Low Photosynthetic Photon Flux Research Articles

Acclimation of Photosynthesis and Dark Respiration of a Submersed Angiosperm beneath Ice in a Temperate Lake.

Ceratophyllum demersum L. remained physiologically active beneath ice of a southeastern Michigan lake. The effect of seasonally low photosynthetic photon flux density (PPFD) and cold but nonfreezing temperature on whole-plant physiology was studied. Net photosynthesis was measured at six temperatures and 12 PPFDs. Net photosynthesis, soluble protein concentration, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein concentration, and Rubisco activity of winter plants were 32, 31, 33, and 70% lower, respectively, than those of plants collected in the summer. Optimum temperatures for net photosynthesis of winter and summer plants were 5 and 30[deg]C, respectively. Dark respiration of winter plants was up to 313% greater than that of summer plants. Reduced Rubisco activity and increased dark respiration interacted to reduce net photosynthesis. Interaction of reduced net photosynthesis and increased dark respiration increased CO2 and light compensation points and the light saturation point of winter plants. Growth of C. demersum was limited by the ambient phosphorus concentration of lake water during summer. Apical stem segments of winter-collected plants had 54 and 35% more phosphorus and nitrogen, respectively, than summer-collected plants. Physiologically active perennation beneath ice enabled C. demersum to accumulate phosphorus during the winter when it was most abundant. Partial uncoupling of phosphorus acquisition from utilization may reduce phosphorus limitation upon growth during the summer when phosphorus concentration is seasonally the lowest.

Effects of shading on gas exchange, specific leaf weight and chlorophyll content in four kiwifruit cultivars under field conditions

Diurnal changes of incident photosynthetic photon flux density (PPFD), transpiration rate (E), photosynthetic rate (Pn), stomatal conductance (Cs) and water use efficiency (WUE), together with a number of environmental factors, were measured on individual leaves at well exposed and shaded positions within the canopy for four cultivars of kiwifruit (Actinidia sinensis, P.). Pn and E rates were'much lower in the shaded leaves for all cultivars due to extremely low PPFD values reaching these leaves. Cs followed a trend similar to that for Pn. Exposed leaves showed a significantly higher WUE in the morning, while no difference was observed between exposed and shaded leaves around midday. Specific leaf weight (SLW) was lower in shaded leaves, which had a greater chlorophyll content than exposed leaves.

Respiration of geranium and petunia in response to low night temperature

The morphology and rate of development of some bedding plant species is altered by growth at low night temperatures. This cultural practice is of interest because it results in lower greenhouse energy costs and higher productivity at low photosynthetic photon flux. Low night temperatures and changes in the carbohydrate content of the plants may alter energy losses from maintenance respiration. To test this hypothesis, seedlings of geranium (cultivar ‘Red Elite’) and petunia (cultivar ‘Snow Cloud’) were grown at day temperatures of 25°C and night temperatures of either 7°C or 15.5°C. Thereafter, CO 2 evolution in the dark at 21°C was measured and starch, soluble sugar and total nitrogen levels were determined. The respiration rates at 21°C of plants grown at 7°C night temperature were consistently higher than those of plants grown at 15.5°C night temperature. Plants grown at the low night temperature had higher starch and soluble sugar levels, even after 112 h in darkness. Effects of night temperature in winter on maintenance respiration and reserve carbohydrate accumulation are discussed.

Differential effects of drought and light levels on accumulation of citric and malic acids during CAM in Clusia

ABSTRACTNight‐time citrate accumulation has been proposed as a response to stress in CAM plants. To address this hypothesis, gas exchange patterns and nocturnal acid accumulation in three species of Clusia were investigated under controlled conditions with regard to water stress and responses to low and high photosynthetic photon flux density (PPFD). Under high PPFD, leaves of Clusia nocturnally accumulated large amounts of both malic and citric acids. Under low PPFD and well‐watered conditions, substantial night‐time citrate accumulation persisted, whereas malate accumulation was close to zero. Malate accumulation and night‐time CO2 uptake from the atmosphere declined in all three species during prolonged drought periods, whereas citrate accumulation remained similar or increased. Recycling of respiratory CO2 was substantial for both well‐watered and water‐stressed plants. The suggestion that citrate accumulation is energetically more favourable than malate accumulation is not supported if the source of CO2 for the formation of malate is respiratory CO2. However, the breakdown of citric acid to pyruvate in the light period releases three molecules of CO2, while the breakdown of malic acid releases only one CO2 per pyruvate formed. Thus, citric acid should be more effective than malic acid as a mechanism to increase CO2 concentration in the mesophyll and may help to prevent photoinhibition. Organic acid accumulation also affected the vacuolar pH, which reached values of 2·6–3·0 at dawn. At these pH values, the transport of 2H+/ATP is still feasible, suggesting that it is the divalent form of citrate which is being transported in the vacuoles. Since citrate is a well‐known buffer, and Clusia spp. show the largest day‐night changes in organic acid levels measured in any CAM plant, it is possible that citrate increases the buffer capacity of the vacuoles. Indeed, malate and titratable acidity levels are positively related to citrate levels. Moreover, Clusia species that show the highest nocturnal accumulation of organic acids are also the ones that show the greatest changes in citric acid levels.

Paraheliotropic Leaf Movements in Common Bean under Different Soil Nutrient Levels

The primary objective of this study was to investigate the possible physiological consequences of paraheliotropic leaf movements in response to soil nutrient levels in well‐watered common bean (Phaseolus vulgaris L. cv. Cahone). In pot‐grown plants, leaflets from low‐fertilizer treatment (LF) plants oriented away from the sun's direct rays, receiving a significantly lower photosynthetic photon flux density (PPFD) than leaflets from high‐fertilizer treatment (HF) plants. Leaf conductance and transpiration rate were significantly lower in LF treatment plants than in HF plants. Interactions among incident PPFD, transpiration rate, and leaf area resulted in equivalent leaf temperatures under different fertilizer levels. Leaf water potential was not significantly different between these treatments Carbon isotope discrimination of HF plants was significantly lower than that of LF plants, indicating that intercellular CO2 concentrations were lower and long‐term water‐use efficiency higher for HF plants. The variations in water‐use efficiency most likely resulted from a lower ratio of photosynthesis to leaf conductance in LF plants than in HF plants. Similar tendencies were observed in field‐grown plants. Photosynthetic light‐saturation point was lower for LF plants than for HF plants. As a result of paraheliotropic leaf movements, incident PPFD on leaflets remained close to the photosynthetic light‐saturation points in both treatments.

PHOTOAUTOTROPHIC MICROPROPAGATION OF CYMBIDIUM PLB

Cymbidium (cv. Lisa rose) PLB (protocorm-like bodies) were cultured in liquid 1/2 MS medium with/without 20 mg g-1 sucrose under continuous lighting conditions. The vessels were shaken at 100 rpm under PPF (photosynthetic photon flux) of 20 and 140 μmol m-2 s-1 and CO2 concentrations outside the vessel (Cout) of 450 and 2000 μmol mol-1 conditions. Photoautotrophic growth was obtained at high PPF and high Cout. The chlorophyll content of the PLB in the medium without sucrose at high PPF and high Cout was almost 3 times that with sucrose at low PPF and low Cout. The number of newly developed PLB with sucrose at low PPF and low Cout was 1.6 times that without sucrose at high PPF and high Cout; the dry weight per unit PLB with sucrose at low PPF and low Cout was almost 3 times that without sucrose at high PPF and high Cout. Photoautotrophic growth of the PLB might be further promoted at higher CO2 concentration (> 1%).

Regulation of the photosynthetic capacity of primary bean leaves by the red:far‐red ratio and photosynthetic photon flux density of incident light

The main objective of the present work was to examine the effects of the red:far‐red ratio (R:FR) prevailing during leaf development on the photosynthetic capacity of mature leaves. Plants of Phaseolus vulgaris L. cv. Balin de Albenga were grown from time of emergence in a controlled environment room, 25 ± 3°C, 12‐h photoperiod, with different light treatments:a) high photosynthetic photon flux density (PPFD) = 800 μmol m−1 s−1+ high R:FR= 1.3;b) low PPFD= 300 μmol m−2 s−1+ high R:FR= 1.3; c) high PPFD=800 μmol m−2 s−1+ low R:FR= 0.7; d) low PPFD= 300 μmol m−2s−1+ low R:FR=0.7. With an R:FR ratio of 1.3, a decrease in irradiance during leaf growth reduced photosynthesis when measured at moderate to high PPFD; but when measured at low PPFD, leaves expanded under low irradiance actually had photosynthesis rates higher than those of leaves grown in high irradiance. A low R:FR ratio during development reduced the photosynthetic capacity of the leaves. In leaves expanded under R:FR = 0.7 and high irradiance photosynthesis was reduced by 42 to 89%, depending on the PPFD at which measurements were made, whereas for leaves developed at R:FR = 0.7 and low irradiance photosynthesis decreased by 21 to 24%, compared to leaves under R:FR = 1.3 and similar irradiance. The reduced photosynthetic capacity under R:FR = 0.7 and high irradiance. In natural environments, leaves may experience low R:FR conditions temporarily during their development, and this may affect their future photosynthetic capacity in full sunlight.

The Role of Ribulose-1,5-Bisphosphate Regeneration in the Induction Requirement of Photosynthetic CO2 Exchange under Transient Light Conditions

The temporally variable light environment of natural plant canopies presents distinct limitations to carbon assimilation, partially as a result of the photosynthetic induction requirement that develops when leaves are shaded. This study was undertaken with soybean (Glycine max L.) leaves to further identify factors contributing to the activation state of the fast component of induction during low photosynthetic photon flux density (PPFD) periods. Determination of pool sizes of carbon reduction cycle intermediates at low light and upon return to saturating light indicated that different limitations to photosynthetic activity arise over the time course of a 10-minute low PPFD period. Photosynthetic activity upon reillumination was limited by the regeneration of ribulose 1,5-P(2). There was an increase in the levels of fructose 1,6-P(2), sedoheptulose 1,7-P(2), triose-P, ribose 5-P, and ribulose 5-P pools, indicating inactivation of stromal enzymes, most notably fructose 1,6-bisphosphatase, sedoheptulose 1,7-bisphosphatase, and ribulose 5-P kinase. The fast-induction component was the most important factor limiting assimilation during rapid, brief light transients, during which the decay of the slow component was minimal. This may be particularly significant for upper leaves in soybean canopies that generally experience very rapid light transients.

Morphogenesis and tissue culture of sweet orange (Citrus sinensis (L.) Osb.): Effect of temperature and photosynthetic radiation

Both incubation temperature and photosynthetic radiation affected morphogenesis, callus culture and plantlet culture of sweet orange (Citrus sinensis) cultured in vitro. Bud culture from nodal stem segments, regeneration of shoots and buds from internode stem segments and induction of primary callus were near optimal at incubation temperatures between 21–30°C. The optimal temperature for root formation was 27°C with temperatures above and below being clearly deleterious. Incubation in the dark or under low photosynthetic photon flux density (PPFD) was beneficial for callus induction and growth and also favored the production of rooted plantlets from bud cultures. Incubation in the dark improved considerably the regeneration of shoots and buds from internode segments and the recovery of whole plants. No off-types, as determined by protein and isoenzyme analysis, were observed among plantlets recovered from bud cultures or from regeneration of shoots from internode stem segments.

Effects of ozone concentration on growth of tomato at various light, air humidity and carbon dioxide levels

The effect of ozone (O 3) concentration on the growth of Lycopersicon esculentum was studied at different photosynthetic photon flux densities (PPFD), relative air humidities (RH) and carbon dioxide (CO 2) concentrations. Increasing the O 3 concentration from < 10 to 85 nl 1 −1 for 6 h per day reduced the shoot dry weight 35% at 70% RH and 62% at 90% RH. Increasing the PPFD from 100 to 350 μmol m −2s −1 significantly reduced the effect of O 3 in one of two experiments. The most pronounced interaction between RH, PPFD and O 3 was found on plant height. High O 3 levels generally decreased plant height at low PPFD and had no, or a stimulating, effect at high PPFD. Raising the RH from 70 to 90% significantly increased the negative effect of O 3 on height. Increasing the O 3 concentration from <10 to 65 nl 1 −1 significantly decreased plant height at low CO 2 concentration (300–340 μl 1 −1), but small effects were found at high CO 2 concentration (700–800 μl 1 −1).

Leaf Gas Exchange of Eastern Redbud (Cercis canadensis L.) Grown Under Sun and Shade

Abstract Leaf physiology of eastern redbud (Cercis canadensis L.) was assessed under natural photoperiod when grown in 100% sun or under polyethylene shade with a light transmittance of 69%, 47%, or 29% sun. Net CO2 assimilation rate (A) was similar under 100%, 69%, and 47% sun; A was reduced under 29% sun. Adaptations to shade included a near perpendicular leaf orientation to the sun, reduction in specific leaf weight (SLW), and a decreased chlorophyll a: chlorophyll b ratio. Conversely, eastern redbud adapted to 100% sun by manifesting an increased SLW and a vertical orientation of leaves that curled inward toward the midrib. Light response curves were similar for sun- and shade-acclimatized plants. When all data were analyzed collectively, A was most closely related to photosynthetic photon flux (PPF) (R2 = 0.52), whereas stomatal conductance to water vapor (gs) was primarily influenced by vapor pressure deficit (VPD) (R2 = 0.75). Hence, A and gs were not well correlated (R2 = 0.41). The lack of strong coupling between A and gs allowed the stomates to remain open under low PPF, resulting in an elevated intercellular CO2 concentration. Thus, A was stimulated above what might have normally occurred under low PPF.

INFLUENCE OF IRRADIANCE ON CELL VOLUME AND CARBON QUOTA FOR TEN SPECIES OF MARINE PHYTOPLANKTON1

ABSTRACTTen species of marine phytoplankton were grown under a range of photosynthetic photon flux densities (PFDs) and examined for variation in cell volume and carbon quota. Results suggest that in response to low PFDs phytoplankton generally reduce their cell volume and frequently reduce their carbon quota. A significant linear relationship between the log of PFD (I) and cell volume (in nine of ten species) and log I and carbon quota (four of ten species) was demonstrated.When exposed, to a transient in light intensity, Thalassiosira pseudonana (Hustedt, clone 3H) Hasle and Heimdal underwent a rapid adaptation in cell volume and carbon quota. Cells going from low light to high light reached maximum mean cell volume within 5 h, and cells going from high light to low light reached a minimum mean cell volume within 12 h. The resulting kinetic constant (k; a measure of the rate of adaptation) was considerably larger than previously reported k values. Ditylum brightwellii (West) Grunow increased in length but did not increase in width during a transient to increased irradiance.Nutrient limitation was shown to override PFD in determining cell volume and carbon quota for Heterosigma akashiwo Hada. Cells grown at equivalent irradiances but N‐limited, were smaller than light‐limited and nutrient‐saturated cells. Therefore, cell volume and carbon quota do not have the same relationship with PFD when factors other than PFD control growth rate.The ecological implications of reduced cell volumes and carbon quotas with decreasing PFD include possible impacts on CO2 budgets, an influence on sinking rates, potential changes in predation rates, and surface area/cell volume benefits.

Elevated Atmosphere Partial Pressure of CO2 and Plant Growth. III. Interactions Between Triticum aestivum (C3) and Echinochloa frumentacea (C4) During Growth in Mixed Culture Under Different CO2, N Nutrition and Irradiance Treatments, With Emphasis on Below-Ground Responses Estimated Using the δ13C Value of Root Biomass

Wheat (Triticum aestivum L.), a C3 species, and Japanese millet (Echinochloa frumentacea Link), a C4 species, were grown in pots in monoculture and mixed culture (2 C3 : 1 C4 and 1 C3:2 C4) at two ambient partial pressures of CO2 (320 and 640 μbar), two photosynthetic photon flux densities (PPFDs) (daily maximum 2000 and 500 �mol m-2 s-1) and two levels of nitrogen nutrition (12 mM and 2 mM NO3-). Growth of shoots of both components in mixed culture was measured by physical separation, and the proportions of root biomass due to each component were calculated from δ13C value of total root biomass. In air (320 μbar CO2) at high PPFD and with high root zone-N, the shoot biomass of C3 and C4 components at the first harvest (28 days) was in proportion to the sowing ratio. However, by the second harvest (36 days) the C4 component predominated in both mixtures. Under the same conditions, but with low PPFD, C3 plants predominated at the first harvest but C4 plants had over- taken them by the time of the second harvest. Elevated atmospheric CO2 (640 μbar) stimulated shoot growth of Triticum in 15 of 16 treatment combinations and the stimulation was greatest in plants provided with low NO3-. Root growth of the C3 plants was generally stimulated by elevated CO2, but was only occasionally sensitive to the presence of C4 plants in mixed culture. However, growth of the C4 plants was often sensitive to the presence of C3 plants in mixed culture. In mixed cultures, elevated CO2 plants stimulated growth of C4 plants at high PPFD, high-N and in all low-N treatments but this was insufficient to offset a marked decline in shoot growth with increasing proportion of C3 plants in mixed cultures. The unexpected stimulation of growth of C4 plants by elevated CO2 was correlated with more negative δ13C values of C4 root biomass, suggesting a partial failure of the CO2 concentrating mechanism of C4 photosynthesis in Echinochloa under low-N. These experiments show that for these species nitrogen was more important than light or elevated pCO2 in determining the extent of competitive interactions in mixed culture.

Control of the Acclimation of Photosynthesis to Light and Temperature in Relation to Partitioning of Photosynthate in Developing Soybean Leaves

Photosynthetic acclimation was examined by exposing third trifoliolate leaves of soybeans to air temperatures of 20 to 30°C and photosynthetic photon flux densities (PPFD) of 150 to 950μmol photons m−2 s−1 for the last 3 d before they reached maximum area. In some cases the environment of the third leaf was controlled separately from that of the rest of the plant. Photosynthesis, respiration and dry mass accumulation were determined under the treatment conditions, and photosynthetic capacity, and dry mass and protein content were determined at full expansion. Photosynthetic capacity, the light-saturated rate of net carbon dioxide exchange at 25°C and 34 Pa external partial pressure of carbon dioxide, could be modified between 21 and 35 μmol CO2 m−2 s−1 by environmental changes after leaves had become exporters of photosynthate. Protein per unit leaf mass did not differ between treatments, and photosynthetic capacity increased with leaf mass per unit area. Photosynthetic capacity of third leaves was affected by the PPFD incident on those leaves, but not by the PPFD on other leaves on the plant. Photosynthetic capacity of third leaves was affected by the temperature of the rest of the plant, but not by the temperature of the third leaves. Photosynthetic capacity was linearly related to carbon dioxide exchange rate in the growth regimes, but not to daytime PPFD. At high PPFD, and at 25 and 30°C, mass accumulation was about 28% of the mass of photosynthate produced. At lower PPFD, and at 20°C, larger percentages of the photosynthate produced accumulated as dry mass. The results suggest that photosynthate supply is an important factor controlling leaf structural growth and, consequently, photosynthetic acclimation to light and temperature.

RuBPCO kinetics and the mechanism of CO2 entry in C3 plants

Abstract. The CO2 partial pressure in the chloroplasts of intact photosynthetic C3 leaves is thought to be less than the intercellular CO2 partial pressure. The intercellular CO2 partial pressure can be calculated from CO2 and H2O gas exchange measurements, whereas the CO2 partial pressure in the chloroplasts is unknown. The conductance of CO2 from the intercellular space to the chloroplast stroma and the CO2 partial pressure in the chloroplast stroma can be calculated if the properties of photosynthetic gas exchange are compared with the kinetics of the enzyme ribulose 1,5‐bisphosphate carboxylase/oxygenase (RuBPCO). A discrepancy between gas exchange and RuBPCO kinetics can be attributed to a deviation of CO2 partial pressure in the chloroplast stroma from that calculated in the intercellular space. This paper is concerned with the following: estimation of the kinetic constants of RuBPCO and their comparison with the CO2 compensation concentration; their comparison with differential uptake of 14CO2 and 12CO2; and their comparison with O2 dependence of net CO2 uptake of photosynthetic leaves. Discrepancy between RuBPCO kinetics and gas exchange was found at a temperature of 12.5 °C, a photosynthetic photon flux density (PPFD) of 550 μmol quanta m−2 s−1, and an ambient CO2 partial pressure of 40 Pa. Consistency between RuBPCO kinetics and gas exchange was found if CO2 partial pressure was decreased, temperature incresed and PPFD decreased. The results suggest that a discrepancy between RuBPCO kinetics and gas exchange is due to a diffusion resistance for CO2 across the chloroplast envelope which decreases with increasing temperature. At low CO2 partial pressure, the diffusion resistance appears to be counterbalanced by active CO2 (or HCO3) transport with high affinity and low maximum velocity. At low PPFD, CO2 partial pressure in the chloroplast stroma appears to be in equilibrium with that in the intercellular space due to low CO2 flux.

Environmental control of photosynthetic gas exchange characteristics of fynbos species representing three growth forms

The effects of irrigation and different combinations of temperature and irradiance levels on net photosynthesis of three fynbos growth forms were investigated. The photosynthetic light saturation of Erica plukenetii (ericoid growth form) was recorded at a lower photosynthetic photon flux density (800 μmol m − 2 s − 1 ) than for a proteoid and a restioid species (1 000 μmol m − 2 s − 1 ). Thamnochortus lucens (restioid growth form) changed its photosynthetic temperature optimum from early summer to late summer (30 °C to 20 °C) while Erica plukenetii and Protea laurifolia (proteoid growth form) maintained constant photosynthetic temperature optima (20 °C) during both measuring periods. Irrigation during summer resulted in a 20 to 40% increase in net photosynthetic rate of T. lucens. P. Iaurifolia and E. plukenetii , however, showed no photosynthetic responses upon the addition of water.

Phototropic Leaflet Reorientation in Alfalfa Germplasm Selected for Leaf Color and Size

AbstractSolar tracking has been described in several agronomic and wild species, yet variation within a given species has not been reported. To address that issue, photoperiod leaflet reorientation in response to vectorial light was measured in three alfalfa (Medicago sativa L.) germplasm pools selected from the cultivar Ladak 65. Dark‐leaf germplasm responded to all photosynthetic photon flux densities (PPFD) (100 to 700 μmol/m2/s) with no detectable lag period. Following a 24‐h dark treatment only high PPFD (600 to 700μmol/m2/s) elicited reorientation. Pale‐leaf germplasm responded to intermediate (300 to 400 μmol/m2/s) and high PPFD in a similar manner; however, the low (100‐125μmol/m2/s) PPFD treatment had little effect on reorientation. The 24‐h dark treatment decreased reorientation at all PPFD. Small‐leaf germplasm responded equally to all PPFD and was not affected by the 24‐h dark treatment. The results suggest that available energy levels may influence the reorientation response. Carbonyl cyanide m‐chlorophenylhydrazone, an inhibitor of photophosphorylation, and 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea, an inhibitor of non‐cyclic photosynthetic electron transport, reduced reorientation only in the pale‐leaf type. These results suggest that stored energy reserves were adequate to maintain leaflet reorientation in the absence of photosynthesis in all but the pale‐leaf germplasm. In that germplasm photosynthetically generated energy may be required to support reorientation.

Response of Oxypetalum caeruleum to Irradiance, Temperature, and Photoperiod

Incremental increases in temperature from 14 to 22 to 30C resulted in linear increases in stem length and node number and decreases in stem diameter and stem strength of Oxypetalum caeruleum (D. Don.) Decne. Higher temperatures also resulted in additional flower abortion, reduced time to flowering, and fewer flowering stems per inflorescence. Reduction in the photosynthetic photon flux (PPF) from 695 to 315 μmol·s-1·m-2 had similar effects as increasing the temperature on vegetative characteristics, but had little effect on reproductive ones. The rate of stem elongation was greatest at low PPF for all temperatures and at high temperature for all PPF treatments. Net photosynthesis rose between 14 and 22C and declined at 30C for all PPF treatments. Long photoperiods (12 or 14 hours) resulted in longer internodes, longer stems, and more flowers per cyme than short photoperiods (8 or 10 hours), but photoperiod had little effect on flowering time. Treatments to reduce latex coagulant and silver thiosulfate treatments had no significant effect on vase life.

Effects of light quality and quantity on growth of the clonal plant Eichhornia crassipes.

Plant canopy shade reduces photosynthetic photon flux density (PPFD) and ratio of red to far-red light (z). Both effects can cause plants to increase potential for light acquisition through vertical growth and leaf area expansion. Clonal plants such as Eichhornia crassipes might alternatively increase light interception via horizontal growth of stolons or rhizomes and placement of new ramets in less shaded microsites. Effect of simulated canopy shade and component effects of PPFD and z were tested by filtering or adding light uniformly, to a whole group of connected ramets, or locally, to individual ramets within a group. In uniform treatments, low PPFD reduced total growth but low z did not. Low PPFD and low z independently reduced stolon and ramet production and caused etiolation of petioles; effect of low PPFD plus low z on ramet production was greater than that of either factor alone. Lateral clonal growth thus did not seem to be a response to uniform shading; instead, uniformly low PPFD or low z increased partitioning to established ramets. Low z changed partitioning without changing total growth. In local treatments, reduction of growth of individual ramets due to low PPFD and inhibition of new ramet production attributable to spectral composition of light were mitigated when connected ramets were unshaded; plants may respond differently to patchy than to uniform shade.

LIGHT, TEMPERATURE AND PHOTOPERIOD AFFECT THE USEFULNESS OF OXYPETALUM CAERULEUM, TWEEDIA, AS A CUT FLOWER CROP.

A progressive increase in temperature from 14 to 30C resulted in linear increases in stem length and node number and decreases in stem diameter and stem strength. Higher temperatures also resulted in additional flower abortion, reduced time to flowering and fewer flowering stems per inflorescence. Reduction in the photosynthetic photon flux (PPF) from 695 to 315 μmole m-2s-1 had similar effects as increasing the temperature on vegetative parameters but had little effect on reproductive parameters. The rate of stem elongation was greatest at low PPF for all temperatures and at high temperature for all PPF treatments. Net photosynthesis rose between 14 and 22C and declined at 30C for all PPF treatments. Long photoperiods (12, 14 hr.) resulted in longer internodes, longer stems and more flowers per cyme than short photoperiods (8, 10 hr) but photoperiod had little effect on flowering time.