Saturating Light Intensity Research Articles

Influence of rhizomania disease on gas exchange and water relations of a susceptible and a tolerant sugar beet variety

Gas exchange measurements were undertaken on healthy and rhizomania-infected sugar beet plants ( Beta vulgaris L.) by comparing susceptible (KWM) and tolerant (Rizor) cultivars grown on contaminated and uncontaminated soil. Whole day cycle measurements in trial fields under natural conditions showed that the susceptible cultivar on contaminated soil had lower net CO 2 uptake and transpiration when compared with the tolerant cultivar. As a result light saturation curves gave lower net CO 2 uptake at saturating light intensity, higher dark respiration and the highest light compensation point for the infected plants of KWM. Measurement of water relations showed that the plants of the susceptible variety in contaminated soil exhibit a stressed water status which was indicated by a lower leaf conductance and a higher xylem tension. Because of the diminished transpiration, leaf temperatures of infected plants of the susceptible variety were up to 2·3 °C higher, than those of the tolerant one. For this reason leaf temperature can be used as a criterion for selection of less susceptible plants for breeding of more tolerant hybrids.

Replacement of aspartic acid-96 by asparagine in bacteriorhodopsin slows both the decay of the M intermediate and the associated proton movement.

The photocycle, electrical charge translocation, and release and uptake of protons from the aqueous phase and release and uptake of protons from the aqueous phase were investigated for bacteriorhodopsin mutants with aspartic acid-96 replaced by asparagine or glutamic acid. At neutral pH the main effect of the Asp-96----Asn mutation is to slow by 2 orders of magnitude the decay of the M intermediate and the concomitant charge displacement associated with the reprotonation of the Schiff base from the cytoplasmic side of the membrane. The proton uptake measured with the indicator dye pyranine is likewise slowed without affecting the stoichiometry of proton pumping. The corresponding results for the Asp-96----Glu mutant, on the other hand, are very close to those for the wild-type protein. These results provide a kinetic explanation for the fact that at pH 7 and saturating light intensities the steady-state proton pumping is almost abolished in the Asp-96----Asn mutant but is close to normal in the Asp-96----Glu mutant. Thus, the pump is simply turning over much more slowly in the Asp-96----Asn mutant. The time constants of the decay of M and the associated charge translocation increase strongly with increasing pH for the Asp-96----Asn mutant but are virtually pH-independent for the Asp-96----Glu mutant and wild-type bacteriorhodopsin. At pH 5 the M decay of the Asp-96----Asn mutant is as fast as for wild type. These results suggest that Asp-96 serves as an internal proton donor in the proton-uptake pathway from the cytoplasm to the Schiff base.

Phytoplankton, Nutrients, and Primary Production in Fertilized and Natural Lakes at Saqvaqjuac, N.W.T.

Whole-lake phosphorus (P) and nitrogen (N) addition experiments at Saqvaqjuac, N.W.T. (63°N in the central Canadian arctic), showed that the lakes were P limited but required both P and N for increased production. Photosynthetic response to 0.1 g P and 1.0 g N∙m−2∙yr−1 was immediate (15→30 g C∙m−2∙yr−1), with simultaneous increases in protozoa, while oligotrophic chrysophyte assemblages gave way to volvocalean greens. Cyanophytes were not important during P-only or P and N additions or in oligotrophic lakes, but formed permanent blooms in several naturally mesotrophic lakes near sea level. Retention of P was naturally low, but high during P addition. Silicon (Si) retention was always very high. Chlorophyll: P ratios were similar to those of subarctic and north-temperate lakes. Saturation light intensity (Ik) tracked surface light flux with a 2- to 3-wk delay, averaging 15 E∙m−2∙s−1 in winter and peaking at 140 E∙m−2∙s−1 in July. Comparison of our data with those for lakes in the Experimental Lakes Area (northwestern Ontario, 50°N) and Char Lake (75°N) shows that with increasing latitude, in small lakes, (a) phytoplankton production decreases, (b) phytoplankton production per unit light decreases less sharply, and (c) the proportion of primary production occurring beneath ice cover increases.

Single leaf photosynthesis of rice plants under field conditions. IV. Variations in single-leaf photosynthesis among several successive days, and the interactive effects of irradiance and stomatal conductance on CER.

近接した日の間にみられる水稲個葉の光合成速度 (CER) の変動とその生理的基礎, 並びに, 葉面光強度および気孔伝導度 (gs) の2要因がCERに及ぼす効果の相互関係について検討した。(1) CERの日による変動; (i) 圃場条件下で, 数日間連続して, 午前のほぼ同じ時刻に, 飽和光下で同一葉のCERを測定したところ, その値は日によって異なり, この期間の最大値と最小値の開きは5～10 mg CO2 dm-2h-1に達することが認められた。(ii) CERの日による変動はgsのそれと並行的であった。(iii) gsの日による変動は蒸発計蒸発速度と密接な関連を示し, この値が大きい日にはgsが小さいという傾向がみられた。(2) 葉面光強度とgsのCERに及ぼす効果の相互関係; (i) 連続した数日間においていろいろな時刻に測定した同一葉位の葉のCERと葉面光強度およびgsとの関係を検討したところ, 両要因の効果には相互作用が存在することがわかった。すなわち, (ii) 弱光下ではCERはgsに無関係に, ひとつの光-光合成曲線であらわされた。(iii) 光飽和点はgsが大きいほど高かった。(iv) 強光下ではCERは光強度に影響されずgsの増大に伴い増加した。

Single leaf photosynthesis of rice plants under field conditions. II. Individual variations in single-leaf photosynthesis and the physiological basis for it.

A group of leaves, each of which was situated at the equivalent position on a stem (for instance, the second leaf counted from the uppermost fully expanded leaf of each stem), were chosen and their photosynthetic rates were estimated under field conditions. A rather large variation in single-leaf photosynthetic rates (CER) was observed among them even though the measurements were conducted at approximately the same time of a day and under the same light regime (saturation light intensity). The variation in CER amounted to 9-12 mg CO2 dm-2h-1 when it was expressed as the difference between the largest and the smallest values found in a group of leaves. This variation was named individual variation in single-leaf photosynthesis (CER); the nature and physiological basis of this variation were examined. (1) The variation in CER was closely correlated with variations in stomatal conductance (gs). (2) Leaves having large gs values at a certain time of a day also showed large gs values at other times throughout the measuring period. Therefore, it seemed that the rank in gs of a leaf among leaves was kept rather constant over a period of time. (3) In the case where the gs value of the first leaf was large, the values of the second and the third leaves on the same stem also tended to be large. (4) The rate of bleeding from the section of basal part of a leaf was measured after sunset and compared with gs values of the leaf in the daytime. The leaves which showed large gs values in the daytime tended to exhibit large rates of bleeding after sunset. (5) In the case where the bleeding rate of the first leaf was high, those for the second and the third leaves on the same stem also tended to be high. (6) The obtained results suggested that the basis for variation in CER was a variation in water supplying ability to leaves, which, in turn, varied from stem to stem, probably due to a difference in the amount and activity of roots formed on stem.

Inorganic carbon and ammonium uptake by phytoplankton in nitrogen depleted waters in Lake Suwa.

Inorganic carbon and NH4+ uptake by phytoplankton was studied during a period of nitrogen depletion in early fall of 1985 in Lake Suwa to investigate a possible effect of NH4+ supply from the bottom sediments on phytoplankton production. Phytoplankton samples from the experimental enclosures were, after being size-fractionated using 40 μm netting, incubated with the simultaneous addition of 13C inorganic carbon at tracer level (6.515.6% in excess of ambient DIC) and 15NH4+ in substrate saturated concentration (541330% in excess of ambient NH4+) under saturation light intensity for 34 hr. The 40 μm fraction, exclusively composed of large colonial blue-green algae Microcystis spp. There was little difference in specific carbon uptake rates and no difference in photosynthetic activity between the two size fractions. It was suggested that during a period of nitrogen depletion in Lake Suwa a large supply of NH4+ from the bottom sediments could more favorably affect the small-sized phytoplankton than the large colonial Microcystis.

Interaction between Sucrose Synthesis and CO 2 Fixation III. Response of biphasic induction kinetics and oscillations to manipulation of the relation between electron transport, calvin cycle, and sucrose synthesis

This article investigates the influence of sucrose synthesis on the light response curve of photosynthesis, on induction kinetics and on photosynthetic oscillations. The occurrence of oscillations and analysis of chlorophyll fluorescence quenching at saturating light intensities shows there is surplus electron transport capacity which cannot be used due to a limitation in carbon metabolism. This limitation is increased when high fructose 2,6-bisphosphate and low sucrose phosphate synthase activity are restricting the rate of sucrose synthesis. After feeding phosphate, raised levels of metabolites allowed higher rates of sucrose synthesis even though sucrose phosphate synthase deactivated and fructose-2,6-bisphosphate rose. The maximum rate of photosynthesis is increased, the light intensities needed to saturate photosynthesis are increased and the transients and oscillations become less marked. Feeding mannose to inhibit sucrose synthesis has the opposite effect on all these parameters. Chemicals which inhibit electron transport (dichlorophenylmethyl urea), energisation and stromal alkalinisation (dinitrophenol, acetate) and ribulose-1,5-bisphosphate carboxylase (cyanide) abolished the oscillations without reducing the maximum steady state rate of photosynthesis. They also, with the exception of cyanide, decreased the initial slope of the light saturation curve. It is discussed how changes in the induction kinetics, in oscillations and in the response of the light response curve can be combined with metabolite analysis to provide a way of screening for chemicals or conditions which modify sucrose synthesis. As an example, it is shown that lithium inhibits photosynthesis via a selective restriction of sucrose synthesis, probably acting by inhibiting the cytosolic fructose-1,6-bisphosphatase.

The link between the electrogenic and redox functions of the plasmalemma and the energy metabolism of the cell

A dependence of the plasmalemma redox activity, determined by the reduction of external electron acceptors (ferricyanide, nitro‐blue tetrazolium), on the energy state of the cell, which was modified by light conditions or introduction of glucose into the media, was shown on leaves of Elodea canadensis Rich. Glucose (10 mM) and light (40 W m‐2) caused hyperpolarization of the membrane potential and stimulated the redox activity of the plasmalemma. 3‐(3,4‐Dichlorophenyl)‐1,1‐dimethylurea (DCMU) completely inhibited the light activation of electrogenic and redox functions of the plasmalemma. The light saturation intensity for membrane potential and ferricyanide reductase activity was 10–30% of the light saturation of photosynthesis. Membrane potential, K+ transport and plasmalemma redox activity changed in parallel in response to light and darkness and when DCMU was added. Ferricyanide reductase activity is suggested to be a simple parameter for characterizing the energy state of the cell. The functional significance of the light‐induced hyperpolarization of the membrane potential is discussed.

Nitrogen Fixation on the Tropical Volcano, La Soufrière (Guadeloupe): Nitrogen Fixation, Photosynthesis and Respiration During the Prevailing Cloud/Shroud Climate By Stereocaulon Virgatum

AbstractStereocaulon virgatum Ach. has colonized lava flows deposited on the west flank of the volcano La Soufrière. The mean annual rate of acetylene reduction was 43·4 nmol C2H4 gdw−1 h−1 with maximum rates during the prevalent cloud/shroud meteorology of 101 and minimum rates during rare high insolation events of 0·63 nmol gdw−1 h−1. Percentage thallus moisture was the major variable controlling nitrogenase activity. During cloud/shroud conditions the upper 90% of the lichen canopy reduced 85% of the acetylene. Canopy shading reduced intra-canopy temperatures allowing the basal 10% of the canopy to fix nitrogen during insolation shocks. Basal portions of pseudopodetia exhibited reduced rates of ethylene production when exposed to canopy surface light intensities during cloud/shroud conditions. The recovery pattern of nitrogenase following desiccation during an insolation shock is presented. Rates of photosynthesis during cloud/shroud conditions were high, reaching 50% of those attained during saturating light intensities.

The Polyphasic Rise of Chlorophyll Fluorescence upon Onset of Strong Continuous Illumination: I. Saturation Characteristics and Partial Control by the Photosystem II Acceptor Side

Applying a rapid modulation system for measurement of chlorophyll fluorescence yield (U. Schreiber. Photosynth. Res. 9. 261-272 (1986)) the induction kinetics upon onset of strong actinic illumination previously studied by Delosme (Biochim, Biophys, Acta 143, 108-128 (1967)) are reinvestigated. With increasing actinic intensity the fluorescence rise is changed from the typical O-I-P characteristic to a more complex rise curve with two intermediary levels I1 and I2 both of which show saturation at high intensity. The typical kinetics at saturating light intensity (O-I1-D-I2-P) are observed in a variety of plant species. The properties of the kinetics with respect to light intensity, temperature, electron acceptors and PS II inhibitors suggest that the O-I1 phase is controlled by photochemical charge separation (photochemical phase), while the I1-D-I2-P transients are limited by dark reactions (thermal phases). Dichlorophenyl-dimethylurea (DCMU) eliminates the thermal phases by raising I, to the original I level. While in principal the previous findings by Delosme are confirmed, there, is the new aspect of two distinct components in the thermal part of the rise curve, which display different properties. Electron acceptors suppress only the I2-P phase, which appears to parallel the reduction of the plastoquinonc pool, which is a fluorescence quencher when oxidized. While the DCMU effect suggests quenching control during I1-I2 by reoxidation of PS II acceptors, this suggestion is contradicted by the observed saturation of I1 with light intensity and at low temperatures. The relevance of these results with respect to quenching analysis of chlorophyll fluorescence by the saturation pulse method is discussed.

Thylakoid membrane protein phosphorylation modifies the equilibrium between Photosystem II quinone electron acceptors

Chloroplast thylakoid membrane protein phosphorylation inhibits Photosystem-II-mediated electron transfer at saturating light intensities. Measurements of room temperature chlorophyll fluorescence have been used to monitor electron transfer reactions between the primary (Q A) and secondary (Q B) quinone acceptors of Photosystem II. The initial transfer from Q − A to Q B is not affected by the phosphorylation, although a second slow reoxidation phase is even slower after phosphorylation. After equilibrium is achieved between Q − AQ B and Q AQ − B there appears to be a higher concentration of Q − A in the phosphorylated membranes. This is explained by a phosphorylation-induced destabilisation of the anionic semiquinone, Q − B, as seen from the DCMU-stimulated fluorescence increase.

The adaptation of the electron transfer chain of Rhodopseudomonas capsulata to different light intensities

(1) Cells of Rhodopseudomonas capsulata (wild-type) were grown photoheterotrophically in a turbidostat under very high and very low light intensity. Membranes were isolated from cells adapted to the respective light conditions and fractionated by sucrose density centrifugation. The molar ratios of ubiquinone and cytochromes c 2, c 1, b-561 and b-566 per reaction center were 3-fold to 5-fold higher in high-light than in low-light membranes. (2) Most of the Cyt( c 1 + c 2) and Cyt b-561 detected in dark redox titrations undergoes light-induced redox changes, both in high- and in low-light membranes. (3) The fractions of the total photooxidizable reaction center and Cyt( c 1 + c 2) oxidized under continuous light in the absence of antimycin are higher in membranes from low-light- than from high-light-grown cells. (4) From these data and results of kinetic studies it is proposed that cyclic electron flow under saturating light intensities is faster in high-light-grown cells.

Photosynthesis and Transpiration of Trees in a Eucalypt Forest Stand: CO2, Light and Humidity Responses

Exchange of water vapour and CO2 was measured on a small group of trees in a 12-year-old regenerating forest dominated by Eucalyptus spp. in Kioloa State Forest, south-eastern New South Wales. The trees were growing in a weighing lysimeter (10.3 m2 ground area) with the dominant tree about 10 m high. A 12 m high enclosure made of polyethylene film was erected to enclose the trees on the lysimeter. Air was impelled in from the bottom of the enclosure by four fans at a total flow rate of 4.6 m3 s-1. Air samples for infrared gas analysers were taken 1, 4, 7 and 10 m above the ground. Pure CO2 (up to 100 litres min-1) was injected to obtain the response of rates of photosynthesis and transpiration at various levels of CO2 in the enclosure environment. At normal ambient partial pressure of CO2, the photosynthetic rate of the canopy (28 �mol m-2 ground area s-1) was found to be saturated at about half of full sunlight. At midday, the foliage in the layers 1-4 m, 4-7 m and 7-10 m from the ground level contributed 20.7, 44.3 and 35% of the total carbon assimilated by the canopy, respectively. Canopy conductance was reduced with increasing vapour pressure difference between leaves and the air on the basis that internal partial pressure of CO2 was decreased. Light intensity required to saturate photosynthesis increased with increasing ambient partial pressure of CO2. At 680 �bar ambient partial pressure of CO2 and at saturating light intensity, there was a 50% increase in photosynthetic rate and a 30% reduction in transpiration rate, resulting in a reduction in transpiration ratio of about 80%. The apparent quantum yield derived at 340 and 680 �bar CO2 was 0.049 and 0.071, respectively. The light compensation point also decreased at higher ambient partial pressure of CO2.

Photosynthesis and transpiration of euphorbia geniculata compared with corn and euphorbia hirta.

Photosynthesis and transpiration activities of E. geniculata were examined under several conditions in comparison with E. hirta and corn plants. The results obtained are as follows:1) E. geniculata, a C3 plant, had a lower saturation light intensity for photosynthesis, ca. 50klux, while those of E. hirta and corn, both C4 plants, were over 70klux and 75klux, respectively. The apparent photosynthetic rate under a strong light was limited by mesophyll resistance, rm, in E. geniculata, but by stomatal resistance, rs, in E. hirta and corn. The highest transpiration rate of E. geniculata was about two times that of E. hirta and corn. Stomatal closing reaction due to darkening was slower in E. geniculata than in the other two species.2) A young leaf of E. geniculata had a higher saturation light intensity, maximum transpiration rate and maximum apparent photosynthetic rate than an old leaf did.3) When grown under shading treatment, saturation light intensity for apparent photosynthesis was lower in E. geniculata than in corn. The degree of decrease in apparent photosynthesis under a strong light condition was larger in E. geniculata than in corn. The decreased apparent photosynthesis was attributable to increased rm in E. geniculata, but to both increased rm and rs in corn. Shading treatment decreased transpiration rate in corn but not in E. geniculata.4) The short-term water requirement of E. geniculata was more than twice that of corn under normal light, and further increased to approximately three times that of corn by shading treatment.5) Maximum apparent photosynthesis in E. geniculata became smaller when day-length was increased. The highest maximum transpiration rate was obtained from 12-hour day-length, giving the lowest water use efficiency. Stomatal closing reaction due to darkening was far quicker in 8-hour than in longer hour plots.

Seasonal changes in photosynthetic characteristics of Pachysandra terminalis (Buxaceae), an evergreen woodland chamaephyte, in the cool temperate regions of Japan.

Seasonal changes in photosynthetic capacity, and photosynthetic responses to intercellular CO2 concentration and irradiance were investigated under laboratory conditions on intact leaves of Pachysandra terminalis. Photosynthetic capacity and stomatal conductance under saturating light intensity and constant water vapor pressure deficit showed almost the same seasonal trend. They increased from early June just after the expansion of leaves, reached the maximum in late-Septemer, and then decreased to winter. In over-wintering leaves they recovered and increased immediately after snow-melting, reached a first maximum in late April, and then decreased to early July in response to the reduction of light intensity on the forest floor. There-after, they increased from mid August, reached a second maximum in late September, and then decreased to winter. The parallel changes of photosynthesis and stomatal conductane indicate a more or less constant intercellular CO2 concentration throughout the year. The calculated values of relative stomatal limitation of photosynthesis were nearly constant throughout the year, irrespective of leaf age. The results indicate that the seasonal changes in light-saturated photosynthetic capacity are not due to a change of stomatal conductance, but to a change in the photosynthetic capacity of mesophyll. Indeed, carboxylation efficiency assessed by the inital slope of the Ci-photosynthesis curve changed in proportion to seasonal changes of the photosynthetic capacity in both current-year and over-wintered leaves. High photosynthetic capacity in current-year leaves as compared with one-year-old leaves was also due to the high photosynthetic capacity of mesophyll. Nevertheless, stomatal conductance changed in proportion to photosynthetic capacity, indicating that stomatal conductance is regulated by the mesophyll photosynthetic capacity such that the intercellular CO2 concentrations are maintained constant. The quantum yield also changed seasonally parallel with that in the photosynthetic capacity.

The electrical and chemical components of the protonmotive force in chloroplasts as measured with capillary and pH-sensitive microelectrodes

After impaling an isolated giant chloroplast of Peperomia metallica with a glass-insulated antimony pH-microelectrode, the potential difference between this microprobe and the external reference electrode was monitored during light activation and compared with the light-induced change of the membrane potential. The signal recorded with the pH-sensitive microelectrode during illumination was composed of a change of the membrane potential and an additional component, consisting of a slow (20–30 s) voltage increase up to +80–+120 mV with respect to the initial dark level. The superposition of two components was interpreted as being due to the location of the tip of the pH-sensitive microelectrode inside the thylakoid. The slow component in the pH-microelectrode signal reflects presumably an acidification of the internal thylakoid by 1.8 pH units at saturating light intensity. An addition of 5 mM NH 4Cl to the medium resulted in a suppression of the slow component, but it also produced a significant increase in the stationary level of the membrane potential. The results are consistent with the suggestion that a decrease of a pH gradient in the presence of NH 4Cl is compensated by an increase in the membrane potential. It appears that the photo-induced voltage recorded with pH-sensitive antimony electrode inside the chloroplast reflects the time-course of the protonomotive force, provided that the change of pH in the chloroplast stroma is relatively small.

Adaptation of photosynthetic electron-transport rate to growth temperature in pea

Pea (Pisum sativum L. cv. Feltham First) plants were germinated and grown under two temperature regimes, one chilling (6-8° C) and one non-chilling (16-18° C), which are referred to as "cold-grown" and "warm-grown", respectively. It was found that: (1) At saturating light intensity and with excess CO2, cold-grown leaves exhibited faster rates of oxygen evolution than warm-grown leaves when measured below 15° C. However when measurements were carried out above this temperature, the reverse relationship was observed. (2) Full-chain electron-transport measurements on thylakoids showed that those isolated from cold-grown plants had greater light-saturated uncoupled rates than their warm-grown equivalents at all temperatures between 3 and 19° C. (3) This difference was apparently not due to a greater activity of photosystem I or II in the thylakoids from cold-grown plants, but rather to a more rapid turnover of a dark step within the electron-transport chain. These results are interpreted in terms of a previously reported apparent homeoviscous adaptation of the pea thylakoid membrane to growth temperature (J. Barber, R.C. Ford, R.A.C. Mitchell, P.A. Millner, 1984, Planta 161, 375-380).

Aspects of nitrogen fixation in Sargassum communities off the coast of Florida

The magnitude and character of nitrogen fixation in Sargassum communities was studied for three different species, S. fluitans Borgesen, S. natans J. Meyen and S. filipendula C. Agardh. Nitrogen fixation activity was measured using the acetylene reduction technique. The character of epiphytic populations on the surface of Sargassum was investigated by scanning electron microscopy. All three species of Sargassum exhibited the potential for high levels of acetylene reduction. Mean rates of up to 7.1 μmol C 2H 4 produced·g −1 ( Sargassum dry wt.)·h −1 were observed at one location. Nitrogen fixation activity was strongly light dependent. Saturation light intensity for nitrogen fixation was low, i.e. < 100 μE·m −2·s −1, and no photoinhibition was observed under full sunlight intensity (i.e. photon flux of ≈ 2000 μE·m −2·s −1). Results indicated that cyanobacteria were responsible for nitrogen fixation. Both Calothrix and LPP type cyanobacteria were commonly represented on the surface of the Sargassum. Activity associated with the benthic species S. filipendula exhibited significant seasonal variability. Nitrogen fixation activity in pelagic samples was variable but high throughout the year. The contribution of nitrogen fixation to the nitrogen budget of Sargassum communities appears to be particularly pronounced in the pelagic environment.

PHOTOSYNTHESIS IN THE FEMALE GAMETOPHYTE OF GINKGO BILOBA

The female gametophyte of Ginkgo biloba is the only seed plant gametophyte known to contain chlorophyll. Measurements of photosynthetically active radiation (PAR) indicate that a gametophyte growing within an ovule can receive significant quantities of light (70 μmol photons m–2 s–1). Under the conditions of our experiments, whole gametophytes dissected free from ovules were capable of gross photosynthesis, but not net photosynthesis. On a dry wt basis, the maximum rate of carbon fixation under near saturating light intensities was 3.64 × 10–3 μmol CO2g–1 s–1. The unique ability of the female gametophyte of Ginkgo biloba to produce chlorophyll and engage in photosynthesis appears to result from its exposure to sufficient levels of light and a predisposition to react to this stimulus by the development of a functional photosynthetic apparatus.

Photosynthesis in the Female Gametophyte of Ginkgo biloba

The female gametophyte of Ginkgo biloba is the only seed plant gametophyte known to contain chlorophyll. Measurements of photosynthetically active radiation (PAR) indicate that a gametophyte growing within an ovule can receive significant quantities of light (70 μmol photons m–2 s–1). Under the conditions of our experiments, whole gametophytes dissected free from ovules were capable of gross photosynthesis, but not net photosynthesis. On a dry wt basis, the maximum rate of carbon fixation under near saturating light intensities was 3.64 × 10–3 μmol CO2g–1 s–1. The unique ability of the female gametophyte of Ginkgo biloba to produce chlorophyll and engage in photosynthesis appears to result from its exposure to sufficient levels of light and a predisposition to react to this stimulus by the development of a functional photosynthetic apparatus.