Study on the Daily Change of Photosynthetic Rate for Prunus domestica × armeniacain Different Seasons

[Objective] The aim of this study was to explore the daily change of photosyntheticratefor Prunus domestica ×armeniacain different growing seasons. The study can provide theoretical basis for arid area high yield and quality cultivation.[Method] The photosynthetic physiological properties of leaves of different types of Prunus domestica × armeniaca were measured by the Li-6400 portable photosynthesis system indifferent seasons. By this method could analysis of photosyntheticcharacteristicsfor different types of Prunus domestica×armeniaca in different seasons.[Result] Daily change of photosyntheticrate（Pn） for Prunus domestica×armeniaca in differentseasons showed a ＂double-peak＂ curve. The peak values were at 10：00 and16：00. The Pn of ‘Fengweihuanghou＇, ‘Konglongdan＇, ‘Weihou＇, ‘Weiwang＇ and‘Weidi＇ reached the maximum in July, theywere 13.75, 14.76, 12.96, 13.3, and 11.9μmol/（m^2·s）, respectively. The Pn of Prunus domestica×armeniaca reached minimumin August, they were 9.78, 10.71, 12.02, 10.43 μmol/（m^2·s）. The Pn overall average of ‘Konglongdan＇ was highest,it reached 12.65 μmol/（m^2·s）.The Pn overall average of ‘Weiwang＇ was lowest, it reached 11.31μmol/（m^2·s）. There were extremely significant positive correlation between the Pn and Gs（P0.01）. [Conclusion] Daily change of photosyntheticrate for Prunus domestica ×armeniaca in differentseasons showed a ＂double-peak＂ curve, showing significant phenomenon of ＂midday depression＂.The photosynthesis intensity of Prunus domestica ×armeniaca was strongest in July, and the photosynthesis intensity was weakest in August. ‘Konglongdan＇showed the strongest photosynthesis capacity, ‘Weihou＇ and ‘Weiwang＇, followed.There were highest correlation between the Pn and stoma conductance（Gs）.

Post Cryopreservation Growth Kinetic and Photosynthetic Assessment of an Acid Tolerant Strain of Stichococcus bacillaris

Preserving microbial diversity has become a strategic undertaking. Thus, ex situ microalgal culture conservation results in strategic and functional resource in both biodiversity protection and application domains. Cryopreservation of microalgae has been practiced since the 1960s and is now considered the optimal preservation strategy. Furthermore, the overall monitoring during growth of cultures after freezing/thawing protocols was hardly investigated and there is poor evaluation related to preserve especially the photosystem apparatus. The present study focuses on Stichococcus bacillaris as case study for short-term cryopreservation at −80 °C storage. Various freezing pretreatments using cryoprotective agents, and two thawing methods were compared introducing a novel variable to evaluate viability recovery and assessing growth kinetics of cultures immediately after thawing and after a series batch cultivation. Photosynthetic rate and pigments assessment were proposed to evaluate hidden metabolic cell damage. Results underline cryoprotective agents can increase the kinetic recovery of preserved cells in terms of reduction of lag phase during batch cultivation tests: the use of dimethyl sulfoxide and glycerol granted a growth comparable to unpreserved cells when sudden thawing occurs after 24 hours of storage, but recovery after preservation is less sensitive to cryoprotective agents when gradual thawing and 1 month of storage is considered. However, cells are always able to restore their physiological pathways even without agents, so their kinetic effect has been proved and quantified. Interestingly, both the photosynthetic efficiency and the ratio between total chlorophyll and carotenoids are comparable (0.75 Fv/Fm, 2.2 ± 0.25 g/g) to unpreserved cells and they are unsensitive to chosen agents, but the ratio between chlorophyll a and chlorophyll b was clearly altered (up to 10 times), suggesting that photoactive pigments relative proportions can result in similar growth kinetic performances. Long-term studies will be carried out to assess whether the differences found could cause chronic damage to photosystem efficiency of S. bacillaris cultures.

Linear Relationships between Photosynthetic Rate and Photochemical Energy Expressed by <i>PAR</i>×<i>Fv/Fm</i>

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.

Comparison of the S-, N- or P-Deprivations’ Impacts on Stomatal Conductance, Transpiration and Photosynthetic Rate of Young Maize Leaves

Seven-day-old maize (Zea mays) plants were grown hydroponically for ten days in deprived nutrient solutions against the corresponding control grown under full nutrition;the effects of S-, N- or P-deprivation on laminas’ mean stomatal conductance (gs), transpiration rate (E) and photosynthetic rate (A) were monitored, along with the impact on the laminas’ total dry mass (DM), water amount (W), length and surface area (Sa). Furthermore, a time series analysis of each parameter’s response ratios (Rr), i.e. the treatment’s value divided by the corresponding control’s one, was performed. Under S-deprivation, the Rr of laminas’ mean gs, E, and A presented oscillations within a ±15% fluctuation zone, notably the “control” zone, whilst those of laminas’ total DM, water amount, surface area, and length included oscillation during the first days and deviation later on, presenting deviation during d10. Under the N-deprivation conditions all Rr time courses except the A one, included early deviations from the control zone without recovering. The deviation from the control zone appeared at d4. Under P-deprivation, all Rr time courses represented oscillations within the control zone. P-deprivation’s patterns resembled those of S-deprivation. Compared to the one of the S-deprivation, the P-one’s oscillations took place within a broader zone. Linear relationships among the various Rr patterns were found between gs-E, gs-A, E-A, DM-W and DM-Sa. In conclusion, the impact of P-deprivation appeared in an early stage and included an alleviation action, the one of N-deprivation appeared early with no alleviation action, whilst that of S-deprivation appeared later, being rather weaker when compared to the impact of the P-deprivation’s impact.

Preliminary Study on the Photosynthetic Characteristics of Super Hybrid Rice P88S/0389

[Objective] The aim of this study is to understand the high yield often photosynthetic mechanism of super hybrid rice P88S/0389.[Method] Chlorophyll and carotenoid contents were measured via the method of Wellburn and Lichtenthaler,and the photosynthetic rate and chlorophyll fluorescence parameters were measured using portable photosynthesis system.Through comparison of above measured indices between P88S/0389 and its parents,the photosynthetic characteristics were obtained.[Result] The pigment content of P88S/0389 lied between their parents,and detailedly was closer to its female parent at earlier growth stage and to its male parent at latter stage,respectively.The Pn,Fv/Fo,Fv/Fm and qP of the flag leaf of P88S/0389 were all higher than those of its parents,while the degradation rates of these parameters were slower than those of its parents.The qN of flag leaf was lower than those of its parents,and the increasing rate of qN of P88S/0389 was also slower than those of its parents,indicating the great heterosis of P88S/0389 to its parents in photosynthetic capacity.The data show that P88S/0389 is endowed with stronger capacities in light energy absorption,transformation and carbon fixation compared to its parents.[Conclusion] The light energy absorbed by hybrid mainly contributes to photochemical reaction,and little for heat dissipation.For ensuring high yield,the rice cultivars selected for production should synchronously possess higher photosynthetic rate and longer high value duration.

Leaf Photosynthetic Rate and Related Influencing Factors in Super High-yield Combination C Liangyou H255

[ Objective] Study on the photosynthesis and influencing factors in super high-yield combination C Liangyou H255. [ Method] The photosynthetic characteristies were measured at a hot and muggy day under natural conditions. [ Result] A respective single peak at 11：30 was observed in diurnal variation curves of net photosynthetic rate （Pn） and transpiration rate（Tr）. Correlation analysis shows that Pn presents an extremely significant correlation with photon flux densities （PFD） of photosynthetically active radiation, in comparison assumes a significant correlation either with stomatal conductance （Gs） or with ambient CO2 concentration （Ca）. [ Conclusion] Gs followed by PFD presented most influence on Pn of super high-yield combination C Liangyou H255.

Nitrogen in Relation to Photosynthetic Capacity and Accumulation of Osmoprotectant and Nutrients in Brassica Genotypes Grown Under Salt Stress

Different strategies of the application of nutrients are required to overcome the adverse effects of mustard （Brassica juncea L.） in response to NaCl stress. The objective of the present study was to determine if different added levels of nitrogen （N） in growth medium could alleviate the adverse effects of salt stress on photosynthetic capacity and accumulation of osmoprotectants and nutrients. 14 days mustard seedlings of salt-sensitive （cv. Chuutki） and salt-tolerant （cv. Radha） genotypes were fed with： （i） 0 mmol L^-1 NaCl ＋ 0 mg N kg^-1 sand （control）, （ii） 90 mmol L^-1 NaCl ＋ 30 mg N kg^-1 sand, （iii） 90 mmol L^-1 NaCl ＋60 mg N kg^-1 sand, （iv） 90 mmol L^-1 NaCl ＋90 mg N kg^-1 sand and （v） 90 mmol L^-1NaCl＋ 120 mg N kg^-1 sand. Under the condition of salinity stress, N application caused a significant ameliorative effect on both genotypes with respect to growth attributes [fresh weight （FW） and dry weight （DW）] and physio-biochemical parameters [percent water content （WC）, net photosynthetic rate （PN）, stomatal conductance （gs）, total chlorophyll （Ch1）, carbonic anhydrase （CA） activity and malondialdehyde （MDA）, nitrogen （N）, potassium （K） and sodium （Na） contents, and K/Na ratio] and yield attributes （number of pods/plant, seeds/pod and seed yield/plant）. The salt-tolerant genotype exhibited maximum value for growth, physio-biochemical and yield attributes at 60 mg N kg 1 sand than that of salt-sensitive genotype. These results suggest that application of N may ameliorate most of the attributes and prove to be a physiological remedy to increase the tolerance against the ill effects of salt stress in Brassicas.

Relationship between plant canopy characteristics and photosynthetic productivity in diverse cultivars of cotton(Gossypium hirsutum L.)

Genotype and plant type affect photosynthetic production by changing the canopy structure in crops.To analyze the mechanism of action of heterosis and plant type on canopy structure in cotton(Gossypium hirsutum L.),we had selected two cotton hybrids(Shiza 2,Xinluzao 43) and two conventional varieties(Xinluzao 13,Xinluzao 33) with different plant types in this experiment.We studied canopy characteristics and their correlation with photosynthesis in populations of different genotypes and plant types during yield formation in Xinjiang,China.Canopy characteristics including leaf area index(LAI),mean foliage tilt angle(MTA),canopy openness(DIFN),and chlorophyll relative content(SPAD).The results showed that LAI and SPAD peak values were higher and their peak values arrived later,and the adjustment capacity of MTA during the flowering and boll-forming stages was stronger in Xinluzao 43,with the normal-leaf,pagoda plant type,than these values in other varieties.DIFN of Xinluzao 43 remained between0.09 and 0.12 during the flowering and boll-forming stages,but was lower than that in the other varieties during the boll-opening stage.Thus,these characteristics of Xinluzao 43 were helpful for optimizing the light environment and maximizing light interception,thereby increasing photosynthetic capability.The photosynthetic rate and photosynthetic area were thus affected by cotton genotype as changes in the adjustment range of MTA,increases in peak values of LAI and SPAD,and extension of the functional stage of leaves.Available photosynthetic area and canopy light environment were affected by cotton plant type as changes in MTA and DIFN.Heterosis expression and plant type development were coordinated during different growth stages,the key to optimizing the canopy structure and further increasing yield.

Comparison of Net Photosynthetic Rate in Leaves of Soybean with Different Yield Levels

A total of nine soybean （Glycine max （L.） Merr.） cultivars were divided into three yield levels which were planted under the same environmental condition. The net photosynthetic rate was measured by LI-6400 portable photosynthesis system. The chlorophyll content and specific leaf weight were measured with regular methods. The results showed that the specific leaf weight, chlorophyll content and net photosynthetic rate of high yield varieties were higher than those of low yield varieties. The yield had a significantly positive correlation with the net photosynthetic rate. With the improvement of modem technology, the net photosynthetic rate could be measured quickly and exactly. Hence, net photosynthetic rate could be used as an effective index in the selection of high yield soybean.

Effects of Chilling Stress on Photosynthetic Rate and Chlorophyll Fluorescence Parameter in Seedlings of Two Rice Cultivars Differing in Cold Tolerance

A cold-tolerant cultivar, Xiangnuo 1, and a cold-sensitive cultivar, IR50, were used to study the influence of chilling on photosynthetic rate and chlorophyll fluorescence parameters in rice seedlings. The photosynthetic rates declined dramatically during chilling, and decreased by 48.7% and 67.5% in Xiangnuo 1 and IR50 seedlings, respectively, after being subjected to chilling treatment for two days. Chlorophyll fluorescence measurements showed that relatively higher qp and qNP in Xiangnuo 1 were maintained to dissipate the redundant excitation energy and protect the reaction centers from chill injury; accordingly, redundant excitation energy accumulated less in the reaction centers, and antenna systems were less injured by chilling in Xiangnuo 1. On the contrary, in IR50, qp and qNP declined rapidly while Eg increased, as the chilling persisted. This result indicated that the reaction centers and antenna systems in IR50 were damaged severely by chilling, which led to the lower photosynthetic rate.

Effect of Nitrogen Fertilizer on Photosynthetic Rate of Leymus chinensis in Grassland of Different Degrading Degrees

[Objective] This study aimed to investigate the effect of nitrogen fertilizer on photosynthetic rate of Leymus chinensis in the grasslands of different degrading degrees. [Method] With the L. chinensis in Inner Mongolia Baiyinxile Ranch as the research object, different rations of nitrogen fertilizer were applied to the grassland （0, 30, 50, 80 g/m^2）. The effect of different gradients of nitrogen fertilizer on photo- synthetic rate of Leymus chinensis, and the effect on grasslands of different degrading degrees were analyzed. [Result] The photosynthetic rate of L. chinensis in- creased with the increase of nitrogen gradients; in the grassland communities with different degrading degrees, the responses of the photosynthetic rate of L. chinensis to nitrogen fertilizer were different, and the response in the grassland with severe degradation was the best. [Conclusion] Nitrogen fertilizer played an important role in enhancing the restoration degree of grassland.

Relationship Between Diurnal Changes of Net Photosynthetic Rate and Influencing Factors in Rice under Saline Sodic Stress

The net photosynthetic rate of flag leaves and influencing factors under saline sodic soil conditions were investigated at the full heading stage of rice. The net photosynthetic rate of rice leaves showed a double-peak curve in a day in both non-saline sodic and saline sodic soil treatments. The first peak of the net photosynthetic rate appeared at 9：00-10：00 and 9：00 in the saline sodic and non-saline sodic soil treatments, respectively, whereas the second peak both at 14：00. The midday depression of the net photosynthetic rate always appeared regardless of non-saline sodic or saline sodic soil conditions. In addition, the net photosynthetic rate significantly decreased in all day under saline sodic conditions compared with that under non-saline sodic conditions. Some differences were observed in correlation characters between the net photosynthetic rate and all influencing factors during 9：00-13：00. Under non-saline sodic conditions, the diurnal changes of the net photosynthetic rate in a day were mainly caused by stomatal conductance, and the limitation value and the stomatal factors served as determinants; whereas under saline sodic stress, the diurnal changes of the net photosynthetic rate in a day were mainly caused by non stomatal factors including light intensity and air temperature.

One-time application of controlled-release bulk blending fertilizer enhances yield,quality and photosynthetic efficiency in late japonica rice

Controlled-release urea(CRU)releases nitrogen(N)at the same pace that rice takes it up,which can effectively improve N use efficiency,increase rice yield and improve rice quality.However,few studies have described the effects of CRU application on the photosynthetic rate and endogenous enzyme activities of rice.Accordingly,a twoyear field trial was conducted with a total of seven treatments:CK,no N fertilizer;BBF,regular blended fertilizer;RBBF,20%N-reduced regular blended fertilizer;CRF1,70%CRU+30%regular urea one-time base application;CRF2,60%CRU+40%regular urea one-time base application;RCRF1,CRF1 treatment with 20%N reduction;and RCRF2,CRF2 treatment with 20%N reduction.Each treatment was conducted in triplicate.The results showed that the N recovery efficiency(NRE)of the controlled-release bulk blending fertilizer(CRBBF)treatments was significantly greater over the two years.There were significant yield increases of 4.1–5.9%under the CRF1treatment and 5.6–7.6%under the CRF2 treatment compared to the BBF treatment,but the differences between the reduced-N treatments RBBF and RCRF2 were not significant.Photosynthetic rates under the CRF1 and CRF2treatments were significantly higher than under the other treatments,and they had significantly greater RuBPCase,RuBisCO,glutamate synthase(GOGAT)and glutamine synthetase(GS)enzyme activities.Additionally,the soil NH_(4)^(+)-N and NO_(3)^(–)-N contents under the CRBBF treatments were significantly higher at the late growth stage of rice,which was more in-line with the fertilizer requirements of rice throughout the reproductive period.CRBBF also led to some improvement in rice quality.Compared with the BBF and RBBF treatments,the protein contents under the CRBBF treatments were reduced but the milling,appearance,eating and cooking qualities of the rice were improved.These results showed that the application of CRBBF can improve the NRE,photosynthetic rate and endogenous enzyme activities of rice,ensuring sufficient N nutrition and photosynthetic material production during rice growth and thereby achieving improved rice yield and quality.

Effects of elevated CO_2 on net photosynthetic rate of trees in Changbai Mountain

Net photosynthetic rates (NPRs) of four species seedlings, Pinus koraiensis, Ptrius Syvestriformis,Fraxinus mandshuthe and Phellodendron amurense, were measured at different CO2 concentrations and time respectively in Changbai Mountain during the growing season in 1999. The seedlings were cultivated in open-top chambers (OTCs), located outdoors and exposed to natural sunlight. The experimental objects were divided into four groups by tree species. CO2 concentrations in chambers were kept at 500 μL-L-1 and 700 μL-L-1 and contrast chamber and contrast field were set. The results showed that the effects of elevated CO2 on NPR of the trees strongly depended on tree species and time. NPRs of Pin us koreaipsis and Pinus syvestriformis seedfings increased with the rising of CO2 concentration, while that of Phellodron amurense and haus mandshurica increased at some time and decreased at another time.

Photosynthetic Characteristics of Flag Leaves in Rice White Stripe Mutant 6001 During Senescence Process

Physiological, biochemical and electron microscopy analyses were used to investigate the photosynthetic performance of flag leaves in rice white stripe mutant 6001 during the senescence process. Results showed that the chlorophyll content at the heading and milk-ripe stages in rice mutant 6001 were about 34.78% and 3.00% less than those in wild type 6028, respectively. However, the chlorophyll content at the fully-ripe stage in rice mutant 6001 was higher than that in wild type 6028. At the heading stage, the net photosynthetic rate （Pn） in rice mutant 6001 was lower than that in wild type 6028. Rice mutant 6001 also exhibited a significantly slower decrease rate of Pn than wild type 6028 during the senescence progress, especially at the later stage. Furthermore, Ca2^-ATPase, Mg~^-ATPase and photophosphorylation activities exhibited the similar trends as the Po. During the senescence process, the 68 kDa polypeptide concentrations in the thylakoid membrane proteins exhibited a significant change, which was one of the critical factors that contributed to the observed change in photosynthesis. We also observed that the chloroplasts of rice mutant 6001 exhibited higher integrity than those of wild type 6028, and the chloroplast membrane of rice mutant 6001 disintegrated more slow during the senescence process. In general, rice mutant 6001 had a relatively slower senescence rate than wild type 6028, and exhibited anti-senescence properties.

Study on the Photosynthetic Characteristics in Spring Maize (Zea mays L.)of Different Quality Type

common maize synthetic rate（ Photosynthetic characteristics were probed by sweet maize, waxy maize, high starch maize and The results revealed that leaf area index （LAI）, chlorophyll a content, chlorophyll b content,photo-PR） showed single peak curve at the whole growth stage. The stages of peak were different according to different varieties. NEAUS4 had the lowest peak and while SIDAN 19 had the highest among all stages. Ratio of chlorophyll a to b was low at seedling stage, reached the peak atjointing stage and then declined. SIDAN 19 had the lower level at the last stages.

Diurnal changes in net photosynthetic rate of Pueraria lobataand its impact factors

In order to identify the relationship between diurnal changes in the net photosynthetic rate（Pn） of Pueraria lobata and environmental factors,diurnal changes in the Pn of leaves of two P.lobata cultivars were measured using a CIRAS-1 portable photosynthesis measurement system（PP-Systems,UK）.The results show that diurnal changes in Pn of both cultivars could be interpreted as double-peak curves,indicating the occurrence of an obvious midday depression.Further analyses indicate that the correlation between Pn and stomatal conductance was positive and extremely significant（p 〈 0.01）.The correlations of Pn with intercellular CO2 concentration and transpiration rate were positive and significant（p 〈 0.05）,while the correlations of Pn with air and leaf temperatures were negative and significant（p 〈 0.05）.The results indicate that among the factors affecting photosynthetic properties,some can be grouped as stomatal limitations while others are non-stomatal limitations.

Proline Accumulation,Photosynthetic Abilities and Growth Characters of Sugarcane(Saccharum officinarum L.) Plantlets in Response to Iso-Osmotic Salt and Water-Deficit Stress

The aim of this study was to investigate the biochemical, physiological and morphological responses of sugarcane to iso- osmotic salt and water-deficit stress. Disease-free sugarcane plantlets derived from meristem cuttings were photo- autotrophically grown in MS media and subsequently exposed to -0.23 （control）, -0.67 or -1.20 MPa iso-osmotic NaCl （salt stress） or mannitol （water-deficit stress）. Chlorophyll a （Chl a）, chlorophyll b （Chl b）, total carotenoids （Cx＋c）, maximum quantum yield of PSII （Fv/Fm）, photon yield of PSII （ΦPSII）, stomatal conductance （Gs） and transpiration rate （E） in the stressed plantlets were significantly reduced when compared to those of plantlets of the control group （without mannitol or NaCl）, leading to net-photosynthetic rate （Pn） and growth reduction with positive correlation. In addition, physiological changes and growth parameters of plantlets in the salt stress conditions were more sharply reduced than those in waterdeficit stress conditions. On the other hand, the proline content and non-photochemical quenching （NPQ） in the leaves of stressed plantlets increased significantly, especially in response to iso-osmotic salt stress. The chlorophyll pigments in iso-osmotic stressed leaves were significantly degraded （r^2 = 0.93）, related to low water oxidation （r^2 = 0.87）, low net- photosynthetic rate （r^2 = 0.81）, and growth reduction （r^2 = 0.97）. The multivariate biochemical, physiological and growth parameters in the present study should be further used to develop salt, or drought, tolerance indices in sugarcane breeding programs.

QTL analysis of leaf photosynthetic rate and related physiological traits in rice(Oryza sativa L.)

Photosynthesis is one of the most important factors that influence the biomass and yield. Recently, more attention has been paid to genetic study on rice photosynthesis and rice breeding for the physiological traits related to high efficient photosynthesis. Chlorophyll content, stomatal resistance, and transpiration rate were very important physiological traits related to photosynthesis. But until now, no genetic study on these traits has been reported. A DH population derived from anther culture of ZYQ8/JX17, a typical indica/japonica hybrid was developed,

Improving Photosynthetic Responses during Recovery from Heat Treatments with Brassinosteroid and Calcium Chloride in Indian Bread Wheat Cultivars

Climate change is expected to unleash severe and frequent heat waves in future, adversely affecting crop productivity. The aim of this study was to examine the effect of two separate episodes of heat stress, mimicking heat wave conditions on the physiology of four Indian bread wheat cultivars and to study the ameliorating effects of epibrassinolide (BR) and calcium chloride on the recovery of these cultivars. The two thermo-tolerant cultivars C306 and K7903 suffered less inhibition of photosystem II efficiency as compared to the two thermo-susceptible cultivars HD2329 and PBW343. Application of BR and calcium chloride resulted in faster recovery in all the four cultivars. Measurement of the minimum fluorescence (Fo) versus temperature curves revealed a higher inflection temperature of Fo (Ti) for the two tolerant cultivars as compared to the susceptible cultivars, emphasizing greater thermo stability of the photosynthetic apparatus. The two thermo-tolerant cultivars showed higher photochemistry (ΦPSII) relative to the two susceptible cultivars. An increase in the steady state fluorescence was observed in both the susceptible cultivars as compared to the tolerant cultivars. Expression analysis revealed faster recovery of the transcripts involved in photosynthesis in tolerant cultivars as compared to susceptible cultivars. Exogenous application of the ameliorating compounds resulted in faster recovery of transcripts in all the cultivars. The result suggested that under severe stress conditions tolerant cultivars showed faster recovery and a better thermo-stability of its photosynthetic apparatus as compared to susceptible cultivars and application of epibrassinolide and calcium chloride could ameliorate the damaging effect of severe temperature stress to a considerable level in all the four cultivars under study.