK). For more detailed information, please see S4 Table.

Mathematical fitting and model testing
To obtain the equation parameters (i.e., α, β, γ, K i , and Rd), mathematical fitting of NIMM was performed using 1stOpt software (7D-Soft High Technology Inc. Beijing, China) with the Levenberg-Marquardt method. In addition, mathematical fitting of the relationship of Pn and I and that of Pn and PAR were performed to obtain the equation parameters using the same software and method as in the previous case. The relationship between the Pn and PAR of T. pratense response to different concentrations of phenol in our pot-culture experiment was calculated according to the mathematical fitting results to test the NIMM. The relationship between the Pn and PAR of W. trilobata response to different concentrations of Cu 2+ was calculated using the same method.

Experimental results
The Pn in our pot-culture experiments was measured with a Ciras-2 under conditions of natural ambient CO 2 at different PAR. Our results were similar to the references [22][23][24][25]. Clearly, the PI curves of the plants were saturation curves. The results also showed that, either in W. trilobata or in T. pratense, the Pn increased with PAR increasing below the PAR sat (i.e., light saturation point, % 1000 μmol photon m -2 s -1 in T. pratense, % 1400 μmol photon m -2 s -1 in W. trilobata), while decreased as PAR increasing above PAR sat . The results also showed that the pollutant obviously negatively affected the PI curves. For more detailed information, please see S5 and S6 Tables.

Effect of a pollutant on the normalized Pn of plants
The normalized Pn of plants decreased with increasing concentrations of the pollutant under 1000 μmol photon m -2 s -1 PAR (Fig 1). Akaike's information criterion (AIC) was proposed by Akaike [31,32] and defined as Eq 9, Where N is the number of experimental data points, p is the number of parameters in an estimated model, and R e is the residual sum of squares. In addition, the model with the lowest AIC is regarded as the best representation of a curve [32]. The Pn values for all five species were normalized to the pollutant-free control value of Pn, and the normalized Pn were regressed with respect to I using linear (Eq 10), power (Eq 11), exponential (Eq 12), and hyperbolic (Eq 13) functions, Pn in these equations (Eqs 10~13), Pn 0 is the normalized net photosynthetic rate, a and b are coefficients, I is the relative pollution index.
And the results showed that all functions (Eqs 10, 11, 12 and 13) were significant (P< 0.01), and the hyperbolic function (Eq 13) was the optimal function based on having the greatest goodness-of-fit (R 2 ) of 0.5983 and the lowest AIC of -9.0 (Fig 1a). The normalized Pn of each species was regressed with respect to I using a hyperbolic (Eq 13) function, and the results were all significant (P< 0.01) (Fig 1b).

Mathematical fitting of PI curves using different models
The Pn of T. pratense, Z. mays seedling, C. sinensis Osbeck, Z. mays, P. asiatica, W. trilobata were respectively regressed with respect to PAR usingan EF [8,10], HTF [1], NHM [11,13], RHM [17,18], BRF [9,13], and MM [14]. The R 2 was significant for all models (P< 0.001). In T. pratense or P. asiatica, the three largest R 2 values (associated with the lowest AIC) of models were for HTF, NHM, and MM (Fig 2a and 2b). In C. sinensis Osbeck or Z. mays seedling, the three largest R 2 values (associated with the lowest AIC) of models were for HTF, BRF, and MM (Fig 2c and 2d). In Z. mays, the three largest R 2 values (associated with the lowest AIC) of models were for RHM, NHM, and MM (Fig 2e). In W. trilobata, the three largest R 2 values (associated with the lowest AIC) of models were for EF, HTF, and MM (Fig 2f). The MM and BRF were both better than other models at describing the photoinhibition phenomenon at high PAR (Fig 2).

Evaluation of NIMM
The Pn of each species was regressed on PAR and I using NIMM, and the results are shown in Table 2. The R 2 values were greater than 0.95 except for Z. mays. For Cu pollution, the K i of W. trilobata was greater than that of Citrus sinensis Osbeck. The K i of Cu to W. trilobata was greater than that of the phenol to T. pratense. For Z. mays, the K i of Cd was greater than that of Pb.
The NIMM was tested using our pot-culture experimental data. Either in T. pratense, or in W. trilobata, the R 2 values were all significant (P< 0.001) under different pollution levels ( Fig  3, Table 3). Either in T. pratense, or in W. trilobata, the light saturation point (PAR sat ) and the light compensation point (PAR com ) both increased with worsening pollution, while the maximum photosynthetic rate (P m ), quantum efficiency at PAR com (φ c ), and intrinsic quantum efficiency (φ 0 ) all decreased ( Table 3). The φ c represents the light energy use efficiency at PAR com , the φ 0 represents the intrinsic light energy use efficiency at darkness, i.e., the optimal light use potential of plant. The results suggested that the pollutant inhibited the light use potential of plant. In order to analyze the credibility of the assessment results, we performed paired samplest test analysis, and the results showed that in T. pratense, the calculated P m was no significant difference to the measured P m (t = -1.975, df = 3, P 2-tailed = 0.143), in W. trilobata, the calculated P m was also no significant difference to the measured P m (t = -1.777, df = 3, P 2-tailed = 0.174).

Discussion
All of the above mentioned existing models (i.e., EF; HTF; NHM; RHM; BRF; and MM) provide useful protocols for PI curve assessment. Jassby and Platt reported that, from zero light up to the onset of photoinhibition, the PI curve for natural populations of coastal phytoplankton is best described by HTF, and they recommended its use as an operational model for the elucidation of physiological parameters in photosynthesis-light experiments and for the theoretical investigation [1]. The shape of PI curve described by EF suggests that a linear relation holds only for low light intensities, then the photosynthetic rate tends towards a maximum valuewhen the light intensity is increasing [8,10]. The NHM was found to be objective to calculate the photosynthetic parameters of the PI curve [9,11,13,20], the PI curve could also be described by BRF [13,33], but the BRF could not be used to calculate thequantum  efficiencyand explain that the predicted Pn declines quickly when PAR excesses the light saturation point [13]. In addition, the BRF has the shortcoming of sometimes inferring a positive dark respiration rate, which has no biological significance. The RHM can be obtained from the NHM by putting θ = 0, it is a special case of the NHM [20]. And the RHM is preferred to the NHM by some workers on the grounds of simplicity [18,20], though it is rather tedious to take the limit as θ ! 0 in the NHM equation [20]. Our experimental results showed that the shapes of PI curves were similar to that of the literatures. Our experimental results also showed that the PI curves have photoinhibition phenomenon at high irradiance, i.e., the Pn decreased when the PAR exceeded light saturation point. These results were fully consistent with that of the literature [8,11,13,14,23,34]. Although the HTM, EF, NHM and RHM have been extensively applied [11,14,17,18,20,[34][35][36][37][38], they do not consider the photoinhibition of plants. The MM, which is based on the RHM, is useful to study photoinhibition and photosynthetic behavior at high irradiance and, especially, is the best model to describe the PI curve because its fitted values wereclose to the measured data [14]. Therefore, the MM (Eq 6) was the optimal modelfor predicting the relationship of Pn and PAR. Moreover, based on the lowest AIC values [31,32], the HTF, NHM, and MM are more suitable for characterizing the PI curve (Fig 2). Temperature, intensity of irradiation, and concentration of carbon dioxide in the surrounding medium are the three important controlling factors could influence the rate of photosynthesis in plant, and of the three controlling factors, the most important is the temperature [11,17]. However, the concentration of carbon dioxide in the atmosphere remains relatively constant, and it is unlikely to be a major factor effecting variations in the rate of photosynthesis, simultaneously, the temperature could not influence the shape of the PI curve of plant, therefore, temperature and concentration of carbon dioxide need not appear explicitly in a PI curve model [11]. On the other hand, the shapes of PI curves in our pot-culture experiments (Fig 3) were fully consistent with that of the literatures [8,11,13,14,[22][23][24][25]34], and showed that α and P m both decreased along with the increasing concentrations of pollutant, but the convexity [11,13,37], or the sharpness of the knee [20] of the PI curve described by the NHM increased along with increasing pollutant concentrations. It indicated that the pollutants negatively affected on the photosynthesis of plants, and the impact degree increased with rising pollutant concentrations. This conclusion was similar to that of the literature [17]. The literature [17] reported that a poison may materially to reduce the rate of photosynthesis, because the poison may either decrease the velocity of the Blackman reaction, or decrease the velocity of the primary photosynthetic reaction by being preferentially adsorbed by the chlorophyll a and thus preventing the latter from adsorbing or combining with hydrated carbon dioxide. So, pollutant was significant and necessary appear explicitly in a PI curve model. And even though some metals, such as zinc and copper, are essential trace elements for plants as the natural active sites of an enzyme, plant growth and development only need low concentrations of these metals of around 10 μg g -1 dry plant tissue [39,40]. Some studies [41,42] have also shown that pollutants  Then, how to integrate the I into the MM? The relationship of normalized Pn and I were respectively regressed using linear (Eq 10), power (Eq 11), exponential (Eq 12), and hyperbolic (Eq 13) functions. And, the effect of pollutants on the Pn of plants (Fig 1) indicated that the hyperbolic function (Eq 13) was optimal for predicting the relationship of Pn and I. Thus, we integrated the I into the MM as: Eq 14 can be converted into: Further, Eq 15 can be converted into: If b = K i and d ¼ b a , Eq 16 can be expressed as: Where δ is a non-zero coefficient, Eq 17 is equivalent to the NIMM, i.e., Eq 7. Further, our mathematical fitting results showed that the NIMM was suitable for predicting the relationship of Pn, PAR, and I because of their high R 2 ( Table 2) and their significance at the P< 0.001 level (Table 3), that is, the NIMM was suitable for fitting the PI curve of plant responses to pollution (Fig 3, Table 3). The NIMM showed that the Pn is a function of PAR and I, thus, the Eq 18 denotes the influence rate of I on Pn, and the Eq 19 denotes the influence rate of PAR on Pn, Where @Pn @I and @Pn @PAR are partial derivative, denotes the influence rate of I on Pn, and the influence rate of PAR on Pn respectively; α, β, γ, K i , PAR, and I are the same as above mentioned.
Pollutants (metals) are harmful to plants because they inhibit various metabolic processes [41][42][43]. Some metal pollutants directly affect enzymes of the chlorophyll biosynthesis pathway [44][45][46], and some affect the proper assembly of the photosynthetic pigment-protein complexes [47,48]. Some metalsreplace the central Mg ion in chlorophyll molecules, destroying the chlorophyll [49]. Conversely, some studies have not found that metal pollutants directly affect the biosynthesis of pigments or influence the photosynthetic machinery, and have claimed that the metal pollutants interfere with cell division and chloroplast replication, thus decreasing the number of chloroplasts and ultimately lowering the photosynthetic efficiency [50]. Thus, regardless of whether elevated concentrations of pollutants in contaminated environments bind equally well to enzymes, they will already have negatively affected plant growth and development through the inhibition of photosynthetically related enzyme activity. Our mathematical fitting results indicate that the elevated concentrations of pollutants not only inhibited α (i.e., photosynthetic potential, light use efficiency, or the slope of the PI curve), but also lowered Pn (Tables 2 and 3, Figs 1 and 3). The former (i.e., decreased α associated with increasing pollutant concentrations) suggested that the pollution decreased the activity of the photosynthetically related enzyme. Ourpot-culture experimental results showed that in W. trilobata, the pollutant (Cu 2+ ) did not significantly affect the pigment content, above-ground biomass, or belowground biomass, but did significantly affect the Pn (Please see S7 Table). Our pot-culture experimental results also showed that the pollutant (phenol) significantly affected the biomass and Pn of T. pratense, but did not affect its pigment contents (Please see S8 Table). The results indicate that the pollutants acted as a non-competitive inhibitor because they varied the Pn of plants (which is equivalent to the maximum enzymatic reaction rate in the Michaelis-Menten model). Combining with the above-mentioned relationship between individual gross photosynthesis and PAR following the Michaelis-Menten model [21], that is, our results were similar to the literature [21]. And, the NIMM was suitable for reasonably predicting the relationships of Pn, PAR, and I.
To compare the three Michaelis kinetics (i.e., non-competitive, competitive, and un-competitive inhibition), we integrated the pollution factor into the MM in different ways, and performed mathematical fitting using our pot-culture experimental data for T. pratense. The result for un-competitive inhibited Michaelis Both AIC values were greater than 242.5 (i.e., the AIC of the NIMM). We also tested the two models (UIMM and CIMM) using our pot-culture experimental data for T. pratense; the results are shown in Table 5. Based on the UIMM, it's unreasonable that the φ 0 increased but the calculated P m decreased with the increasing phenol pollution. Based on the CIMM, we performed paired samples t test analysis, and the results showed that the calculated P m was significant higher than the measured P m (t = -5.184, df = 3, P 2-tailed = 0.014), i.e., the calculated P m deviated greatly from the measured P m . So, the UIMM and CIMM were both unsuitable for predicting the relationship of Pn, PAR, and I. The NIMM, however, was suitable for predicting the relationship of Pn, PAR, and I because the calculated P m values were close to the measured P m (Table 3), and the fitted results were close to measured data (Fig 3). Interestingly, pollutants play a role in the inhibition of photosynthetically related enzyme activity; the K i decreased with the combination of the pollutant with the photosynthetically related enzyme. The mathematical fitting results (Table 2) indicate that W. trilobata is tolerant of Cu pollution [51].
Finally, we put forward a perspective that the field investigation still needs to be further done for model validation. The published results [22][23][24][25] and the present study showed that the pollution factor could affect the PI curve in controlled experiment. In natural environment, many other uncontrolled variables such as temperature, humidity, CO 2 concentrations and so on, can also affect photosynthetic parameters. Therefore, it is important to justify and reveal the accuracy of the NIMM in practice.
Supporting Information S1