Maximum Photosynthetic Photon Flux Research Articles

Photomorphogenesis of wheat sprouts with LED irradiation of different intensities

Definition of the growth and development characteristics of plants in varied light conditions is a key factor for the creation of highly efficient light facilities for plant cultivation. Experimental research was conducted using an LED irradiation facility with photosynthetic photon flux densities ranging from 0 to 261 μmol m−2 s−1 and a continuous spectrum with maxima at 445 and 600 nm. Under the maximum photosynthetic photon flux density (261 μmol m− 2 s−1) wheat germs demonstrated diminishing leaf surface with high values of specific leaf area, enhanced pubescence of ground tissues, increases in the number of stomata on the upper epidermis and palisade, and an increase in the thickness of the leaves as well as an increase in carotenoids but a decrease in the chlorophyll a+b/carotenoids relation. It was revealed that the optimum level of photosynthetic photon flux density for the referred spectrum was in the range from 82 to 100 µmol m−2 s−1, which may enable a reduction of irradiance under specific conditions during early development with no harm to the plants while minimizing energy consumption during cultivation.

Spectral Characterization of Difusse Par Irradiance under Tipuana Tipu Shading

Tipa (Tipuana tipu) is a common tree in gardens and carparks, although the shading effect of its canopy must be still characterised to assess the decrease of temperature and quality of irradiance. This work is a preliminary study aimed to assess the effect of shading of tree canopies on the diffuse irradiance pattern received at the soil level in comparison to other conditions. The shade provided by a group of Tipa trees, located at the Universitat Politècnica de València (Valencia, Spain), was evaluated in this experiment and compared to cloudy days and direct sun. Photosynthetic Photon Flux Density (PPFD) and red/near infrared ratios were recorded with a portable spectrometer. Measurements were recorded in January and February 2017, at 10h, 13h and 16h. Depending on the region of the spectrum and time of the day, PPFD values ranged from 0.05 to 0.42, 0.40 to 1.14, and 0.94 to 3.90 μmol·m-2·s-1, for Tipa shade, cloudy days and direct sun, respectively. The spectral analysis of PPFD in cloudy days revealed maximum values in the green region and minimum at near infrared region, while maximum PPFD for tipa shade was mostly found at near infrared, revealing higher importance of this spectral region compared to cloudy days.

A method for micropropagation of Cornus wilsoniana: An important biofuel plant

Cornus wilsoniana Wangerin is cultivated in China as an important biofuel plant due to its high oleic and linoleic acids derived from fruit. Recently, high yield cultivars have been developed, and there is increasing need for vegetative propagation of these new cultivars for commercial production. This study establishes a method for micropropagation of a selected new cultivar Xiang Lin G1 through shoot culture. Shoots were collected from field-grown ‘Xiang Lin G1’. After surface sterilization, shoot explants were cultured on Driver and Kuniyuki Walnut medium (DKW) supplemented with 300mgL−1 polyvinylpyrrolidone (PVP), 0.5mg L−1 zeatin, and 0.1mgL−1 indole-3-butyric acid. Axillary bud break occurred and more than 4 shoots were produced per explant. The axillary shoots could either be used as explants for additional shoot production or be rooted in DKW medium containing 300mgL−1 PVP and 1.0mgL−1 potassium salt of indole butyric acid. Resultant plantlets were transplanted into a substrate and grew vigorously in a shaded greenhouse under a maximum photosynthetic photon flux density of 200μmolm−2 s−1. Since the propagation is based on existing meristems, no somaclonal variants were observed in the micropropagated population. This established method could lead to rapid propagation of the high yield cultivars on a year-round basis and quickly increase their production hectares.

Spatial, Vertical, and Temporal Variability of Ambient Environments in Strawberry and Tomato Greenhouses in Winter

Strawberry and Tomato Greenhouses in Winter Myong-Jin Ryu, Dong-Ki Ryu, Sun-Ok Chung*, Yun-Kun Hur, Seung-Oh Hur, Soon-Jung Hong, Je-Hoon Sung, Hak-Hun Kim Department of Biosystems Machinery Engineering, Chungnam National University, Daejeon, Korea Rural Development Administration, Gyeonggi-do, Korea Chungnam Agriculture Research & Extension Services, Chungcheongnam-do, Korea Received: November 18th, 2013; Revised: February 8th, 2014; Accepted: February 19th, 2014 Purpose: In protected crop production facilities such as greenhouse and plant factory, farmers should be present and/or visit frequently to the production site for maintaining optimum environmental conditions and better production, which is time and labor consuming. Monitoring of environmental condition is highly important for optimum control of the conditions, and the condition is not uniform within the facility. Objectives of the paper were to investigate spatial and vertical variability in ambient environmental variables and to provide useful information for sensing and control of the environments. Methods: Experiments were conducted in a strawberry-growing greenhouse (greenhouse 1) and a cherry tomato-growing greenhouse (greenhouse 2). Selected ambient environmental variables for experiment in greenhouse 1 were air temperature and humidity, and in greenhouse 2, they were air temperature, humidity, PPFD (Photosynthetic Photon Flux Density), and CO2 concentration. Results: Considerable spatial, vertical, and temporal variability of the ambient environments were observed. In greenhouse 1, overall temperature increased from 12:00 to 14:00 and increased after that, while RH increased continuously during the experiments. Differences between the maximum and minimum temperature and RH values were greater when one of the side windows were open than those when both of the windows were closed. The location and height of the maximum and minimum measurements were also different. In greenhouse 2, differences between the maximum and minimum air temperatures at noon and sunset were greater when both windows were open. The maximum PPFD were observed at a 3-m height, close to the lighting source, and CO2 concentration in the crop growing regions. Conclusions: In this study, spatial, vertical, and temporal variability of ambient crop growing conditions in greenhouses was evaluated. And also the variability was affected by operation conditions such as window opening and heating. Results of the study would provide information for optimum monitoring and control of ambient greenhouse environments.

EFFECTS OF SELENIUM ENRICHMENT OF TOMATO PLANTS ON RIPE FRUIT METABOLISM AND COMPOSITION

EFFECTS OF SELENIUM ENRICHMENT OF TOMATO PLANTS ON RIPE FRUIT METABOLISM AND COMPOSITION

Rhamnus alaternus growth under four simulated shade environments: Morphological, anatomical and physiological responses

Light management and species light plasticity are critical factors for proper crop production in nurseries. So, to assess the light necessities of Rhamnus alaternus, which is a promising native evergreen Mediterranean shrub of ornamental and ecological interest, it was exposed to five shade levels (0%, 32%, 48%, 84% and 93%) for one year (0% of shade received annual mean maximum photosynthetic photon flux density [PPFD] of 1293 ± 415 μmol m −2 s −1 with a summer mean maximum PPFD of 1541 ± 225 μmol m −2 s −1 [mean ± s.d.]). The treatments providing 84% and 94% shading produced less compact plants, with greater height and less stem diameter, greater specific leaf area and higher shoot/root index. In general, the shading produced dark green leaves except the 94% treatment, in which the leaves were lighter because the relative chlorophyll content was reduced. Leaf thickness was reduced in all the shading treatments compared with the control. The stomatal density and pore area were reduced with the 84% and 93% shading treatments, as was photosystem II (PSII) efficiency and the leaf soluble sugar content (falling to 60% of the control level); in contrast, non-photochemical quenching increased. The 93% treatment caused irreversible damages in PSII because dark-adapted PSII efficiency was reduced. The results show that solar radiation can be used as a tool to improve plant quality during the nursery period. We conclude that shading screens providing more than 84% shade are not recommendable in terms of photochemical activity, soluble sugar content and overall plant quality. Shading screens of 32% and 48% improve R. alaternus quality in spring and summer, respectively. As regards its interest as an understory species for reforestation programs, we also conclude that this shrub can be adapted to a wide range of shade intensities, although its proper development would be compromised in very strong shading conditions.

Regeneration of Dracaena surculosa Through Indirect Shoot Organogenesis

This study established a method of regenerating Dracaena surculosa Lindl. ‘Florida Beauty’ through indirect shoot organogenesis. Bud, leaf, and stem explants were cultured on a Murashige and Skoog basal medium supplemented with N6-(2-isopentyl) adenine (2iP) at 12.3 and 24.6 μM with 3-indoleacetic acid (IAA) at 0, 1.1, and 2.3 μM, respectively, and 2iP at 36.9, 49.2, 61.5, and 73.8 μM with IAA at 1.1 and 2.3 μM, respectively. Calluses were induced from leaf explants but failed to produce adventitious shoots. Calluses were also induced from stem and bud explants cultured on the basal medium containing 12.3 μM 2iP and 2.3 μM IAA, 24.6 μM 2iP or higher with either 1.1 or 2.3 μM IAA. The highest callus induction frequency was 63.2% from stem explants and 69.6% from bud explants when they were cultured on the basal medium supplemented with 49.2 μM 2iP and 2.3 μM IAA. The highest shoot formation frequency was 65.7% from stem-derived callus cultured on the basal medium containing 61.5 μM 2iP and 1.1 μM IAA and 88% from bud-derived callus cultured with 49.2 μM 2iP and 1.1 μM IAA. The highest number of shoots per piece of stem- and bud-derived calluses was 3.8 and 6.7, respectively. Adventitious shoots developed better root systems in the basal medium supplemented with 2.0 μM IAA. Plantlets after transplantation into a soilless substrate grew vigorously in a shaded greenhouse under a maximum photosynthetic photon flux density of 300 μmol·m−2·s−1. Neither disease incidence nor somaclonal variants were observed in the regenerated population. This established method could be used for efficient micropropagation of D. surculosa, and the availability of tissue-cultured liners could reduce the dependency on imported cuttings, which often bring new or invasive pests into the United States.

Intermittent Light from a Rotating High-pressure Sodium Lamp Promotes Flowering of Long-day Plants

A technology for long-day (LD) lighting was evaluated for commercial production of ornamentals using a stationary high-pressure sodium (HPS) lamp with an oscillating aluminum parabolic reflector (rotating HPS lamp). We performed an experiment with four LD species (Campanula carpatica Jacq., Coreopsis grandiflora Hogg ex Sweet, Petunia ×hybrida Vilm.-Andr., and Rudbeckia hirta L.) to compare the efficacy of a rotating HPS lamp in promoting flowering with night-interruption (NI) lighting using incandescent (INC) lamps. Seedlings were grown under natural short-day (SD) photoperiods (12 h or less) and NI treatments were delivered from a 600-W rotating HPS lamp mounted at one gable end of the greenhouse or from INC lamps that were illuminated continuously for 4 h or cyclically for 6 min every 30 min for 4 h. Plants were grown at lateral distances of 1, 4, 7, 10, or 13 m from the rotating HPS lamp, which provided a maximum photosynthetic photon flux of 25.4 μmol·m−2·s−1 (at 1 m) to 0.3 μmol·m−2·s−1 (at 13 m). Control plants were grown under an uninterrupted 15-h skotoperiod. Within 16 weeks, 80% or greater of the plants within each species that received NI lighting had a macroscopic visible flower bud or inflorescence, whereas all species but Petunia ×hybrida remained vegetative under the SD. Flowering of all species grown at 13 m from the rotating HPS lamp was delayed by 14 to 31 d compared with those under continuous INC. The weekly operational costs to provide NI lighting to a 139-m2 greenhouse with one 600-W rotating HPS lamp or a standard cyclic INC lamp installation was estimated to be 80% to 83% lower compared with INC lighting for the entire 4-h NI. These results indicate that a rotating HPS lamp can be used to efficiently deliver LD lighting, but flowering time was delayed and flower number reduced in some species when the maximum NI light intensity was less than 2.4 μmol·m−2·s−1.

Effects of Light Intensity and Paclobutrazol on Growth and Interior Performance of Pachira aquatica Aubl.

Pachira aquatica Aubl. has recently been introduced as an ornamental foliage plant and is widely used for interiorscaping. Its growth and use under low light conditions, however, have two problems: leaf abscission and accelerated internode elongation. This study was undertaken to determine if production light intensity and foliar application of paclobutrazol [β-(4-chlorophenyl)methyl-α-(1,1-dimethylethyl)-1H- 1,2,4- triazole-1-ethanol] improved plant growth and subsequent interior performance. Two-year-old P. aquatica trunks were planted in 15-cm diameter plastic pots using a peat-based medium and were grown in a shaded greenhouse under three daily maximum photosynthetic photon flux densities (PPFD) of 285, 350, and 550 μmol·m−2·s−1. Plant canopy heights, average widths, and internode lengths were recorded monthly over a 1-year production period. Two months after planting, the plant canopy was sprayed once with paclobutrazol solutions at concentrations of 0, 50, and 150 mg·L−1, ≈15 mL per plant. Before the plants were placed indoors under a PPFD of 18 μmol·m−2·s−1 for 6 months, net photosynthetic rates, quantum yield, and light saturation and compensation points were determined. Results showed that lowering production light levels did not significantly affect canopy height, width, or internode length but affected the photosynthetic light response curve and reduced the light compensation point. Foliar application of paclobutrazol reduced internode length, thereby resulting in plants with reduced canopy height and width and more compact growth form. Paclobutrazol application also reduced the light compensation point of plants grown under 550 μmol·m−2·s−1. Plants with the compact growth form did not grow substantially, dropped fewer leaflets, and thus maintained their aesthetic appearance after placement indoors for 6 months. These results indicated that the ornamental value and interior performance of P. aquatica plants can be significantly improved by producing them under a PPFD range between 285 and 350 μmol·m−2·s−1 and foliar spraying of paclobutrazol once at a concentration between 50 and 150 mg·L−1.

PROPAGATION AND PRODUCTION OF ZAMIOCULCAS ZAMIIFOLIA

PROPAGATION AND PRODUCTION OF ZAMIOCULCAS ZAMIIFOLIA

CONTAINER OPACITY AND MEDIA COMPONENTS INFLUENCE ROOTING OF POTTED PHALAENOPSIS AND DORITAENOPSIS ORCHIDS

CONTAINER OPACITY AND MEDIA COMPONENTS INFLUENCE ROOTING OF POTTED PHALAENOPSIS AND DORITAENOPSIS ORCHIDS

EFFECT OF TEMPERATURE AND PSEUDOBULB MATURITY ON FLOWERING OF THE ORCHID MILTONIOPSIS AUGRES 'TRINITY'

ISHS XXVII International Horticultural Congress - IHC2006: International Symposium on Ornamentals, Now! EFFECT OF TEMPERATURE AND PSEUDOBULB MATURITY ON FLOWERING OF THE ORCHID MILTONIOPSIS AUGRES 'TRINITY'

(50) Evaluating Low Light Level Effects on Chlorophyll Content and Commercial Quality in Several Species of Bambuseae

Bamboo has increasingly become a popular exterior ornamental plant because of its durability, versatility, and evergreen qualities in conditions of extreme temperature and moisture variations. Use as an interior foliage plant has been limited due to the difficulty of finding species adaptable to lower light levels. Nineteen species from seven genera (Bambusa, Cephalostachyum, Dendrocalamus, Gigantochloa, Schizostachyum, Thyrsostachys, and Vietnamosasa) were evaluated. Fifteen plants from each species were potted in like conditions (50% leaf mold; 50% topsoil; 5 g of 14–14–14 controlled-release fertilizer) and grown under a maximum daily photosynthetic photon flux density (PPFD) range between 1200 to 2000 μmol·m-2·s-1 for 6 weeks. Chlorophyll content of leaves was measured. The commercial quality of leaves, culms, and general appearance was also recorded. Light was then limited to a maximum PPFD of 150 to 300 μmol m-2s-1 for 6 weeks and all measurements were again recorded. Five species had significant increases in chlorophyll content after the 6-week period of reduced light levels. Species with a larger maturity size had a greater mortality percentage as well as lower quality leaf and overall appearance when grown under reduced light levels. Culm quality remained constant in 18 of the 19 species after the 6-week period. Vietnamosasa ciliata showed the greatest increase in chlorophyll levels as well as highest commercial quality of leaf and overall appearance.

(130) Dieffenbachia Calcium Oxalate Crystal Formation Affected by Cultivars, Nitrogen Rates, and Light Intensity

Tissue-culturedexplantsofDieffenbachiamaculate`Exotic Perfection', D.`Snow Flake', and D. × `Tropic Breeze' were grown on ebb-and-flow trays subirrigated with nitrogen (N) at 50, 200, or 800 mg·L-1 using a water-soluble fertilizer 17N–2.1P–15.7K for 10 weeks in a shaded greenhouse under a maximum photosynthetic photon flux density of 285 μmol·m-2·s-1. Plants were then transferred to interior rooms under a light level of 8 μmol·m-2·s-1. Samples of the midrib were taken from the first mature leaf of plants before being placed indoors and also from the first mature leaf of plants 8 months after growing indoors. Counts of calcium oxalate crystal idioblasts in cross-sections of the basal midrib using polarized light microscopy showed that the number of crystal idioblasts was higher in all three cultivars fertigated with 200 mg·L-1 N than those fertigated with either 50 or 800 mg·L-1 N. The number of crystal idioblasts in each cultivar grown under 8 μmol·m-2·s-1 was about 50% of the number detected when plants were grown under 285 μmol·m-2·s-1. `Snow Flake' had the highest number of crystal idioblasts with counts up to 60 per cross-section, whereas `Exotic Perfection' had the lowest with only 30 per cross-section. This study shows that in addition to cultivar differences, light intensity and N can significantly affect calcium crystal formation, and the highest number of crystal idioblasts occurred when Dieffenbachia cultivars were grown under optimum conditions.

FLOWERING OF THE ORCHID MILTONIOPSIS AUGRES ´TRINITY´ IS INFLUENCED BY PHOTOPERIOD AND TEMPERATURE

FLOWERING OF THE ORCHID MILTONIOPSIS AUGRES ´TRINITY´ IS INFLUENCED BY PHOTOPERIOD AND TEMPERATURE

Plasticity in Leaf and Photosynthetic Characteristics of Ficus benjamina Ggrown under Different Lght Regimes

Ficus benjamina is considered to have a high degree of morphological and physiological plasticity in response to light levels. In this study, leaf area and thickness, specific leaf area (SLA), chlorophyll content, and photosynthetic characteristics of Ficus benjamina `Common'; grown in a shaded greenhouse under four maximum photosynthetic photon flux densities (PPFDs) of 150, 250, 450, or 650 μmol·m-2·s-1 were investigated. Results showed that plants grown under 450 and 650 PPFDs had higher SLA and leaf thickness but smaller leaf areas than those grown under 150 and 250 PPFDs. Total chlorophyll content per unit leaf area decreased as PPFDs increased. Net photosynthetic rates (Pn) increased from 2.7 μmol·m-2·s-1 under 150 PPFD to 5.7 μmol·m-2·s-1 under 450 PPFD, then slightly decreased to 5.5 μmol·m-2·s-1 under 650 PPFD. The highest net photosynthetic rate was not associated with higher intercellular CO2 concentrations (Ci) and stomatal conductance (gs) as plants grown under 250 PPFD had the highest (Ci) (259 ppm) and gs (0.1 mol·m-2·s-1), which suggests that photosynthetic enzymes could play a increasing role under 450 PPFD. Plant quality, however, was not necessarily correlated with the Pn because only those grown under 250 PPFD had appropriate heights, large and dark green leaves, and well-spread branches, and thus were graded higher than plants grown under the other PPFDs. This study shows that fine-tuning production light level is important for high quality Ficus benjamina production.

TEMPERATURE AND PHOTOPERIODIC EFFECTS ON GROWTH AND FLOWERING OF ZYGOPETALUM REDVALE 'FIRE KISS' ORCHIDS

TEMPERATURE AND PHOTOPERIODIC EFFECTS ON GROWTH AND FLOWERING OF ZYGOPETALUM REDVALE 'FIRE KISS' ORCHIDS

Operation of the xanthophyll cycle and degradation of D1 protein in the inducible CAM plant, Talinum triangulare, under water deficit.

Changes in photochemical activity induced by water deficit were investigated in Talinum triangulare, an inducible CAM plant. The aim was to analyse the interactions between C3 photosynthesis, induction and activity of CAM, photosynthetic energy regulation and the mechanisms responsible for photoprotection and photoinhibition under water stress. Gas exchange, chlorophyll a fluorescence, titratable acidity, carotenoid composition and relative contents of the PSII reaction centre protein (D1) were measured. A decrease in xylem tension (psi) from -0.14 to -0.2 MPa substantially decreased daytime net CO2 assimilation and daily carbon gain, and induced CAM, as shown by CO2 assimilation during the night and changes in titratable acidity; a further decrease in psi decreased nocturnal acid accumulation by 60%. Non-photochemical quenching of chlorophyll a fluorescence (NPQ) increased with water deficit, but decreased with a more severe drought (psi below -0.2 MPa), when CAM activity was low. NPQ was lower at 0900 h (during maximum decarboxylation rates) than at 1400 h, when malate pools were depleted. Down-regulation of PSII activity related to the rise in NPQ was indicated by a smaller quantum yield of PSII photochemistry (phiPSII) in droughted compared with watered plants. However, phiPSII was larger at 0900 h than at 1400 h. The de-epoxidation state of the xanthophyll cycle increased with drought and was linearly related to NPQ. Intrinsic quantum yield of PSII (FV/FM) measured at dusk was also lower in severely stressed plants than in controls. Under maximum photosynthetic photon flux and high decarboxylation rates of organic acids, the D1 content in leaves of droughted plants showing maximal CAM activity was identical to the controls; increased drought decreased D1 content by more than 30%. Predawn samples had D1 contents similar to leaves sampled at peak irradiance, with no signs of recovery after 12 h of darkness. It is concluded that under mild water stress, early induction of CAM, together with an increased energy dissipation by the xanthophyll cycle, prevents net degradation of D1 protein; when water deficit is more severe, CAM and xanthophyll cycle capacities for energy dissipation decline, and net degradation of D1 proceeds.

Photosynthetic depression in relation to plant architecture in two alpine herbaceous species

The Qinghai-Tibet Plateau is characterized by extremely high radiation, which may induce down-regulation of photosynthesis in plants living in this alpine ecosystem. To clarify whether photoinhibition occurs in the alpine environment and to discern its underlying mechanisms, we examined photosynthetic gas exchange and fluorescence emission in response to the changes in photosynthetic photon flux density (PPFD) and leaf temperature under natural regimes for two herbaceous species: prostrate Saussurea superba and erect-leaved Saussurea katochaete from altitude 3250 m on the Qinghai-Tibet Plateau. S. superba intercepted a higher maximum PPFD and experienced much higher leaf temperature than the erect-leaved S. katochaete. S. superba exhibited a much higher light saturation point for photosynthesis than S. katochaete. Under controlled conditions, the former species had higher CO 2 uptake rates and neither species showed obvious photosynthetic down-regulation at high PPFD. Under natural environmental conditions, however, apparent photoinhibition, indicated by reduced electron transport rate (ETR), was evident at high PPFD for both species. After a night frost, the photochemistry of S. katochaete was depressed markedly in the early morning and recovered by mid-day. After a frost-free night, it was high in the morning and low at noon due to high radiation. S. superba did not respond to the night frost in terms of daytime photochemical pattern. In both species, photochemical depression was aggravated by high leaf temperature and the erect species was more sensitive to high temperature. This study suggests that the high radiation on the Qinghai-Tibet Plateau is likely to induce rapidly reversible photoinhibition, which is related closely to plant architecture. Photochemistry in the prostrate species seems able to tolerate higher PPFD, without obvious suppression, than the erect species.

Moderate shade can increase net gas exchange and reduce photoinhibition in citrus leaves.

Daily variations in net gas exchange, chlorophyll a fluorescence and water relations of mature, sun-acclimated grapefruit (Citrus paradisi Macfady.) and orange (Citrus sinensis L. Osbeck) leaves were determined in tree canopies either shaded with 50% shade screens or left unshaded (sunlit). Mean daily maximum photosynthetic photon flux density (PPFD) under shade varied from 500 to 700 micromol m-2 s-1 and was sufficient to achieve maximum net CO2 assimilation rates (A CO2). Responses of grapefruit and orange leaves to shading were remarkably similar. At midday, on bright clear days, the temperatures of sunlit leaves were 2-6 degrees C above air temperature and 1-4 degrees C above the temperatures of shaded leaves. Although midday depressions of stomatal conductance (gs) and A CO2 were observed in both sunlit and shaded leaves, shaded leaves had lower leaf-to-air vapor pressure differences (D) along with higher gs, A CO2 and leaf water-use efficiency than sunlit leaves. Estimated stomatal limitation to A CO2 was generally less than 25% and did not differ between shaded and sunlit leaves. Leaf intercellular CO2 partial pressure was not altered by shade treatment and did not change substantially with increasing D. Radiation and high temperature stress-induced non-stomatal limitation to A CO2 in sunlit leaves was greater than 40%. Reversible photoinhibition of photosystem II efficiency was more pronounced in sunlit than in shaded leaves. Thus, non-stomatal factors play a major role in regulating A CO2 of citrus leaves during radiation and high temperature stress.