Photosynthetic rate ( Pn ) of plants is simultaneously affected by photosynthetically active radiation ( PAR ) and maximum yield of primary photochemistry ( Fv / Fm ). In order to explore the quantitative relationship between Pn , PAR and Fv / Fm , those parameters were simultaneously measured for different plant species (maize, sunflower, daylily and alfalfa), growth stages and irrigation treatments. Results indicated that the diurnal variation of Pn had no significant correlation with that of Fv / Fm . Mean diurnal values of Pn were linearly correlated with those of Fv / Fm among the different irrigation treatments of alfalfa ( p < 0.05), but this linear correlation was not observed among the different species. There was a positive relationship between Pn and Fv / Fm only at midday (12:00 and 14:00) ( p < 0.01). A significant linear relationship was observed between the diurnal variation of Pn and PAR × Fv / Fm ( p < 0.05), this law was suitable for the different species, and the different growth stages and the different irrigation treatments of the same species. This study confirms that Pn is significantly related to the photochemical energy ( PAR × Fv / Fm ), the light energy directly used in photochemical reactions of plants.
The power driving photosynthesis in plants is light which mainly comes from solar radiation. Many studies have documented a positive relation between the net photosynthetic rate (Pn) and photosynthetically active radiation (PAR) [
The rate of photosynthesis of plants is not only related with the intensity of PAR, but also affected by the efficiency of light quantum chemistry [
Actually, Pn of plants is simultaneously affected by PAR and Fv/Fm. Our study found a positive correlation between Pn of alfalfa and the product of PAR and Fv/Fm (PAR × Fv/Fm) [
This study was conducted at Jiazhuang, a village of Hunyuan County, Shanxi Province (39˚53'N and 113˚32'E). The site is located in the northeast of Loess Plateau. The altitude is 1091.9 m above sea level. It is a temperate continental semi-arid monsoon climate with mean annual temperature of 6.2˚C, precipitation of 436.2 mm, water evaporation of 1828 mm, sunshine hours of 2700 h and frost-free period of 110 ~ 140 days. The soil type is kastanozems rich in fine sand. The top soil at 0 ~ 20 cm depth contained 11.2 g/kg of organic matter, 8.1 of pH, 24 mg/kg of available P and 101.1 mg/kg of exchangeable K.
Maize (Zea mays L.), sunflower (Helianthus annulus L.), daylily (Hemerocallis fulva L.) and alfalfa (Medicago sativa L.) were selected for this study. The local cultivated maize variety Yongfeng 1# and alfalfa variety Ameristand 210+Z introduced from US were selected. Sunflower and daylily were planted by local farmers. Alfalfa was sown in July 2003 with seeding rate 15 kg∙ha−1, and applied nitrogen 13.8 kg∙N∙ha−1∙year−1, phosphorus 105 kg∙P2O5∙ha−1∙year−1. Maize was sown in May 2004 with density of 55000 plants∙ha−1 and chemical fertilizer application was 300 kg∙N∙ha−1 and 90 kg∙P2O5∙ha−1. Sunny days were selected to observe for this work. The days selected are shown in
There were 4 levels for alfalfa irrigation procedures including irrigated 0 time, 1 time, 2 times and 3 times for each harvest expressed by W0, W1, W2 and W3, respectively. The irrigation quota for each time was 75 mm controlled by water meter. Border irrigation method was employed. The irrigation scheme is shown in
Pn and PAR were measured with Li-6400 portable photosynthetic system using the natural light source. Three typical plants sampling for each treatment were selected for the measurements. Maize, sunflower and daylily were determined on the middle of the first fully expanded leaf on the top of the plants, and alfalfa was determined on the middle leaflet of the first three fully expanded leaves on the top of plants. The measurements were operated in 2 h interval from 6:00 to 18:00. The light saturation point was determined by PAR corresponding to the maximum photosynthetic rate.
The chlorophyll fluorescence parameters were observed with Fim-1500 portable chlorophyll fluorescence meters. The leaves were subjected to darkness for 20 - 30 minutes prior to each measurement, and then the initial fluorescence (F0) was measured. A saturating flash light was used to determine the maximal fluorescence (Fm). The variable fluorescence (Fv) and maximum quantum efficiency of PSII photochemistry (Fv/Fm = (Fm − F0)/Fm) were calculated according to Kitajima and Buter [
Date | Growth period of plants | |||
---|---|---|---|---|
Maize | Sunflower | Daylily | Alfalfa | |
2005-06-09 | Seedling stage | |||
2005-06-22 | Jointing stage | Budding stage | Budding stage | Renewable period after the first harvest |
2005-07-23 | Huge bellbottom stage | Beginning of flowering |
Date of irrigation | Irrigation scheme of the first crop | Date of Irrigation | Irrigation scheme of the first crop | ||||
---|---|---|---|---|---|---|---|
2005-04-26 | W1 | W2 | W3 | 2005-06-12 | W1 | W2 | W3 |
2005-05-13 | W3 | 2005-06-27 | W3 | ||||
2005-05-23 | W2 | 2005-07-04 | W2 | ||||
2005-05-26 | W3 | 2005-7-12 | W3 |
The soil water content was measured with a time-domain-reflectometry (TDR) system [
Significance test and correlation analysis were carried out through SAS statistical software.
Under non-irrigation condition in the semi-arid region, Pn of maize, sunflower, daylily and alfalfa to changes in PAR had a similar response pattern, which presented an initially rapid rise as the increase of PAR, then a slow rise and a pronounced decline at high light (
Different crop species possessed different maximum Pn and light saturation point. The maximum Pn was ranked maize (25.7 μmol∙m−2∙s−1) > daylily (23.9 μmol∙m−2∙s−1) > sunflower (23.5 μmol∙m−2∙s−1) > alfalfa (10.1 μmol∙m−2∙s−1), and the light saturation point was ranked as the same order as Pn, i.e. maize (1539 μmol∙m−2∙s−1) > daylily (1356 μmol∙m−2∙s−1) > sunflower (1238 μmol∙m−2∙s−1) > alfalfa (984 μmol∙m−2∙s−1) (
The response curves of Pn to PAR were different for different growth stages of maize (
(p < 0.01). The occurrence of the light saturation of maize was related to drought stress because the leaves wilted due to lower soil moisture at the elongation and huge bellbottom stages. Light saturation point of maize decreased with drought stress intensifying. The light saturation point of maize was 1539 μmol∙m−2∙s−1 at the elongation stage with soil water content of 16.8%, and reduced to 1343 μmol∙m−2∙s−1 at the huge bellbottom stage with soil water content of 15.1% comparing to that of 17.1% at the seedling stage.
Water is a very important factor influencing Pn of plants. Relationships between Pn of alfalfa and PAR showed marked differences for different irrigation treatments (
For different plant species (maize, sunflowers, daylily and alfalfa), maize at different growth stages and alfalfa under different irrigation treatments, the diurnal variation of Pn had no significant correlation with that of Fv/Fm (data not shown).
At the different observation moments of daytime, however, there were different relationships between Pn and Fv/Fm among the different species (
Mean diurnal values of Pn among different species were no correlation with those of Fv/Fm (
For the different species, maize at the different growth stages and alfalfa under different irrigation treatments, the diurnal variation of Pn was significantly correlated with the product of Fv/Fm and PAR (PAR × Fv/Fm) with R2 of 0.62 ~ 0.91 (p < 0.01 or p < 0.05) (Figures 6-8). The linear functions obtained for the different species highlighted different slopes, which of alfalfa was the smallest due to the low soil moisture (
At the different observation moments of the daytime, Pn among the different species had a significant positive correlation with PAR × Fv/Fm (p < 0.01) (
Mean diurnal values of Pn were a significant positive correlation with those of PAR × Fv/Fm among the different species and different irrigation treatments of alfalfa, respectively (p < 0.01). The slopes of the linear functions obtained at 10:00 - 16:00 were lower which indicated that drought stress was even more pronounced at this period, especially at 12:00 and 14:00 (
The quantitative relationships between Pn of plants and PAR were unfixed in the fields condition. Under drought stress, Pn and PAR showed a quadratic function which result was in good accordance with some studies [
Photosynthetic response curves to light were different with artificial light source (red and blue light) and natural light source. A logarithmic curve of Pn was usually obtained with the artificial light, and no reduction of Pn was observed even with very high light intensity. However, the results determined with natural light sources revealed that obvious midday depression of photosynthesis occurred [
Several studies revealed that Pn-light response curves simulated were a rectangular hyperbola [
There was no linear correlation between diurnal variation of Pn and that of Fv/Fm of plants. This was because the diurnal changes of Pn and Fv/Fm were not synchronous with rise of PAR. In low light conditions, Pn rapidly increased while Fv/Fm slowly reduced with increase of PAR. In high light conditions, Pn slowly increased while Fv/Fm rapidly dropped with increase of PAR [
Mean diurnal values of Pn among different irrigation treatments of alfalfa were linearly correlated with those of Fv/Fm, but this linear correlation was not observed among different plant species. This finding further confirmed that soil moisture was critical to the photosynthesis, as our previous study had proved that both Pn and Fv/Fm of alfalfa had significant positive correlation with soil moisture content [
A significant linear relationship was observed between the diurnal variation of Pn of plants and the product of Fv/Fm and PAR (PAR × Fv/Fm). This law was suitable for the different species, and the different growth stages and the different soil water conditions of the same species. This is because Pn is not only related to PAR and Fv/Fm, but also more importantly to the light energy directly used in photochemical reactions of plants, i.e. photochemical energy. The product of PAR and Fv/Fm (PAR × Fv/Fm) can reflect the maximum photochemical energy consumed in photosynthesis. When light intensity is low, majority of solar radiation energy is absorbed and used for the photochemical reaction in plants, which reveals the photochemical efficiency is high, but the total energy used for the photochemical reactions is actually small, so that Pn is relatively low. When the light intensity is increasing, more and more solar radiation is used for fluorescence emission and heat dissipation of plants themselves, so that the proportion of light energy being absorbed and used for the photochemical reactions is relatively fall, which makes Fv/Fm (the photochemical efficiency) relatively lower, but the increase of light intensity can compensate the reduction of the photochemical efficiency in PSII, so that the total amount of energy used for photochemical reactions is still increasing, so Pn of plants is also rising as a result. When light intensity at midday is enough strong and exceeds the capability of light energy utilization of plants, the photosynthesis of plants may be inhibited, majority of the solar radiation is not used for the photosynthesis but for the fluorescence emission and heat dissipation of plants themselves, which makes Fv/Fm (the photochemical efficiency) dramatically drop, so the total energy used for the photochemical reactions declines which leads to the reduction of Pn. This phenomenon was particularly true when the plants are exposed to the environmental stresses [
of photosynthetic flux density (PFD) and the effective quantum yield of PSII in the illuminated leaf had demonstrated that Pn of plants was significantly correlated with the product (ΔF/Fm’) (PFD × ΔF/Fm’) (p < 0.01) [
This study was supported by the Science and Technology Innovation Project of Chinese Academy of Agricultural Sciences (No. CAAS-XTCX2016019), the National Natural Science Foundation of China (31401344) and the 12th five-year plan of National Key Technologies R&D Program (No. 2012BAD09B01).
Sun, D.B. and Wang, Q.S. (2018) Linear Relationships between Photosynthetic Rate and Photochemical Energy Expressed by PAR × Fv/Fm. American Journal of Plant Sciences, 9, 125-138. https://doi.org/10.4236/ajps.2018.92011