A coupled model of stomatal conductance and photosynthesis for winter wheat

The model couples stomatal conductance (g _s) and net photosynthetic rate (P _N) describing not only part of the curve up to and including saturation irradiance (I _max), but also the range above the saturation irradiance. Maximum stomatal conductance (g _smax) and I _max can be calculated by the coupled model. For winter wheat (Triticum aestivum) the fitted results showed that maximum P _N (P _max) at 600 μmol mol⁻¹ was more than at 350 μmol mol⁻¹ under the same leaf temperature, which can not be explained by the stomatal closure at high CO₂ concentration because g _smax at 600 μmol mol⁻¹ was less than at 350 μmol mol⁻¹. The irradiance-response curves for winter wheat had similar tendency, e.g. at 25 °C and 350 μmol mol⁻¹ both P _N and g _s almost synchronously reached the maximum values at about 1 600 μmol m⁻² s⁻¹. At 25 °C and 600 μmol mol⁻¹ the I _max corresponding to P _max and g _smax was 2 080 and 1 575 μmol m⁻² s⁻¹, respectively.     

Stomatal and non-stomatal limitations to photosynthesis in field-grown grapevine cultivars

Diurnal changes of photosynthesis in the leaves of grapevine (Vitis vinifera × V. labrusca) cultivars Campbell Early and Kyoho grown in the field were compared with respect to gas exchanges and actual quantum yield of photosystem 2 (Φ_PS2) in late May. Net photosynthetic rate (P_N) of the two cultivars rapidly increased in the morning, saturated at photosynthetic photon flux density (PPFD) from 1200 to 1500 μmol m⁻² s⁻¹ between 10:00 and 12:00 and slowly decreased after midday. Maximum P_N was 13.7 and 12.5 μmol m⁻² s⁻¹ in Campbell Early and Kyoho, respectively. The stomatal conductance (g_s) and transpiration rate changed in parallel with P_N, indicating that P_N was greatly affected by g_s. However, the decrease in P_N after midday under saturating PPFD was also associated with the observed depression of Φ_PS2 at high PPFD. The substantial increase in the leaf to air vapour pressure deficit after midday might also contribute to decline of g_s and P_N.     

Irradiance influences tea leaf (<Emphasis Type="Italic">Camellia sinensis</Emphasis> L.) photosynthesis and transpiration

Rates of net photosynthesis (P _N) and transpiration (E), and leaf temperature (T_L) of maintenance leaves of tea under plucking were affected by photosynthetic photon flux densities (PPFD) of 200–2 200 μmol m⁻² s⁻¹. P _N gradually increased with the increase of PPFD from 200 to 1 200 μmol m⁻² s⁻¹ and thereafter sharply declined. Maximum P _N was 13.95 μmol m⁻² s⁻¹ at 1 200 μmol m⁻² s⁻¹ PPFD. There was no significant variation of P _N among PPFD at 1 400–1 800 μmol m⁻² s⁻¹. Significant drop of P _N occurred at 2 000 μmol m⁻² s⁻¹. PPFD at 2 200 μmol m⁻² s⁻¹ reduced photosynthesis to 6.92 μmol m⁻² s⁻¹. PPFD had a strong correlation with T_L and E. Both T_L and E linearly increased from 200 to 2 200 μmol m⁻² s⁻¹ PPFD. T_L and E were highly correlated. The optimum T_L for maximum P _N was 26.0 °C after which P _N declined significantly. E had a positive correlation with P _N.     

Photosystem 2 activity of <Emphasis Type="Italic">Citrus volkameriana</Emphasis> (L.) leaves as affected by Mn nutrition and irradiance

Citrus volkameriana (L.) plants were grown for 43 d in nutrient solutions containing 0, 2, 14, 98, or 686 μM Mn (Mn₀, Mn₂, Mn₁₄, Mn₉₈, and Mn₆₈₆, respectively). To adequately investigate the combined effects of Mn nutrition and irradiance on photosystem 2 (PS2) activity, irradiance response curves for electron transport rate (ETR), nonphotochemical quenching (q_N), photochemical quenching (q_P), and real photochemical efficiency of PS2 (Φ_PS2) were recorded under 10 different irradiances (66, 96, 136, 226, 336, 536, 811, 1 211, 1 911, and 3 111 μmol m⁻² s⁻¹, I₆₆ to I₃₁₁₁, respectively) generated with the PAM-2000 fluorometer. Leaf chlorophyll content was significantly lower under Mn excess (Mn₆₈₆) compared to Mn₀; its highest values were recorded in the treatments Mn₂-Mn₉₈. However, ETR and Φ_PS2 values were significantly lower under Mn₀ compared to the other Mn treatments, when plants were exposed to irradiances ≥96 μmol m⁻² s⁻¹. Furthermore, Mn₀ plants had significantly higher values of q_N and lower values of q_P at irradiances ≤226 and ≥336 μmol m⁻² s⁻¹, respectively, than those grown under Mn₂-Mn₆₈₆. Irrespective of Mn treatment, the values of Φ_PS2 and q_N decreased, while those of q_P increased progressively by increasing irradiance from I₁₃₆ to I₃₁₁₁. Finally, Mn₂-Mn₉₈ plants were less sensitive to photoinhibition of photosynthesis (≥811 μmol m⁻² s⁻¹) than the Mn₆₈₆ (≥536 μmol m⁻² s⁻¹) and Mn₀ (≥336 μmol m⁻² s⁻¹) ones.     

Leaf Photosynthesis of the Mangrove Avicennia Germinans as Affected by NaCl

In leaves of the mangrove species Avicennia germinans (L.) L. grown in salinities from 0 to 40 ‰, fluorescence, gas exchange, and δ¹³C analyses were done. Predawn values of F_v/F_m were about 0.75 in all the treatments suggesting that leaves did not suffer chronic photoinhibition. Conversely, midday F_v/F_m values decreased to about 0.55-0.60 which indicated strong down-regulation of photosynthesis in all treatments. Maximum photosynthetic rate (P _max) was 14.58 ± 0.22 μmol m^-2 s^-1 at 0 ‰ it decreased by 21 and 37 % in plants at salinities of 10 and 40 ‰, respectively. Stomatal conductance (g _s) was profoundly responsive in comparison to P _max which resulted in a high water use efficiency. This was further confirmed by δ¹³C values, which increased with salinity. From day 3, after salt was removed from the soil solution, P _max and g _s increased up to 13 and 30 %, respectively. However, the values were still considerably lower than those measured in plants grown without salt addition. This revised version was published online in June 2006 with corrections to the Cover Date.     

A new model for relationship between irradiance and the rate of photosynthesis in <Emphasis Type="Italic">Oryza sativa</Emphasis>

The calculated maximum net photosynthetic rate (P _N) at saturation irradiance (I _m) of 1 314.13 μmol m⁻² s⁻¹ was 25.49 μmol(CO₂) m⁻² s⁻¹, and intrinsic quantum yield at zero irradiance was 0.103. The results fitted by nonrectangular hyperbolic model, rectangular hyperbolic method, binomial regression method, and the new model were compared. The maximum P _N values calculated by nonrectangular hyperbolic model and rectangular hyperbolic model were higher than the measured values, and the I _m calculated by nonrectangular hyperbolic model and rectangular hyperbolic model were less than measured values. Results fitted by new model showed that the response curve of P _N to I was nonlinear at low I for Oryza sativa, P _N increased nonlinearly with I below saturation value. Above this value, P _N decreased nonlinearly with I.     

Relationship between net photosynthesis and leaf respiration in Mediterranean evergreen species

The relationship between net photosynthetic (P _N) and leaf respiration (R) rates of Quercus ilex, Phillyrea latifolia, Myrtus communis, Arbutus unedo, and Cistus incanus was monitored in the period February 2006 to February 2007. The species investigated had low R and P _N during winter, increasing from March to May, when mean air temperature reached 19.2 °C. During the favourable period, C. incanus and A. unedo had a higher mean P _N (16.4±2.4 μmol m⁻² s⁻¹) than P. latifolia, Q. ilex, and M. communis (10.0±1.3 μmol m⁻² s⁻¹). The highest R (1.89±0.30 μmol m⁻² s⁻¹, mean of the species), associated to a significant P _N decrease (62 % of the maximum, mean value of the species), was measured in July (mean R/P _N ratio 0.447±0.091). Q₁₀, indicating the respiration sensitivity to short-term temperature increase, was in the range 1.49 to 2.21. Global change might modify R/P _N determining differences in dry matter accumulation among the species, and Q. ilex and P. latifolia might be the most favoured species by their ability to maintain sufficiently higher P _N and lower R during stress periods.     

Gas exchanges of an endangered species <Emphasis Type="Italic">Syringa pinnatifolia</Emphasis> and a widespread congener <Emphasis Type="Italic">S. oblata</Emphasis>

Net photosynthetic rate (P_N), transpiration rate (E), water use efficiency (WUE), stomatal conductance (g_s), and stomatal limitation (L_s) were investigated in two Syringa species. The saturation irradiance (SI) was 400 µmol m^-2s^-1 for S. pinnatifolia and 1 700 µmol m^-2s^-1 for S. oblata. Compared with S. oblata, S. pinnatifolia had extremely low g _s . Unlike S. oblata, the maximal photosynthetic rate (P_max) in S. pinnatifoliaoccurred around 08:00 and then fell down, indicating this species was sensitive to higher temperature and high photosynthetic photon flux density. However, such phenomenon was interrupted by the leaf development rhythms before summer. A relatively lower P_N together with a lower leaf area and shoot growth showed the capacity for carbon assimilation was poorer in S. pinnatifolia.     

Diurnal and seasonal trends in photosynthetic performance of <Emphasis Type="Italic">Dalbergia sissoo</Emphasis> Roxb. and <Emphasis Type="Italic">Hardwickia binata</Emphasis> Roxb. from a semi-arid ecosystem

Diurnal and seasonal trends in net photosynthetic rate (P _N), stomatal conductance (g), transpiration rate (E), vapour pressure deficit, temperature, photosynthetic photon flux density, and water use efficiency (WUE) were compared in a two-year-old Dalbergia sissoo and Hardwickia binata plantation. Mean daily maximum P _N in D. sissoo ranged from 21.40±2.60 μmol m⁻² s⁻¹ in rainy season I to 13.21±2.64 μmol m⁻² s⁻¹ in summer whereas in H. binata it was 20.04±1.20 μmol m⁻² s⁻¹ in summer and 13.64±0.16 μmol m⁻² s⁻¹ in winter. There was a linear relationship between daily maximum P _N and g _s in D. sissoo but there was no strong linear relationship between P _N and g _s in H. binata. In D. sissoo, the reduction in g _s led to a reduction in both P _N and E enabling the maintenance of WUE during dry season thereby managing unfavourable environmental conditions efficiently whereas in H. binata, an increase in g _s causes an increase of P _N and E with a significant moderate WUE.     

SEASONAL VARIATIONS OF PHOTOSYNTHETIC PIGMENTS, TOTAL C, N, AND P CONTENT, AND PHOTOSYNTHESIS IN PHYLLARIOPSIS PURPURASCENS: (PHAEOPHYTA) FROM THE STRAIT OF GIBRALTAR

Photosynthetic pigments, C, N, and P tissue composition, and photosynthetic rate were measured from April to October in the brown alga Phyllariopsis purpurascens (C. Agardh) Henry et South (Laminariales, Phaeophyta) growing at a 30-m depth in the Strait of Gibraltar. Ir-radiance reaching the population ranged from 13.5 to 27.5 mol.m^-2.mo^-1. The available light for this species, expressed as a percentage of the irradiance above the water, was 1.8%. Dissolved inorganic nitrogen forms, NO3-and NH₄+, were constant from April to October, whereas phosphate was depleted in August. Chlorophyll a decreased from 520.0 ± 165.0 to 199.6 ± 159.9 μg.g^-1 dry weight; in contrast, chlorophyll c and carotenoids did not change until September but increased threefold in October. C:N and N:P ratios changed in the same way and in the same range. They were constant until July but increased from 15–17 up to 42 (C:N) and from 14 to 40 (N:P) in October, suggesting a severe P limitation of growth of this species. The dark respiration rate and the light compensation point were constant from April to October (0.5 ± 0.1 μmol O₂. m^-2.s^-1 and 6.5 ± 0.2 μmol.m^-2. s^-1, respectively), whereas the maximum rate of apparent photosynthesis, light onset saturation parameter, and half saturation constant for light were maximum in April to May (3.7 μmol O₂. m^-2.s^-1and 40 and 41.5 μmol.m^-2. s^-1, respectively) and October (3.6 μmol O₂. m^-2.s^-1 and 50 and 53.7 μmol.m^-2. s^-1, respectively). They were minimum in August (1.2 μmol O₂.m^-2.s^-1 and 11.3 and 12 μmol.m^-2.s^-1, respectively). These minimum figures yielded a negative carbon budget in August and 0 in September, whereas it was positive the rest of the year. Photosynthetic efficiency, estimated by the ratio between maximum apparent photosynthesis and light half saturation constant, showed a strong agreement with productivity measured by means of an independent method. These results indicate that lamina expansion in this species is controlled by photosynthetic efficiency.     

Photosynthetic responses of apricot (<Emphasis Type="Italic">Prunus armeniaca</Emphasis> L.) to photosynthetic photon flux density,leaf temperature,and CO<Subscript>2</Subscript> concentration

Two cultivars (Katy and Erhuacao) of apricot (Prunus armeniaca L.) were evaluated under open-field and solar-heated greenhouse conditions in northwest China, to determine the effect of photosynthetic photon flux density (PPFD), leaf temperature, and CO₂ concentration on the net photosynthetic rate (P _N). In greenhouse, Katy registered 28.3 μmol m⁻² s⁻¹ for compensation irradiance and 823 μmol m⁻² s⁻¹ for saturation irradiance, which were 73 and 117 % of those required by Erhuacao, respectively. The optimum temperatures for cvs. Katy and Erhuacao were 25 and 35 °C in open-field and 22 and 30 °C in greenhouse, respectively. At optimal temperatures, P _N of the field-grown Katy was 16.5 μmol m⁻² s⁻¹, 21 % less than for a greenhouse-grown apricot. Both cultivars responded positively to CO₂ concentrations below the CO₂ saturation concentration, whereas Katy exhibited greater P _N (18 %) and higher carboxylation efficiency (91 %) than Erhuacao at optimal CO₂ concentration. Both cultivars exhibited greater photosynthesis in solar-heated greenhouses than in open-field, but Katy performed better than Erhuacao under greenhouse conditions.     

Photosynthetic characteristics of dipterocarp species planted on degraded sandy soils in southern Thailand

To elucidate whether dipterocarp species, dominant late-successional species of tropical forests in Southeast Asia, actually have a disadvantage when planted on open site in terms of their photosynthetic characteristics, we investigated photosynthesis in dipterocarp seedlings planted in the open on degraded sandy soils in southern Thailand. These species were compared with seedlings of Acacia mangium Willd., a fast-growing tropical leguminous tree, which is often planted on degraded open site in Southeast Asia. The dipterocarp seedlings had an irradiance-saturated net photosynthetic rate (P _N), stomatal conductance (g _s), carboxylation efficiency, and photosynthetic capacity comparable to or superior to those of A. mangium. In particular, seedlings of Dipterocarpus obtusifolius Teijsm. ex Miq. showed an irradian-ce-saturated P _N of 21 μmol m⁻² s⁻¹, a value higher than any previously reported for a dipterocarp species, accompanied by high g _s (0.7 mol m⁻² s⁻¹) and high photosynthetic capacity. Thus dipterocarp species do not necessarily have a disadvantage in terms of their photosynthetic characteristics on open sites with degraded sandy soils.     

Photosynthesis of Marine Macroalgae from Antarctica: Light and Temperature Requirements

The photosynthetic performance of macroalgae isolated in Antarctica was studied in the laboratory. Species investigated were the brown algae Himantothallus grandifolius, Desmarestia anceps, Ascoseira mirabilis, the red algae Palmaria decipiens, Iridaea cordata, Gigartina skottsbergii, and the green algae Enteromorpha bulbosa, Acrosiphonia arcta, Ulothrix subflaccida and U. implexa. Unialgal cultures of the brown and red algae were maintained at 0°C, the green algae were cultivated at 10°C. I_K values were between 18 and 53 μmol m^?2 s^?1 characteristic or low light adapted algae. Only the two Ulothrix species showed higher I_K values between 70 and 74 μmol m^?2 s^?1. Photosynthesis compensated dark respiration at very low photon fluence rates between 1.6 and 10.6 μmol m^?2 s^?1. Values of α were high: between 0.4 and 1.1 μmol O₂ g^?1 FW h^?1 (μmol m^?2 s^?1)^?1 in the brown and red algae and between 2.1 and 4.9 μmol O₂ g^?1 FW h^?1 (μmol m^?2 s^?1)^?1 in the green algal species. At 0°C P_max values of the brown and red algae ranged from 6.8 to 19.1 μmol O₂ g^?1 FW h^?1 and were similarly high or higher than those of comparable Arctic-cold temperate species. Optimum temperatures for photosynthesis were 5 to 10°C in A. mirabilis, 10°C in H. grandifolius, 15°C in G. skottsbergii and 20°C or higher in D. anceps and I. cordata. P: R ratios strongly decreased in most brown and red algae with increasing temperatures due to different Q₁₀ values for photosynthesis (1.4 to 2.5) and dark respiration (2.5 to 4.1). These features indicate considerable physiological adaptation to the prevailing low light conditions and temperatures of Antarctic waters. In this respect the lower depth distribution limits and the northern distribution boundaries of these species partly depend on the physiological properties described here.     

Photosynthetic Response of Carrots to Varying Irradiances

Response to irradiance of leaf net photosynthetic rates (P _N) of four carrot cultivars: Cascade, Caro Choice (CC), Oranza, and Red Core Chantenay (RCC) were examined in a controlled environment. Gas exchange measurements were conducted at photosynthetic active radiation (PAR) from 100 to 1 000 μmol m⁻² s⁻¹ at 20 °C and 350 μmol (CO₂) mol⁻¹(air). The values of P _N were fitted to a rectangular hyperbolic nonlinear regression model. P _N for all cultivars increased similarly with increasing PAR but Cascade and Oranza generally had higher P _N than CC. None of the cultivars reached saturation at 1 000 μmol m⁻² s⁻¹. The predicted P _N at saturation (P _Nmax) for Cascade, CC, Oranza, and RCC were 19.78, 16.40, 19.79, and 18.11 μmol (CO₂) m⁻² s⁻¹, respectively. The compensation irradiance (I _c) occurred at 54 μmol m⁻² s⁻¹ for Cascade, 36 μmol m⁻² s⁻¹ for CC, 45 μmol m⁻² s⁻¹ for Oranza, and 25 μmol m⁻² s⁻¹ for RCC. The quantum yield among the cultivars ranged between 0.057–0.033 mol(CO₂) mol⁻¹(PAR) and did not differ. Dark respiration varied from 2.66 μmol m⁻² s⁻¹ for Cascade to 0.85 μmol m⁻² s⁻¹ for RCC. As P _N increased with PAR, intercellular CO₂ decreased in a non-linear manner. Increasing PAR increased stomatal conductance and transpiration rate to a peak between 600 and 800 μmol m⁻² s⁻¹ followed by a steep decline resulting in sharp increases in water use efficiency. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photosynthesis and dark respiration of leaves of terrestrial carnivorous plants

Using CO₂ gasometry, net photosynthetic (P _N) and dark respiration rates (R _D) were measured in leaves or traps of 12 terrestrial carnivorous plant species usually grown in the shade. Generally, mean maximum P _N (60 nmol CO₂ g⁻¹(DM) s⁻¹ or 2.7 μmol m⁻² s⁻¹) was low in comparison with that of vascular non-carnivorous plants but was slightly higher than that reported elsewhere for carnivorous plants. After light saturation, the facultatively heliophytic plants behaved as shade-adapted plants. Mean R _D in leaves and traps of all species reached about 50% of maximum P _N and represents the high photosynthetic (metabolic) cost of carnivory.     

Characterization and comparisons of five N2-fixing Azolla-Anabaena associations,I. Optimization of growth conditions for biomass increase and N content in a controlled environment*

Abstract Biomass increase, C and N content, C₂H₂ reduction, percentage dry weight and chlorophyll a/b ratios were determined for clones of Azolla caroliniana Willd., A. filiculoides Lam., A. mexicana Presl., and A. pinnata R. Br. as a function of nutrient solution, pH, temperature, photoperiod, and light intensity in controlled environment studies. These studies were supplemented by a glasshouse study. Under a 16 h, 26°C day at a light intensity of 200 μmol m^?2 s^?1 and an 8 h, 19° C dark period, there was no significant difference in the growth rates of the individual species on the five nutrient solutions employed. Growth was comparable from pH 5 to pH 8, but decreased at pH 9. Using the same photoperiod and light intensity but constant growth temperatures of 15–40°C, at 5°C intervals, the individual species exhibited maximum growth, nitro-genase (N²ase) activity and N content at either 25° or 30°C. There was no difference in the temperature optima at pH 6 and pH 8. The tolerance of the individual species to elevated temperature was indicated to be A. mexicana> A. pinnata> A. caroliniana> A.filiculoides. At the optimum temperature, growth rates increased with increasing photoperiod at both pH 6 and pH 8 but N₂ase activity was usually highest at a 16 h light period. At photon flux densities of 100, 200, 400 and 600 μmol m^?2 s^?1, during a 16 h light period and optimum growth temperature of the individual species, N₂ase activity was saturated at less than 200 μmol m^?2 s^?1 and growth at 400 μmol m^?2 s^?1.No interacting effects of light and pH were noted for any species, nor were light intensities up to 1700 μmol m^?2 s^?1 detrimental to the growth rate or N content of any species in a 5 week glasshouse study with a natural 14.5 h light period and a constant temperature of 27.5°C. Using the optimum growth temperature, a 16 h light period, and a photon flux density of at least 400 μmol m^?2 s^?1, the Azolla species all doubled their biomass in 2 days or less and contained 5–6% N on a dry weight basis.     

Hybrid poplar stomata unresponsive to changes in environmental conditions

Summary The factors affecting stomatal conductance (g_s) of I-214 (Populus euramericana) and a hybrid poplar, Peace (P. koreana x P. trichocarpa), were examined in the field and under controlled environment conditions. Unusual opening of the stomata was observed with Peace leaves at all positions. Ontogenetic changes in g_s were similar between these two poplar species in the light. However, the dark/light ratio of g_s in Peace poplar varied from 0.58 to 1.23 with the insertion level while that of I-214 poplar was zero except for the third leaf from the top. The g_s of I-214 poplar changed with time of the day, varying from 0.74 mol m^-2s^-1 in the morning to zero at night, while the g_s of Peace poplar changed only from the minimum value of 0.23 mol m^-2s^-1 at night to the maximum of 0.48 mol m^-2s^-1 in the morning. Under severe water stress, below -1.5 MPa, which decreased the g_s of I-214 poplar to almost zero, the g_s of Peace poplar remained about onethird of that observed with well-watered leaves. Exposure to a relatively high concentration of O₃ caused the g_s of I-214 poplar to decrease nearly to zero but had no effect on the g_s of Peace. Stomata of Peace poplar were not affected by ABA and the g_s did not change even with 10^-4 M ABA, while the g_s of I-214 decreased to almost zero on the application of this concentration of ABA.     

Photosynthetic photon flux density,carbon dioxide concentration,and vapor pressure deficit effects on photosynthesis in cacao seedlings

Independent short-term effects of photosynthetic photon flux density (PPFD) of 50–400 μmol m⁻² s⁻¹, external CO₂ concentration (C _a) of 85–850 cm³ m⁻³, and vapor pressure deficit (VPD) of 0.9–2.2 kPa on net photosynthetic rate (P _N), stomatal conductance (g _s), leaf internal CO₂ concentration (C _i), and transpiration rates (E) were investigated in three cacao genotypes. In all these genotypes, increasing PPFD from 50 to 400 μmol m⁻² s⁻¹ increased P _N by about 50 %, but further increases in PPFD up to 1 500 μmol m⁻² s⁻¹ had no effect on P _N. Increasing C _a significantly increased P _N and C _i while g _s and E decreased more strongly than in most trees that have been studied. In all genotypes, increasing VPD reduced P _N, but the slight decrease in g _s and the slight increase in C _i with increasing VPD were non-significant. Increasing VPD significantly increased E and this may have caused the reduction in P _N. The unusually small response of g _s to VPD could limit the ability of cacao to grow where VPD is high. There were no significant differences in gas exchange characteristics (g _s, C _i, E) among the three cacao genotypes under any measurement conditions.     

GROWTH,PHOTOSYNTHESIS AND MAINTENANCE METABOLIC COST IN THE DIATOM PHAEODACTYLUM TRICORNUTUM AT VERY LOW LIGHT LEVELS 1

The compensation point for growth of Phaeodactylum tricornutum Bohlin is less than 1 μmol. m^?2s^?1. Growth at low PFDs (<3.5 μmol. m^?2.s^?1) does not appear to reduce the maximum quantum efficiency of photosynthesis (ø_m) or to greatly inhibit the potential for light-saturated, carbon-specific photosynthesis (P_m^c). The value for ø_m in P. tricornutum is 0.10–0.12 mol O₂-mol photon^?1, independent of acclimation PFD between 0.75 and 200 μmol.m^?2.s^?1 in nutrient-sufficient cultures. P_m^c in cells of P. tricornutum acclimated to PFDs <3.5 μmol m^?2?s^?1 is approximately 50% of the highest value obtained in nutrient-sufficient cultures acclimated to growth-rate-saturating PFDs. In addition, growth at low PFDs does not severely restrict the ability of cells to respond to an increase in light level. Cultures acclimated to growth at lees than 1% of the light-saturated growth rate respond rapidly to a shift-up in PFD after a short initial lag period and achieve exponential growth rates of 1.0 d^?1 (65% of the light- and nutrient-saturated maximum growth rate) at both 40 and 200 μmol.m^?2.s^?1     

The Photosynthesis and Chlorophyll a Fluorescence in Seedlings of Kandelia Candel (L.) Druce Grown under Different Nitrogen and NaCl Controls

Kandelia candel (L.) Druce is the dominant mangrove species on the west coast of northern Taiwan. We have measured the net photosynthetic rate (P _N) and chlorophyll (Chl) a fluorescence of seedlings grown at combinations of two nitrogen (0.01 and 0.1 mM) and two NaCl (250 and 430 mM NaCl) controls. With the same nitrogen level, seedlings grown at higher salinity (HS) had a significantly lower P _N and stomatal conductance (g _s) than those at lower salinity (LS). An increase in nitrogen availability significantly elevated P _N and g _s of the LS-grown seedlings. Compared to dark adapted leaves, the maximum quantum yield of photosystem 2 (PS2) (F_v/F_m) of leaves exposed to PFDs of 1200 and 1600 μmol m^-2 s^-1 for 2 h was significantly reduced. The degree of F_v/F_m reduction differed among leaves of the four types of treated plants. Chl fluorescence quenching analysis revealed differences among the examined plants in coefficients of non-photochemical and photochemical quenching. This revised version was published online in June 2006 with corrections to the Cover Date.