High Quantum Flux Research Articles

LED Blue Light-induced changes in phenolics and ethylene in citrus fruit: Implication in elicited resistance against Penicillium digitatum infection

The objective was to investigate whether LED Blue Light (LBL) induces changes in phenolics and ethylene production of sweet oranges, and whether they participate in LBL-elicited resistance against the most important postharvest pathogen (Penicillium digitatum) of citrus fruit. The expression of relevant genes of the phenylpropanoid and ethylene biosynthetic pathways during elicitation of resistance was also determined. Different LBL (wavelength 450nm) quantum fluxes were used within the 60–630μmolm−2s−1 range. The HPLC analysis showed that the most relevant increase in phenylpropanoids occurred in scoparone, which markedly increased 3days after exposing fruits to a very high quantum flux (630μmolm−2s−1) for 18h. However, phenylpropanoids, including scoparone, were not critical factors in LBL-induced resistance. The genes involved in ethylene biosynthesis were differentially regulated by LBL. Ethylene is not involved in elicited resistance, although high LBL levels increased ethylene production in only 1h.

Effect of LED Blue Light on Penicillium digitatum and Penicillium italicum Strains.

Studies on the antimicrobial properties of light have considerably increased due in part to the development of resistance to actual control methods. This study investigates the potential of light-emitting diodes (LED) blue light for controlling Penicillium digitatum and Penicillium italicum. These fungi are the most devastating postharvest pathogens of citrus fruit and cause important losses due to contaminations and the development of resistant strains against fungicides. The effect of different periods and quantum fluxes, delaying light application on the growth and morphology of P. digitatum strains resistant and sensitive to fungicides, and P. italicum cultured at 20°C was examined. Results showed that blue light controls the growth of all strains and that its efficacy increases with the quantum flux. Spore germination was always avoided by exposing the cultures to high quantum flux (700 μmol m(-2) s(-1) ) for 18 h. Continuous light had an important impact on the fungus morphology and a fungicidal effect when applied at a lower quantum flux (120 μmol m(-2) s(-1) ) to a growing fungus. Sensitivity to light increased with mycelium age. Results show that blue light may be a tool for P. digitatum and P. italicum infection prevention during handling of citrus fruits.

Excitation kinetics during induction of chlorophyll a fluorescence

We present a chlorophyll fluorometer module system which adapts the intensity to the individual leaf sample by adjusting the quantum flux density of the excitation light so that the fluorescence signal is kept constant. This is achieved by means of a feedback power adjustment of the fluorescence exciting laser diode. Thus, the intensity of the excitation light is adapted to the actual need of a particular sample for quantum conversion without applying exaggeratedly high quantum flux density. We demonstrate the influence of the initial laser power chosen at the onset of irradiation and kept constant during fluorescence rise transient within the first second. Examples are shown for measuring upper and lower leaf sides, a single leaf with different pre-darkening periods, as well as yellow, light green and dark green leaves. The novel excitation kinetics during the induction of chlorophyll fluorescence can be used to study the yield and regulation of photosynthesis and its related non-photochemical processes for an individual leaf. It allows not only to sense the present state of pre-darkening or pre-irradiation but also the light environment the leaf has experienced during its growth and development. Thus, the individual physiological capacity and plasticity of each leaf sample can be sensed being of high importance for basic and applied ecophysiological research which makes this new methodology both innovative and informative.

Microclimate of the brown alga Feldmannia caespitula interstitium under zero-flow conditions

The microclimate of the brown alga Feldmannia caespitula (J. Agardh) Knoepffler-Peguy interstitium was stud- ied using microelectrode techniques. Zero water flow and ir- radiances of 170 and 1500 µmol photons m -2 s -1 cause steep O2 gradients peaking 3 to 4 mm below the outer surface of the tufts at 310 and 506% atmospheric saturation, respectively. The mean fluxes of O2 from the interstitium to the surround- ing bulk water were 87 ± 21 and 262 ± 68 nmol cm -2 h -1 at low and high quantum flux density. Except for the outer 2 to 4 mm thick margin, the alga interstitium became anoxic within 52 min after abrupt darkening. The rate of dark oxygen up- take was 52 ± 5 nmol cm -2 h -1 . The tufts were populated by 9 metazoan taxa: nematodes, harpacticoid copepods, ostracods, gastropods, bivalves, polychaetes, amphipods, isopods and halacarids. Our results suggest that the interstitium of fine- textured algal thalli is a microhabitat of variable water chem- istry with temporary anoxia and hyperoxia in an otherwise relatively stable water column. Although the tufts are attrac- tive for meiofauna by providing food and protection from cur- rents and predators, rapid fluctuations in oxygen concentra- tion probably cue temporal emigration of the algal infauna.

Gas exchange characteristics of leaves of four species of grain amaranth

Four species of the C4 genus Amaranthus, A. cruentus, A. caudatus, A. hypochondriacus and A. hybridus, were grown at Salt Lake City, Utah for the determination of gas exchange characteristics of recently matured leaves. The response of CO 2 assimilation to leaf temperature, measured at high quantum flux, was very similar in all species, each exhibiting a temperature optimum of approximately 37°C. Above about 40°C, all species showed high temperature inhibition, manifested as an irrversible time-dependent decline in CO 2 assimilation. All four exhibited high rates of net CO 2 assimilation exceeding 40 μmol m −2 s −1 at quantum fluxes of 2.0 mmol m −2 s −1, but none was completely light-saturated at such intensities. Leaf conductance to water vapor declined with decreasing quantum flux in all species, but less rapidly than assimilation, resulting in increasing values of internal CO 2 concentration ( c i). Quantum yield values in three species ( x=0.048±0.003 mol( CO 2/mol(photons))) were similar to those reported previously for NAD-malic enzyme type C4 dicots, but leaves of A. hybridus, which were deep purple in color, exhibited a lower quantum yield, presumably due to light absorption by betacyanin pigments not involved in photosynthesis. All four species had net photosynthesis versus c i responses typical of C4 species, with a steep initial linear response to c i followed by a fairly abrupt transition to saturation in the region of 100–150 μl l −1. Under near-optimal measurement conditions and high quantum flux, all species maintained c i values close to the region at which saturation was reached ( x=148±17 μl l −1). Data pooled acrossspecies relating net photosynthesis measured at 2.0 mmol m −2 s −1 quantum flux and 35°C to Kjeldahl nitrogen content revealed a linear relationship, the slope of which was not significantly different ( P=0.05) from those reported in two otudies employing C4 species.

Limitation of Photosynthesis by Carbon Metabolism

The occurrence of O(2)-insensitive photosynthesis at high quantum flux and moderate temperature in Spinacia oleracea was characterized by analytical gas exchange measurements on intact leaves. In addition photosynthetic metabolite pools were measured in leaves which had been rapidly frozen under defined gas conditions. Upon switching to low O(2) in O(2)-insensitive conditions the ATP/ADP ratio fell dramatically within one minute. The P-glycerate pool increased over the same time. Ribulose bisphosphate initially declined, then increased and exceeded the pool size measured in air. The pools of hexose monophosphates and UDPglucose were higher at a partial pressure of O(2) of 21 millibars than at 210 millibars. These results are consistent with the hypothesis that the rate of sucrose synthesis limited the overall rate of assimilation under O(2)-insensitive conditions.

The midday depression of CO2 assimilation in leaves of Arbutus unedo L.: diurnal changes in photosynthetic capacity related to changes in temperature and humidity

Parts of attached leaves of the sclerophyllous shrub Arbutus unedo were subjected to simulated mediterranean days. Gas exchange was recorded in order to recognize the causes of the midday depression in CO2 assimilation. Depressions could be induced in part of a leaf: they were local responses. The CO2-saturation curves of photosynthesis, determined during the morning and afternoon maxima of CO2 assimilation and during the minimum at midday, established that depressions in CO2 assimilation were in one-half of the investigated cases totally caused by reversible reductions in the photosynthetic capacity of the leaves, and in the other half almost totally caused by such reductions. An analysis of 37 daily courses showed that morning reductions and afternoon recoveries of stomatal conductance and rate of photosynthesis occurred simultaneously and in proportion to each other, with the result that the partial pressure of CO2 in the intercellular spaces remained more or less constant. Midday depressions occurred also in detached leaves standing in water. The initiation of a midday depression was not caused by a circadian rhythm, nor was high quantum flux or high temperature a requirement. There was no correlation between the rate of water loss from the leaves, or the amount of water lost, with the degree of reduction of the photosynthetic capacity. However, depressions occurred if an apparent threshold in the water-vapor pressure difference between leaf and air was exceeded. This critical value varied between about 20 and 30 mbar, depending on the leaf investigated. The dominating role of humidity in the induction of the midday depression was further demonstrated when leaf temperature was held constant and the vapor-pressure difference was made to follow the pattern of the mediterranean day: depressions occurred. Depressions however were hardly noticeable when the water-vapor pressure difference was held constant and leaf temperature was allowed to vary. In another set of experiments, leaves were subjected to variations in temperature and humidity independent of the time of the day, under otherwise constant conditions. Photosynthetic capacity and stomatal conductance proved to be almost insensitive to changes in temperature (in a range extending from 20 to 37° C) as long as the water vapor-pressure difference was held constant. If it was not, the rate of photosynthesis began to decline with increasing temperature after a threshold water-vapor pressure difference was exceeded. The position of the resulting apparent temperature optimum of photosynthesis depended on the humidity of the air. We suggest that the ability of A. unedo to respond to a dry atmosphere with a reversible reduction of its photosynthetic capacity (by a still unknown mechanism) is the result of a co-evolution with the development of a strong stomatal sensitivity to changes in humidity.

Cotton (Gossypium hirsutum L.): Physiological and morphological responses to water deficits and their relationship to yield

With the aid of a rainout shelter and different irrigation treatments, a range of soil and plant water deficits were imposed on cotton (Gossypium hirsutum L.) in order to study the physiological and morphological responses to water stress as a step towards maximising yield per unit of available water. Diurnal changes in plant water relations, photosynthetic rate, leaf conductance and leaf expansion were measured on two or three treatments on five occasions during the growing season. Additionally, measurements of soil water content and detailed observations on the development of floral buds, flowers and bolls and the changes in leaf area and yield were made. As the soil water deficits increased, the leaves adjusted osmotically by 0.5 to 0.6 MPa at full turgor: diurnal changes in osmotic adjustment were about 0.1 MPa. The photosynthetic rate and leaf conductance at high quantum flux densities in the middle of the day decreased progressively with leaf water potential between −1.9 MPa and −3.5 MPa. Leaf expansion was more sensitive to soil and leaf water deficits than was leaf photosynthesis. Leaf size decreased during the day when the predawn leaf water potential decreased below −0.8 MPa. Water deficits reduced leaf and fruiting-site development, the number of floral buds and bolls retained, and final yield, but the proportion of flowers that set bolls was not affected except where substantial soil water deficits developed during early flowering. Large floral buds were retained at the expense of small floral buds. We suggest that soil water deficits reduced the capacity of the crop to carry fruit as a result of lower rates of leaf photosynthesis.

A system for measuring leaf gas exchange based on regulating vapour pressure difference

A system for measurement of leaf gas exchange while regulating leaf to air vapour pressure difference has been developed; it comprises an assimilation chamber, leaf temperature controller, mass flow controller, dew point controller and personal computer. A relative humidity sensor and air and leaf temperature sensors, which are all used for regulating the vapour pressure difference, are mounted into the chamber. During the experiments, the computer continuously monitored the photosynthetic parameters and measurement conditions, so that accurate and intenstive measurements could be made.When measuring the light-response curve of CO2 assimilation for single leaves, in order to regulate the vapour pressure difference, the leaf temperature and relative humidity in the chamber were separately and simultaneously controlled by changing the air temperature around the leaf and varying the air flow rate through the chamber, respectively. When the vapour pressure difference was regulated, net CO2 assimilation, transpiration and leaf conductance for leaves of rice plant increased at high quantum flux density as compared with those values obtained when it was not regulated.When measuring the temperature-response curve of CO2 assimilation, the regulation of vapour pressure difference was manipulated by the feed-forward control of the dew point temperature in the inlet air stream. As the vapour pressure difference was regulated at 12 mbar, the maximum rate of and the optimum temperature for CO2 assimilation in rice leaves increased 5 μmolCO2 m(-2) s(-1) and 5°C, respectively, as compared with those values obtained when the vapour pressure difference took its own course. This was reasoned to be due to the increase in leaf conductance and the decrease in transpiration rate. In addition, these results confirmed that stomatal conductance essentially increases with increasing leaf temperature under constant vapour pressure difference conditions, in other words, when the influence of the vapour pressure difference is removed.This system may be used successfully to measure inter- and intra-specific differences and characteristics of leaf gas exchange in plants with a high degree of accuracy.

Effects of CO2 enrichment on growth and photosynthesis in Desmodium paniculatum

Plants of Desmodium paniculatum (Leguminosae) were grown from seeds in normal air (0.035% CO2) and in air containing 0.1% CO2. CO2 enrichment produced an almost twofold increase in dry matter production. Leaf areas were 1.2 times higher at high CO2. Relative growth rates under high CO2 were significantly higher during the early stages of development and then declined. The increase in dry matter production was not associated with an increase in photosynthetic capacity per unit leaf area. Gas exchange measurements indicated that plants from the high CO2 regime had lower light saturation values and lower photosynthetic rates at high quantum flux densities than the controls. An opposite trend was observed if gas exchange rates were expressed on a chlorophyll basis. The depression in photosynthetic rates on a leaf area basis of plants exposed to CO2 enrichment was correlated with increased starch accumulation and reduced grana formation in chloroplasts of leaves continuously exposed to high CO2 concentrations.

Effects of SO2on photosynthesis and ribulose bisphosphate carboxylase in pine tree seedlings

Fumigations of 4–7 week old seelings ofPinus silvestris, with ca 400 μg SO2m‐3for 4–5 days resulted in a decrease both in net photosynthetic rate and in the levels of ribulose bisphosphate carboxylase activity of the primary needles. The effect on net photosynthesis was reversible. No visible damages were observed on the needles. Lower concentrations, ca 200 μg SO2m‐3, for ca 6 days inhibited net photosynthesis reversibly. The effect on net photosynthesis was not the result of stomata closure or of increased respiration. A dose‐response relation for the inhibition of net photosynthesis and ribulose bisphosphate carboxylase is suggested. A similar relation was observed for the chlorophyll content of the needles. The reversible inhibition of photosynthesis was evident only at high quantum flux density, and the quantum yield of photosynthesis at rate limiting quantum flux was not affected. This was also suggested from an experiment with Scots pine in the field.

Leaf Chlorophyll Fluorescence as an Indicator of High Light Stress (Photoinhibition) in Cucumis sativus L

Chlorophyll fluorescence induction kinetics were measured in detached leaves of cucumber plants (Cucumis sativus L.) grown at low quantum flux density (120 �E m-2 s-1) and then irradiated with high quantum flux density (650 �E m-2 s-1). Short-term irradiations (e.g. 1 min) resulted in loss of the variable component of chlorophyll fluorescence which could be fully recovered over a period of 60-80 min of dark adaptation. Longer irradiations wlth high intensity light (up to 5 h), however, caused an irreversible reduction in fluorescence yield indicative of irreversible photoinhibition. Irradiations as short as 4.5 min produced significant photoinhibition. Similar treatments did not cause irreversible changes in the fluorescence induction kinetics of cucumber leaves grown in sunlight. The capacity of low light grown leaves to photooxidize cytochrome f was not affected by photoinhibitory treatments for as long as 5 h, whereas the slow component of the 518 nm change was almost completely lost. These results support previous findings and provide direct evidence that the sensitive site for photoinhibition in vivo is located on the oxidizing side of photosystem II and that photosystem I mediated reactions do not become inhibited. It is suggested that measurements of chlorophyll fluorescence provide a rapid means of estimating photoinhibitory damage to plants.

A Method with Integrated Temperature Control to Measure Potential Net Photosynthesis in Green Plant Tissues at High Quantum Flux Densities

Summary An improved method is described for measuring photosynthetic oxygen evolution (P N ) from green plant tissues under high quantum flux densities (2.2 mE m −2 s −1 , 400–700 nm) at saturating CO 2 levels. The plant material is placed into an assimilation chamber and its O 2 -output is measured by polarographic oxygen sensing at +0.8 V-DC. Both assimilation chamber and oxygen sensor are integrated into a self-contained temperature control system. Because of its efficiency the method is recommended for applied studies in agronomy, ecology, and plant pathology.