Photosynthetic characteristics of C4 trait in chlorina mutant of rice (Oryza sativa L.)

Biao 810S is a chlorina mutant of the thermosensitive genic male sterile (TGMS) rice. We compared photosynthetic characteristics of these two lines. The contents of chlorophylls and carotenoids in Biao 810S were approximately half of those in 810S. However, the net photosynthetic rate (P _N) of Biao 810S was higher than that of 810S under high irradiance or low concentration of carbon dioxide, and the photon quantum efficiency was higher than that of 810S. The activity of ribulose-1,5-bisphosphate carboxylase/oxygenase in Biao 810S was only 69.80 % of that in 810S, but the activities of phosphoenolpyruvate carboxylase and NADP-malic enzyme were 79.50 and 69.06 % higher than those of 810S, respectively, suggesting that the efficiency of photon energy utilization in Biao 810S was enhanced by reduction of thermal dissipation and increase of electron transfer rate to generate sufficient assimilation power for the dark reactions. Consequently, the increased activities of C₄ photosynthetic enzymes lead to more effective fixation of CO₂ and the synergistic effect of light and dark reactions contributed to the higher P _N of Biao 810S.     

水稻叶色突变体叶绿体发育规律研究

从温敏核不育系水稻'810S'中筛选出一个生长发育正常的淡黄绿叶色自然突变株'标810S',其叶绿素含量约为'810S'的50%,光合速率比野生型高.以'810S'为对照,对'标810S'进行叶片形态、叶肉细胞和叶绿体超微结构以及叶绿体蛋白研究.结果显示,'标810S'的叶长、宽和面积与'810S'相似;叶肉细胞和叶绿体发育稍迟缓,片层结构减少;叶绿体蛋白约为对照的55%,并初步鉴定出与光合作用相关的差异蛋白点13个,其中4个缺失蛋白,包括1个RuBP大亚基缺失.推测该水稻突变体叶色变浅与叶绿体基粒片层减少有关.     

Effect of Cd on growth, photosynthetic gas exchange, and chlorophyll fluorescence of wild and Cd-sensitive mutant rice

Growth, photosynthetic gas exchange, and chlorophyll fluorescence characteristics were investigated in wild type (WT) and Cd-sensitive mutant rice (Oryza sativa L.) plants using 50 μM Cd treatment for 12 d followed by a 3-d recovery. Under Cd stress, net dry mass and pigment contents were significantly lower in the mutant plants than in the WT. The mutant had lower net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s) than WT rice, however, it had higher intercellular CO₂ concentration (C _i), indicating that non-stomatal factors accounted for the inhibition of P _N. Maximal photochemical efficiency of photosystem 2 (F_v/F_m), effective quantum yield of PS2 (Φ_PS2), and photochemical quenching (q_P) decreased much in the mutant under Cd stress. Cd content in roots and leaves of the mutant was significantly higher than those in the WT. Hence Cd toxicity was associated with the marked increases in Cd contents of plant tissue. After the recovery for 3 d, the WT rice had higher capacity to recover from Cd injury than the mutant.     

High photosynthetic efficiency of a rice (Oryza sativa L.) xantha mutant

Comparative analysis revealed that a xantha rice mutant (cv. Huangyu B) had higher ratios of chlorophyll (Chl) a/b and carotenoids/Chl, and higher photosynthetic efficiency than its wild type parent (cv. II32 B). Unexpectedly, the mutant had higher net photosynthetic rate (P _N) than II32 B. This might have resulted from its lower non-photochemical quenching (q_N) but higher maximal photochemical efficiency (F_V/F_M), higher excitation energy capture efficiency of photosystem 2 (PS2) reaction centres (F_V′/F_M′), higher photochemical quenching (q_P), higher effective PS2 quantum yield (Φ_PS2), and higher non-cyclic electron transport rate (ETR). This is the first report of a chlorophyll mutant that has higher photosynthetic efficiency and main Chl fluorescence parameters than its wild type. This mutant could become a unique material both for the basic research on photosynthesis and for the development of high yielding rice cultivars.     

Up-regulation of cyclic electron flow and down-regulation of linear electron flow in antisense-<Emphasis Type="Italic">rca</Emphasis> mutant rice

To investigate how excess excitation energy is dissipated in a ribulose-1,5-bisphospate carboxylase/oxygenase activase antisense transgenic rice with net photosynthetic rate (P _N) half of that of wild type parent, we measured the response curve of P _N to intercellular CO₂ concentration (C _i), electron transport rate (ETR), quantum yield of open photosystem 2 (PS2) reaction centres under irradiation (F_v′/F_m′), efficiency of total PS2 centres (Φ_PS2), photochemical (q_P) and non-photochemical quenching (NPQ), post-irradiation transient increase in chlorophyll (Chl) fluorescence (PITICF), and P700⁺ re-reduction. Carboxylation efficiency dependence on C _i, ETR at saturation irradiance, and F_v′/F_m′, Φ_PS2, and q_P under the irradiation were significantly lower in the mutant. However, NPQ, energy-dependent quenching (q_E), PITICF, and P700⁺ re-reduction were significantly higher in the mutant. Hence the mutant down-regulates linear ETR and stimulates cyclic electron flow around PS1, which may generate the ΔpH to support NPQ and q_E for dissipation of excess excitation energy.     

Comparison of Photosynthetic Traits Between Two Typical Shrubs: Legume and Non-Legume in Hunshandak Sandland

In Huanshandak Sandland, China, net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), intercellular CO₂ concentration (C _i), water use efficiency (WUE), photochemical efficiency of photosystem 2 (F_v/F_m), and leaf nitrogen content were compared for Hedysarum fruticosum var. mongolicum (H.f.m.), a nitrogen fixing shrub, and Salix gordejevii (S.g.), a nitrogen non-fixing shrub. P _N, E, and g _s of the two shrubs were similar in trends, i.e. two peaks were observed in diurnal courses. However, except C _i, other parameters of H.f.m. were higher during the measured days than those of S.g. The midday depression of P _N was mainly due to decrease in stomata conductance and to reduction of F_v/F_m at midday. The higher P _N of H.f.m. was consistent with the higher leaf N content and there was a positive relation between them. In addition, several C₄ traits were found in H.f.m., i.e. high saturation irradiance and WUE, low dark respiration rate, and C _i, which partly resulted in higher P _N. This seems to indicate that the C₃ plant H.f.m. may have C₄ photosynthesis pathway or C₄ enzymes.     

Genetics of esterases in Drosophila. VII. Genetic control of the activity level of juvenile hormone (JH)-esterase and heat resistance in D. virilis at high temperatures

The heat-resistant subline 147S was obtained in Drosophila virilis by selecting for viability individuals of heat-sensitive stock 147. It was shown that in the heat-treated 147S pupae the activity of juvenile hormone (JH)-esterase is decreased and, consequently, the titer of juvenile hormone is increased compared with those in the control pupae. These changes are consistent with those observed earlier for resistant stock 101. Heat-resistant stocks 101 and 147S were crossed with heat-sensitive stock 147, whose heat-treated larvae show earlier activation and higher activity of JH-esterase than control larvae. The viability and electrophoretic esterase patterns were analyzed in the F₁ and F₂ hybrids at different temperatures. It was found that the F₁ hybrid is resistant to the effect of high temperature and its activity level of JH-esterase is lower compared with controls. In the F₂ hybrid, there was a 3:1 segregation of viability and a 1:2:1 segregation of the activity level of JH-esterase at high temperatures. It is concluded that the activity level of JH-esterase and heat resistance in D. virilis are monogenically controlled at high temperatures.     

Photosynthetic behaviour of Arabidopsis plants with a Cap Binding Protein 20 mutation under water stress

Under non-stressed conditions the net photosynthetic rate (P _N) of the mutant plants cbp20 of Arabidopsis was similar to that of the wild type (WT). In response to water deprivation, however, P _N started to decrease later in the mutants and remained substantially higher. Thermoluminescence measurements showed that the lipid peroxidation induced by severe water stress was also less pronounced in the mutant than in the WT. Both soil gravimetric and plant water potential data showed that cbp20 mutants lose water more slowly than the WT plants. The drought-induced decline in F_v/F_m, the quantum efficiency of photosystem 2, and photochemical quenching parameters also started later in the cbp20 mutants than in the WT plants. Thus the restricted gas exchange in the cbp20 mutants does not impair the photosynthetic performance of the plant; however, under drought improved water retention provides significant protection for the photosynthetic apparatus.     

Effects of salinity on chlorophyll fluorescence and CO<Subscript>2</Subscript> fixation in C<Subscript>4</Subscript> estuarine grasses

The effects of salinity (sea water at 0 ‰ versus 30 ‰) on gross rates of O₂ evolution (J _O2) and net rates of CO₂ uptake (P _N) were measured in the halotolerant estuarine C₄ grasses Spartina patens, S. alterniflora, S. densiflora, and Distichlis spicata in controlled growth environments. Under high irradiance, salinity had no significant effect on the intercellular to ambient CO₂ concentration ratio (C _i/C _a). However, during photosynthesis under limiting irradiance, the maximum quantum efficiency of CO₂ fixation decreased under salinity across species, suggesting there is increased leakage of the CO₂ delivered to the bundle sheath cells by the C₄ pump. Growth under salinity did not affect the maximum intrinsic efficiency of photosystem 2, PS2 (F_V/F_M) in these species, suggesting salinity had no effect on photosynthesis by inactivation of PS2 reaction centers. Under saline conditions and high irradiance, P _N was reduced by 75 % in Spartina patens and S. alterniflora, whereas salinity had no effect on P _N in S. densiflora or D. spicata. This inhibition of P _N in S. patens and S. alterniflora was not due to an effect on stomatal conductance since the ratio of C _i/C _a did not decrease under saline conditions. In growth with and without salt, P _N was saturated at ∼500 μmol(quantum) m⁻² s⁻¹ while J _O2 continued to increase up to full sunlight, indicating that carbon assimilation was not tightly coupled to photochemistry in these halophytic species. This increase in alternative electron flow under high irradiance might be an inherent function in these halophytes for dissipating excess energy.     

Different Patterns of Gas Exchange and Photochemical Efficiency in Three Desert Shrub Species Under Two Natural Temperatures and Irradiances in Mu Us Sandy Area of China

Field studies of gas exchange and chlorophyll fluorescence of three desert shrub species, Hedysarum fruticosum var. mongolicum, Artemisia ordosia, and Salix pasmmophylla, showed different patterns under different leaf temperature (T₁) and incident photosynthetic photon flux density (PPFD). H. fruticosum var. mongolicum and A. ordosia exhibited higher P _N and g _s than S. pasmmophylla, especially under very high T₁ (>46 °C) and high PPFD (>2 100 µmol m^–2 s^–1) in hot summer. The decreases of P _N with the diurnal course were due mainly to stomata closure. However, P _N of S. pasmmophylla was seriously depressed by very high temperature from midday to evening as shown by the negative P _N in hot summer, whereas none of such depression was found in spring. Maximal spring photochemical efficiency of photosystem 2 (PS2), i.e, F_v/F_m, was the lowest at 16:00, indicating the injury of PS2 by heat at this stage. In hot summer again, all the three shrubs underwent pronounced midday depression of P _N and g _s, while in spring they showed a one-peak response. And the first peak appeared 2 h earlier in hot summer than in spring for all the three shrubs. It was the high temperature that led to the different patterns of gas exchange and the serious depression of P _N in S. pasmmophylla. H. fruticosum var. mongolicum and A. ordosia were much more tolerant to heat and high irradiance than S. pasmmophylla, which fixed most of CO₂ at the fast growing stage in spring. Nevertheless, in hot summer it had to survive the severe hot environment through strong respiration and storage of CO₂ only in the early morning.     

Midday photoinhibition of two newly developed super-rice hybrids

Super-rice hybrids are two-line hybrid rice cultivars with 15 to 20 % higher yields than the raditional three-line hybrid rice cultivars. Response of photosynthetic functions to midday photoinhibition was compared between seedlings of the traditional hybrid rice (Oryza sativa L.) Shanyou63 and two super-rice hybrids, Hua-an3 and Liangyoupeijiu. Under strong midday sunlight, in comparison with Shanyou63, the two super-rice hybrids were less photoinhibited, as indicated by the lower loss of the net photosynthetic rate (P _N), the quantum yield of photosystem 2 (Φ_PS2), and the maximum and effective quantum yield of PS2 photochemistry (F_v/F_m and F_v′/F_m′). They also had a much higher transpiration rate. Hence the super-rice hybrids could protect themselves against midday photoinhibition at the cost of water. The photoprotective de-epoxidized xanthophyll cycle components, antheraxanthin (A) and zeaxanthin (Z), were accumulated more in Hua-an3 and Liangyoupeijiu than in Shanyou63, but the size of xanthophyll cycle pool of the seedlings was not affected by midday photoinhibition. Compared to Shanyou63, the super-rice hybrids were better photoprotected under natural high irradiance stress and the accumulation of Z and A, not the size of the xanthophyll pool protected the rice hybrids against photoinhibition.     

Physiological Basis of the Difference in Net Photosynthetic Rate of Leaves between Two Maize Strains

Significant differences in net photosynthetic rate (P _N) of leaves between two maize (Zea mays L.) strains (Shuang 105 and 40×44) grown in the field were observed. At several growth stages, P _N of 40×44 was higher than that of Shuang 105 (from 10.3 to 32.5 %). Moreover, the strain 40×44 had a higher plant height, larger leaf area, lower chlorophyll content, and higher photochemical efficiency of photosystem 2 (PS2) (F_v/F_m and F/F_m) than strain Shuang 105. Shuang 105, which showed lower P _N, had lower stomatal conductances (g _s) but slightly higher intercellular CO₂ concentrations (C _i) than those of 40×44. Hence the differences in P _N between the two strains did not result from the difference in g _s, but probably from that in light reaction system.     

Protein profiling in F<Subscript>1</Subscript> and F<Subscript>2</Subscript> generations of two tomato genotypes differing in ripening time

Pericarp polypeptide profiles were analyzed at three ripening stages in the F₁ hybrid and the F₂ population from the cross between the accessions: LA1385 (Lycopersicon esculentum var. cerasiforme) and 804627 (L. esculentum, a homozygous genotype for the nor mutant). Six polymorphic polypeptides were observed in LA1385, while no polymorphic polypeptides among ripening stages was observed in 804627. On the other hand, some polypeptides in the F₁ hybrid were not observed in the parents whereas others were present in both parental genotypes and were unnoticeable in the hybrid genotype. From a cluster analysis on the protein profiles of the F₂ population, the differential expression of proteins allowed to distinguish mature green (MG) stage from the others two stages, while for breaker stage (BR) and red ripe stage, the genetic background was more important in forming groups. The differential expression of proteins could be associated with fruit morphology traits such as a 72 kDa polypeptide present in MG stage with fruit diameter, height and mass and a 47 kDa polypeptide found in BR with fruit shelf life.     

Drought effect on photosynthetic activity,osmolyte accumulation and membrane integrity of two <Emphasis Type="Italic">Cicer arietinum</Emphasis> genotypes

Drought was induced in chickpea (Cicer arietinum L.) genotypes (ChK 3226 and ILC 3279) differing in yield capacity. Water stress (S1, RWC around 55–50%; S2, RWC ≤ 40%) drastically reduced stomatal conductance (g _s) and net photosynthetic rate (P _N) in both genotypes. ILC 3279 showed greater photosynthetic capacity (A _max) decreases. Maximum PSII photochemical efficiency (F_v/F_m), photochemical quenching (q_P), total chlorophylls (Chls) and carotenoids (Cars) content showed stability in both genotypes under stress, but in S2 ILC 3279 presented an increase in basal fluorescence (F₀) and a greater reduction in estimation of quantum yield of linear electron transport (Φ_e) than ChK 3226. Membrane damage evaluated by electrolyte leakage occurred earlier and was greater in ILC 3279. It also presented a decrease of total fatty acids (TFA) along drought, while in ChK 3226 greater amounts of TFA were observed in S1. In rehydration, P _N of S1 plants completely recovered (ILC 3279) or remained slightly below control (ChK 3226). As regards S2 plants, ILC 3279 showed stronger P _N and g _s reductions than ChK 3226, despite both genotypes totally recovered A _max and chlorophyll (Chl) a fluorescence. ChK 3226 recovered more efficiently from membrane damage. Under control conditions, greater amounts of most of the studied soluble metabolites occurred in ChK 3226 plants. Malate and citrate decreased with water stress (S2) in both genotypes. Sucrose and pinitol (that had a higher concentration than sucrose in both genotypes) increased in ILC 3279 (S1 and S2), and decreased in ChK 3226 (S2). In ILC 3279 proline and asparagine followed similar patterns. Genotypes showed a similar shoot dry mass (DM) in control plants, but root DM was higher in ChK 3226. Drought reduced root and shoot DM in ChK 3226 already under S1, while in ILC 3279 root DM was unaffected by drought and shoot biomass decreased only in S2. Root/shoot ratio was always higher in ChK 3226 but tended to decrease under stress, while the opposite was observed in ILC 3279. No pods were obtained from control plants of both genotypes, or droughted ILC 3279 plants. ChK 3226 produced pods under S1 (higher yield) and S2. Under stress conditions, ChK 3226 was less affected in photosynthetic activity and membrane integrity, showing a better tolerance to drought. This agrees with the better yield of this genotype under water stress. Distinct strategies seem to underlie the different physiological responses of the two genotypes to water deficit. In spite of its significant solutes accumulation, ILC 3279 was more affected in photosynthetic activity and membrane integrity during water stress than ChK 3226, which showed better yield under drought. A relation could not be established between solutes accumulation of ILC 3279 and yield.     

Diurnal Changes of Gas Exchange,Chlorophyll Fluorescence,and Stomatal Aperture of Hybrid Poplar Clones Subjected to Midday Light Stress

Diurnal changes in net photosynthetic rate (P _N), chlorophyll (Chl) fluorescence, and stomatal aperture of several hybrid poplar clones subjected to midday light stress were measured in July and August of 1996. Midday depression of P _N, photosystem 2 (PS2) efficiency, stomatal conductance (g _s), and stomatal aperture was observed in all clones, though at differing rates among them. Non-uniform stomatal closure occurred at noon and at other times, requiring a modification of intercellular CO₂ concentration (C ₁). A linear relationship was found between g _s and stomatal aperture. More than half of the photons absorbed by PS2 centre dissipated thermally when subjected to light stress at noon. There was a linear relationship between the rate of PS2 photochemical electron transport (PxPFD) and P _N. There was a consensus for two fluorescence indicators (1 – q_P/q_N and (F_m' – F)/F_m') in assessment of susceptibility of photoinhibition in the clones. According to P _N, Chl fluorescence, and stomatal aperture, we conclude that midday depression of photosynthesis can be attributed to both stomatal and non-stomatal limitations.     

Photosynthetic and biochemical activities in flag leaves of a newly developed superhigh-yield hybrid rice (<Emphasis Type="Italic">Oryza sativa</Emphasis>) and its parents during the reproductive stage

Responses of net photosynthetic rates to intercellular CO₂ concentration (P _n/C _i curves) and photochemical characteristics were investigated in flag leaves of newly developed superhigh-yield hybrid rice (Oryza sativa L.) LiangYouPeiJiu (LYPJ) and its maternal PeiAi64S (PA64S) and paternal WuMang9311 (WM9311) lines grown in the field during the reproductive stage. The results showed that photosynthetic functions, such as the electron transport activities of photosystems and photophosphorylation, assessed in vivo from P _n/C _i curves under field conditions declined more or earlier than those obtained in vitro. The degradation of polypeptides of thylakoid membranes was slower than those for P _Ca=360 (light-saturated net photosynthetic rate measured at 360 μmol mol⁻¹) and CE (carboxylation efficiency, obtained from the initial slope of the P _n/C _i curve). The initial inhibition of the PSII electron transport and oxygen-evolving activity induced by senescence occurred before the degradation of the oxygen-evolving complex. In comparison, LYPJ had intermediate photosynthetic functions in the early stage of leaf development, but greater photochemical activities in the mid and late stages. WM9311 showed a similar pattern of changes but lower values, and PA64S had higher values in the early stage but showed a faster rate of senescence than LYPJ. These findings implied that the hybrid LYPJ demonstrated intermediate photosynthetic activities between its parents in the early stage of leaf development, whereas it had higher photosynthetic activities than its parents in the mid and late stages, which may be responsible for its high yield.     

Comparison of Some Photosynthetic Characters Between Two Hybrid Rice Combinations Differing in Yield Potential

Photosynthetic characteristics of two hybrid rice combinations, Peiai 64S/E32 and Shanyou 63, were compared at the panicle differentiation stage. As compared with Shanyou 63, the new combination Peiai 64S/E32 showed a significantly higher net photosynthetic rate (P _N), apparent quantum yield of carbon assimilation (_c), carboxylation efficiency (CE), and photorespiratory rate (R _P) as well as leaf chlorophyll content, but a significantly lower dark respiration rate (R _D) and compensation irradiance (I _c). It also showed a slightly higher photochemical efficiency (F_v/F_m and F/F_m) of photosystem 2, a lower non-photochemical quenching (q_N), and a similar CO₂ compensation concentration () as compared to Shanyou 63.     

Photosystem 2 photochemistry and pigment composition of a yellow mutant of rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) under different irradiances

A yellow leaf colouration mutant (named ycm) generated from rice T-DNA insertion lines was identified with less grana lamellae and low thylakoid membrane protein contents. At weak irradiance [50 μmol(photon) m⁻² s⁻¹], chlorophyll (Chl) contents of ycm were ≈20 % of those of WT and Chl a/b ratios were 3-fold that of wild type (WT). The leaf of ycm showed lower values in the actual photosystem 2 (PS2) efficiency (Φ_PS2), photochemical quenching (q_P), and the efficiency of excitation capture by open PS2 centres 1 (F_v′/F_m′) than those of WT, except no difference in the maximal efficiency of PS2 photochemistry (F_v/F_m). With progress in irradiance [100 and 200 μmol(photon) m⁻² s⁻¹], there was a change in the photosynthetic pigment stoichiometry. In ycm, the increase of total Chl contents and the decrease in Chl a/b ratio were observed. Φ_PS2, q_P, and F_v′/F_m′ of ycm increased gradually along with the increase of irradiance but still much less than in WT. The increase of xanthophyll ratio [(Z+A)/(V+A+Z)] associated with non-photochemical quenching (q_N) was found in ycm which suggested that ycm dissipated excess energy through the turnover of xanthophylls. No significant differences in pigment composition were observed in WT under various irradiances, except Chl a/b ratio that gradually decreased. Hence the ycm mutant developed much more tardily than WT, which was caused by low photon energy utilization independent of irradiance.     

Photosynthetic acclimation and photoinhibition on exposure to high light in shade-developed leaves of Fagus crenata seedlings

The physiological response of leaves developed in low light (L) on Fagus crenata seedlings exposed to different levels of high light (H: high light, M: medium light) was studied. Measurements were conducted on potted seedlings in the F. crenata forest understory. The seedlings with leaves developed in L were transferred to H (L–H) and M (L–M) in summer. On exposure to high light, the photochemical efficiency of dark-adapted PSII (F_v/F_m) immediately decreased and was followed by a subsequent recovery in both L–H and L–M leaves. The mean value of F_v/F_m in L–H leaves was lower than that in L–M leaves through experiments, indicating that the degree of photoinhibition in L–H leaves was greater than that in L–M leaves. About 1 month after transfer, 37% and 5% of leaves had fallen in L–H and L–M seedlings, respectively. This result also indicated the greater photoinhibition in L–H leaves. Moreover, the photosynthetic capacity (P_Nmax) of L–H leaves decreased. In contrast, the P_Nmax of L–M leaves increased, although the P_Nmax was lower than that of M control leaves. An increase in the xanthophyll cycle pool (VAZ), indicating an increase of the photoprotective function, was found in both L–H and L–M leaves. Especially, the VAZ pool in L–M leaves was higher than that in M leaves by the end of experiments. L–M leaves may avoid photoinhibition effectively by the decrease in excess light with the increase of the P_Nmax or VAZ pool, compared to L–H leaves. Thus, the physiological acclimation on exposure to high light depended on the degree of high light. To achieve successful photosynthetic acclimation with slight photoinhibition, the variation of light intensity before and after exposure to high light would be an important factor because of the difference in excess light.     

Deriving room temperature excitation spectra for photosystem I and photosystem II fluorescence in intact leaves from the dependence of <Emphasis Type="Italic">F</Emphasis><Subscript>V</Subscript>/<Emphasis Type="Italic">F</Emphasis><Subscript>M</Subscript> on excitation wavelength

The F ₀ and F _M level fluorescence from a wild-type barley, a Chl b-less mutant barley, and a maize leaf was determined from 430 to 685 nm at 10 nm intervals using pulse amplitude-modulated (PAM) fluorimetry. Variable wavelengths of the pulsed excitation light were achieved by passing the broadband emission of a Xe flash lamp through a birefringent tunable optical filter. For the three leaf types, spectra of F _V/F _M (=(F _M − F ₀)/F _M) have been derived: within each of the three spectra of F _V/F _M, statistically meaningful variations were detected. Also, at distinct wavelength regions, the F _V/F _M differed significantly between leaf types. From spectra of F _V/F _M, excitation spectra of PS I and PS II fluorescence were calculated using a model that considers PS I fluorescence to be constant but variable PS II fluorescence. The photosystem spectra suggest that LHC II absorption results in high values of F _V/F _M between 470 and 490 nm in the two wild-type leaves but the absence of LHC II in the Chl b-less mutant barley leaf decreases the F _V/F _M at these wavelengths. All three leaves exhibited low values of F _V/F _M around 520 nm which was tentatively ascribed to light absorption by PS I-associated carotenoids. In the 550–650 nm region, the F _V/F _M in the maize leaf was lower than in the barley wild-type leaf which is explained with higher light absorption by PS I in maize, which is a NADP-ME C₄ species, than in barley, a C₃ species. Finally, low values of F _V/F _M at 685 in maize leaf and in the Chl b-less mutant barley leaf are in agreement with preferential PS I absorption at this wavelength. The potential use of spectra of the F _V/F _M ratio to derive information on spectral absorption properties of PS I and PS II is discussed.