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)      , which only presents an overall trend in Pn increasing with PAR, or a linear relationship at low light intensity. In general, Pn initially increases as the solar radiation increases, and levels off and reaches light saturation at high radiation, and the response curve of Pn to PAR is a logarithmic function    . However, high light can inhibit photosynthesis   . When plants are exposed to more light than they can utilize, a phenomenon called photoinhibition appears   and the photosynthetic rate decreases so that the relationship between Pn and PAR is a quadratic function, particularly under high temperature  or drought stress    .
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  . Maximum quantum efficiency of PSII photochemistry (Fv/Fm) is the most frequently used parameter   . Pn of Leymus chinensis among different levels of drought stresses was significantly correlated with Fv/Fm  , but Pn had no correlation with Fv/Fm among broad-leaved trees  . The diurnal variation of Pn of alfalfa in different irrigation treatments was no significant correlation with that of Fv/Fm  , and similar result was obtained in the studies of arid sandy shrub   .
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)  , which means the maximum light energy directly used in photochemistry reactions of plants. In order to further explore the universality of this law, Pn, PAR and the chlorophyll fluorescence parameters for different plant species, growth stages and irrigation treatments of the same plant species were examined simultaneously.
2. Materials and Methods
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.
2.1. Plants and Dates Selected
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 Table 1.
2.2. Design of Irrigation
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 Table 2.
2.3. Measurements of Pn and PAR
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.
2.4. Measurements of Chlorophyll Fluorescence Parameters
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  . The chlorophyll fluorescence parameters and Pn were simultaneously measured. Another three typical plants sampling for each treatment were selected for measuring chlorophyll fluorescence parameters. The method of the sampling leaves selected was the same as that for measuring Pn.
Table 1. Date of determination and the growth period of plants.
Table 2. Irrigation scheme for alfalfa. W0, W1, W2 and W3 represent 0 time, 1 time, 2 times and 3 times irrigations for each harvest respectively.
2.5. Measurements of Soil Water
The soil water content was measured with a time-domain-reflectometry (TDR) system  . Observations were performed at intervals of 20 cm to a depth of 0 - 280 cm every 10 days with three duplicates for each treatment. The soil water content was measured only for alfalfa and maize in the experiment station, not for sunflower and daylily in the farmer land.
2.6. Statistical Analysis
Significance test and correlation analysis were carried out through SAS statistical software.
3.1. The Relationship between Pn and PAR
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 (Figure 1). The relationship between Pn and PAR was a significantly quadratic function with the determination coefficient (R2) of 0.72 - 0.94 (p < 0.01).
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) (Figure 1). The difference of Pn and light saturation of the plant species may be due to different soil moisture.
The response curves of Pn to PAR were different for different growth stages of maize (Figure 2). At the seedling stage, Pn increased with rise of PAR and no light saturation was observed, which showed the relationship between Pn and PAR was a logarithmic function with R2 of 0.97 (p < 0.01). At the elongation and huge bellbottom stages, Pn increased initially and then decreased as the increase of PAR, i.e. the light saturation occurred, which showed the relationship between Pn and PAR was a quadratic function with R2 of 0.92 and 0.94, respectively
Figure 1. Relationships between diurnal variation of net photosynthetic rate (Pn) and photosynthetic active radiation (PAR) for different plant species (2005-06-22).
Figure 2. Relationships between diurnal variation of net photosynthetic rate (Pn) and photosynthetic active radiation (PAR) for maize at different growth stages.
(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 (Figure 3). Under low soil water conditions (W0 and W1) which meant alfalfa might be at risk of drought stress, Pn increased initially and then fell with increasing of PAR, i.e. the light saturation occurred, which demonstrated a quadratic function relationship between Pn and PAR with R2 of 0.74 and 0.88, respectively (p < 0.01). Under suitable soil water conditions (W2 and W3) which meant there might be no drought stress to alfalfa, Pn increased with rise of PAR, which presented the relationship between Pn and PAR was a logarithmic function with R2 of 0.88 (p < 0.01) (Figure 3). The aggravation of drought stress to alfalfa significantly lowered the light saturation point. The light saturation point was 984 μmol∙m−2∙s−1 and 1431 μmol∙m−2∙s−1 for W0 and W1, respectively, and there were no light saturations observed for W2 and W3.
Figure 3. Relationships between diurnal variation of net photosynthetic rate (Pn) and photosynthetic active radiation (PAR) for alfalfa among different irrigation treatments (2005-07-23).
3.2. The Relationship between Pn and Fv/Fm
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 (Figure 4) or the different irrigation treatments of alfalfa (Figure 5). Pn was significantly correlated with Fv/Fm only at 12:00 and 14:00 with R2 of 0.63 ~ 0.84 (p < 0.01), but Pn was not significantly correlated with Fv/Fm at the other observation moments, i.e. at 6:00, 8:00 and 10:00 in the morning and at 16:00 and 18:00 in the afternoon.
Mean diurnal values of Pn among different species were no correlation with those of Fv/Fm (Figure 4), but mean diurnal values of alfalfa among different irrigation treatments were significantly positive correlation with Fv/Fm with R2 of 0.84 (p < 0.01) (Figure 5).
3.3. The Relationship between Pn and PAR × 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 (Figure 6). The slopes and intercepts of the linear functions obtained for alfalfa also increased with irrigation times (Figure 8) which indicated that the slope of the linear function can reflect the extent of drought stress to plants.
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) (Figure 9), and Pn of alfalfa among different irrigation treatments also showed the same correlation (p < 0.01 or p < 0.05) (Figure 10).
Figure 4. Relationships between net photosynthetic rate (Pn) and maximum quantum efficiency of PSII photochemistry (Fv/Fm) among different plant species (maize, sunflowers, day lily and alfalfa) at the different observation moments of daytime (2005-06-22).
Figure 5. Relationships between net photosynthetic rate (Pn) and maximum quantum efficiency of PSII photochemistry (Fv/Fm) among different irrigation treatments at the different observation moments of daytime (2005-07-23).
Figure 6. Correlation of diurnal variation of net photosynthetic rate (Pn) and PAR × Fv/Fm for different plant species (2005-06-22).
Figure 7. Correlation of diurnal variation of net photosynthetic rate (Pn) and PAR × Fv/Fm for maize at different growth stages.
Figure 8. Correlation of diurnal variation of net photosynthetic rate (Pn) and PAR × Fv/Fm for alfalfa among different irrigation treatments.
Figure 9. Correlation of photosynthetic rate (Pn) and PAR × Fv/Fm among different species at the different observation moments of daytime (2005-06-22).
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 (Figure 9 and Figure 10).
Figure 10. Correlation of net photosynthetic rate (Pn) and PAR × Fv/Fm among different irrigation treatments of alfalfa at the different observation moments of daytime (2005-07-23).
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   . Under proper water conditions for plants, however, Pn and PAR had a logarithmic function which was also in consistent with many 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     . This was because Pn-light response curves measured with the artificial light source were usually conducted from 9:00 to 11:00 when high temperature or drought stress to the plants had not occurred yet.
Several studies revealed that Pn-light response curves simulated were a rectangular hyperbola   or a non-rectangular hyperbola     . However, both the rectangular hyperbola and non-rectangular hyperbola could neither be used to directly estimate the light saturation point, nor explain the decline of Pn caused by the photoinhibition of plants     .
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  . At the different observation monuments in the morning (6:00, 8:00 and 10:00) and in the afternoon (16:00 and 18:00), Pn of plants had no correlation with Fv/Fm among the different species and the different irrigation treatments of alfalfa, respectively. Zheng & Shangguan (2006) also found that there was no correlation between Pn and Fv/Fm among various broad-leaved trees at the observation moments between 9:00 and 10:30  . At midday (12:00 and 14:00), however, there was a positive relationship between Pn and Fv/Fm among the different species and the different irrigation treatments of alfalfa. That is to say, the substantial decline of Fv/Fm due to severe environmental stresses from high light intensity and high temperature at midday made significant difference of Fv/Fm could be only observed among the different species and the different soil moisture conditions, respectively, because the difference of Fv/Fm among the different treatments was very small in the morning and in the afternoon.
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  . Under proper moisture conditions, plants may absorb and utilize most light energy, and keep the primary photochemical efficiency higher in order to maintain the photosynthesis higher.
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    . On the whole, the diurnal pattern of Pn of plants was in consistent with that of PAR × Fv/Fm (Figure 11). Similar researches had demonstrated that Pn of plants was significantly correlated with the product
Figure 11. Diurnal variation of photosynthetic rate (Pn), maximum yield of primary photochemistry (Fv/Fm), photosynthetic active radiation (PAR) and PAR × Fv/Fm for different species (2005-06-22).
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)    , but this linear relationship was obtained without the environmental stresses to plants   , and had been reported to become non-linear when plants were subject to the environmental stresses    such as strong solar radiation  and drought   .
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).
 Gao, C., Yan, W.D., Tian, D.L. and Zheng, W. (2011) Diurnal Change in Photosynthetic Rate of Eucommia ulmoides and Its Relationship with Environmental Factors. Journal of Central South University of Forestry & Technology, 31, 100-104.
 He, A.N. and Yao, Y. (2002) Analysis of Net Photosynthetic Rate, Transpiration Rate Change and Its Influencing Factors of Saxifrage in Winter. Southwest China Journal of Agricultural Sciences, 24, 1298-1302.
 Zhao, F.H., Wang, Q.F., Wang, J.L., Wang, J.S., Ou, Y.Z. and Yu, G.R. (2011) Photosynthesis-Transpiration Coupling Mechanism of Wheat and Maize during Daily Variation. Acta Ecologica Sinica, 31, 7526-7532.
 Guo, X.Y., Zhou, C., Yang, Y.F., Li, J.D. and Bao, G.Z. (2005) Characteristic on Physiological Ecology of Photosynthesis of Puccinellia hinampoensis in Alkalized Meadow in Songnen Plains of China. Journal of Northeast Normal University (Natural Science Edition), 37, 73-76.
 Li, F.X., Guo, J.P. and Gao, S.H. (2006) Impacts of CO2 Concentration and Radiant Intensity on C. intermedia Photosynthetic Rate under Different Soil Moisture with a Simulation Condition. Journal of Agro-Environment Science, 25, 81-85.
 Powles, S.B. and Thorne, S.W. (1981) Effect of High-Light Treatments in Inducing Photoinhibition of Photosynthesis in Intact Leaves of Low-Light Grown Phaseolus vulgaris and Lastreopsis microsora. Planta, 152, 471-477.
 Maria, R., Ruthl, A., Nikos, L. and Richardj, G. (2008) Photoinhibition of PSII in Emiliania huxleyi (Haptophyta) under High Light Stress: The Roles of Photoacclimation, Photoprotection, and Photorepair. Journal of Phycology, 44, 670-683.
 Grams, T.E.E. and Thiel, S. (2002) High Light-Induced Switch from C3-Photosynthesis to Crassulacean Acid Metabolism Is Mediated by UV-A/Blue Light. Journal of Experimental Botany, 53, 1475-1483.
 Zhou, Y.H., Lan, H.M. and Zhang, J.H. (2007) Inhibition of Photosynthesis and Energy Dissipation Induced by Water and High Light Stresses in Rice. Journal of Experimental Botany, 58, 1207-1217.
 Li, L.Z., Zhang, D.G., Xin, X.P., Yan, Y.C., Yang, G.X., Li, J. and Wang, X. (2009) Photosynthetic Characteristics of Leymus chinensis under Different Soil Moisture Grades in Hulunber Prairie. Acta Ecologica Sinica, 9, 5271-5279.
 Krause, G.H. and Weis, E. (1991) Chlorophyll Fluorescence and Photosynthesis: The Basics. Annual Review of Plant Physiology and Plant Molecular Biology, 42, 313-349.
 Liu, H.F., Gao, Y.B., Zhang, Q., Li, X. and Li, G.L. (2005) Effects of Soil Drought Stress on Photosynthesis and Chlorophyll Fluorescence for Different Leymus chinensis Populations. Journal of Agro-Environmental Science, 24, 209-213.
 Sun, D.B and Wang, Q.S. (2012) Effects of Water on the Photosynthetic Characteristics of Alfalfa (Medicago sativa). Chinese Journal of Plant Ecology, 36, 72-80.
 Jiang, G.M and Zhu, G.J. (2001) Effects of Natural High Temperature and Irradiation on Photosynthesis and Related Parameters in Three Arid Sandy Shrub Species. Acta Phytoecologica Sinica, 25, 525-531.
 Kitajima, M. and Butler, W. (1975) Quenching of Chlorophyll Fluorescence and Primary Photochemistry in Chloroplasts by Dibromothymoquinone. Biochimica et Biophysica Acta—Bioenergetics, 376, 105-115.
 Laurent, J.P., Ruelle, P., Delage, L., Zairi, A., Nouna, B.B. and Adjmi, T. (2005) Monitoring Soil Water Content Profiles with a Commercial Tdr System: Comparative Field Tests and Laboratory Calibration. Vadose Zone Journal, 4, 1030-1036.
 Habermann, G., Machado, E.C., Rodrigues, J.D. and Medina, C.L. (2003) CO2 Assimilation, Photosynthetic Light Response Curves, and Water Relations of “Pera” Sweet Orange Plants Infected with Xylella Fastidiosa. Brazilian Journal of Plant Physiology, 15, 79-87.
 Liu, Y.Q., Sun, X.Y., Wang, Y. and Liu, Y. (2007) Effects of Shades on the Photosynthetic Characteristics and Chlorophyll Fluorescence Parameters of Urtica dioica. Acta Ecologica Sinica, 27, 3457-3464.
 Tang, Y.J. and Yuan, Z.P. (2012) Photosynthetic Dynamic Variation and Its Relationship with the Eco-Physiological Factors in Caryota Urens and Caryota Mitis. Acta Botanica Boreali-Occidentalia Sinica, 32, 995-1001.
 Kyei-Boahen, S., Lada, R., Astatkie, T., Gordon, R. and Caldwell, C. (2003) Photosynthetic Response of Carrots to Varying Irradiances. Photosynthetica, 41, 1-5.
 Valladares, F., Dobarro, I., Sanchez-Gomez, D. and Pearcy, R.W. (2005) Photoinhibition and Drought in Mediterranean Woody Saplings: Scaling Effects and Interactions in Sun and Shade Phenotypes. Journal of Experimental Botany, 56, 411.
 Ogren, E. and Evans, J.R. (1993) Photosynthetic Light-Response Curves: I. The Influence of Carbon Dioxide Partial Pressure and Leaf Inversion. Planta, 189, 182-190.
 Stirling, C.M., Aguilera, C., Baker, N.R. and Long, S.P. (1994) Changes in the Photosynthetic Light Response Curve during Leaf Development of Field Grown Maize with Implications for Modelling Canopy Photosynthesis. Photosynthesis Research, 42, 217-225.
 Koyama, K. and Kikuzawa, K. (2010) Geometrical Similarity Analysis of Photosynthetic Light Response Curves, Light Saturation and Light Use Efficiency. Oecologia, 164, 53-63.
 Colom, M.R. and Vazzana, C. (2003) Photosynthesis and PSII Functionality of Drought-Resistant and Drought-Sensitive Weeping Lovegrass Plants. Environmental and Experimental Botany, 49, 135-144.
 Evans, J.G., Jakobsen, I. and Ogren, E. (1993) Photosynthetic Light-Response Curves. Gradients of Light Absorption and Photosynthetic Capacity. Planta, 189, 191-200.
 Hassan, I.A. (2002) Effects of Water Stress and High Temperature on Gas Exchange and Chlorophyll Fluorescence in Triticum aestivum L. Photosynthetica, 44, 312-315.
 Genty, B., Briantais, J.M. and Baker, N.R. (1989) The Relationship between the Quantum Yield of Photosynthetic Electron Transport and Quenching of Chlorophyll Fluorescence. Biochimica et Biophysica Acta—General Subjects, 990, 87-92.
 Ogaya, R. and Penuelas, J. (2003) Comparative Seasonal Gas Exchange and Chlorophyll Fluorescence of Two Dominant Woody Species in a Holm Oak Forest. Flora, 198, 132-141.
 Fryer, M.J., Andrews, J.R., Oxborough, K., Blowers, D.A. and Baker, N.R. (1998) Relationship between CO2 Assimilation, Photosynthetic Electron Transport, and Active O2 Metabolism in Leaves of Maize in the Field during Periods of Low Temperature. Plant Physiology, 116, 571-580.
 Tsuyama, M., Shibata, M. and Kobayashi, Y. (2003) Leaf Factors Affecting the Relationship between Chlorophyll Fluorescence and the Rate of Photosynthetic Electron Transport as Determined from CO2 Uptake. Journal of Plant Physiology, 160, 1131-1139.
 Flexas, J., Escalona, J.M. and Medrano, H. (1999) Water Stress Induces Different Levels of Photosynthesis and Electron Transpiration Rate Regulation in Grapevines. Plant Cell & Environment, 22, 39-48.
 Massacci, S.M., Nabiev, L., Pietrosanti, S.K., Nematov, T.N., Chernikova, K., Thor, J. and Leipnerd, J. (2008) Response of the Photosynthetic Apparatus of Cotton (Gossypium hirsutum) to the Onset of Drought Stress under Field Conditions Studied by Gas-Exchange Analysis and Chlorophyll Fluorescence Imaging. Plant Physiology and Biochemistry, 46, 189-195.