The Influence of Previous Irradiance on Photosynthetic Induction in Three Species Grown in the Gap and Understory of a Fagus Crenata Forest

Photosynthetic induction responses to a sudden increase in photosynthetic photon flux density (PPFD) from lower background PPFD (0, 25, 50, and 100 mol m^–2 s^–1) to 1 000 mol m^–2 s^–1 were measured in leaves of Fagus crenata, Acer rufinerve Siebold & Zucc., and Viburnum furcatum growing in a gap and understory of a F. crenata forest in the Naeba mountains. In the gap, A. rufinerve exhibited more than 1.2-fold higher maximum net photosynthetic rate (P _Nmax) than F. crenata and V. furcatum. Meanwhile, in the understory F. crenata exhibited the highest P _Nmax among the three species. The photosynthetic induction period required to reach P _Nmax was 3–41 min. The photosynthetic responses to increase in PPFD depended on the background PPFD before increase in PPFD. The induction period required to reach P _Nmax was 2.5–6.5-fold longer when PPFD increased from darkness than when PPFD increased from 100 mol m^–2 s^–1. The induction period was correlated with initial P _N and stomatal conductance (g _s) relative to maximum values before increase in PPFD. The relationship was similar between the gap and the understory. As the background PPFD increased, the initial P _N and g _s increased, indicating that the degrees of biochemical and stomata limitations to dynamic photosynthetic performance decreased. Therefore, photosynthetic induction responses to increase in PPFD became faster with the increasing background PPFD. The differences in time required to reach induction between species, as well as between gap and understory, were mainly due to the varying of relative initial induction states in P _N and g _s at the same background PPFD.     

Different photosynthetic responses of wild and cultivated plants to high irradiance

In addition to other factors, high altitude (HA) environment is characterized by high photosynthetic photon flux density (PPFD). Photosynthetic characteristics of wild and cultivated plants were studied at different irradiances at Losar, India (altitude 4 200 m). Wild plants were tolerant to high PPFDs. Slopes of curve between net photosynthetic rate (P _N) and intercellular CO₂ concentration (C _i) or stomatal conductance (g _s) increased with increase in irradiance suggesting insensitivity or tolerance of these plants to higher PPFD. Cultivated plants, however, were sensitive to higher PPFD, their slopes of curves between P _N and C _i or g _s decreased with increased PPFD. Tolerance or insensitivity to higher PPFD was an important parameter affecting plant performance at HA.     

Susceptibility of green leaves and green flower petals of CAM orchid <Emphasis Type="Italic">Dendrobium</Emphasis> cv. Burana Jade to high irradiance under natural tropical conditions

Photosynthetic rates of green leaves (GL) and green flower petals (GFP) of the CAM plant Dendrobium cv. Burana Jade and their sensitivities to different growth irradiances were studied in shade-grown plants over a period of 4 weeks. Maximal photosynthetic O₂ evolution rates and CAM acidities [dawn/dusk fluctuations in titratable acidity] were higher in leaves exposed to intermediate sunlight [a maximal photosynthetic photon flux density (PPFD) of 500–600 μmol m⁻² s⁻¹] than in leaves grown under full sunlight (a maximal PPFD of 1 000–1 200 μmol m⁻² s⁻¹) and shade (a maximal PPFD of 200–250 μmol m⁻² s⁻¹). However, these two parameters of GFP were highest in plants grown under the shade and lowest in full sun-grown plants. Both GL and GFP of plants exposed to full sunlight had lower predawn F_v/F_m [dark adapted ratio of variable to maximal fluorescence (the maximal photosystem 2 yield without actinic irradiation)] than those of shade-grown plants. When exposed to intermediate sunlight, however, there were no significant changes in predawn F_v/F_m in GL whereas a significant decrease in predawn F_v/F_m was found in GFP of the same plant. GFP exposed to full sunlight exhibited a greater decrease in predawn F_v/F_m compared to those exposed to intermediate sunlight. The patterns of changes in total chlorophyll (Chl) content of GL and GFP were similar to those of F_v/F_m. Although midday F_v/F_m fluctuated with prevailing irradiance, changes of midday F_v/F_m after exposure to different growth irradiances were similar to those of predawn F_v/F_m in both GL and GFP. The decreases in predawn and midday F_v/F_m were much more pronounced in GFP than in GL under full sunlight, indicating greater sensitivity in GFP to high irradiance (HI). In the laboratory, electron transport rate and photochemical and non-photochemical quenching of Chl fluorescence were also determined under different irradiances. All results indicated that GFP are more susceptible to HI than GL. Although the GFP of Dendrobium cv. Burana Jade require a lower amount of radiant energy for photosynthesis and this plant is usually grown in the shade, is not necessarily a shade plant.     

Photosynthetic characteristics of ornamental passion flowers grown under different light intensities

Responses of leaf gas exchange, fluorescence emission, chlorophyll concentration, and morpho-anatomical features to changes in photosynthetic photon flux density (PPFD) were studied in three wild ornamental species of Passiflora L. to select sun and shade species for landscaping projects. Artificial shade was obtained with different shading nylon nets, under field conditions, which allowed the reduction of 25, 50, and 75% of global radiation, along with a control treatment under full sunlight. For Passiflora morifolia the highest mean values of light-saturated net photosynthetic rate (P _Nmax) and light compensation point (LCP) were observed at 50 and 25% shade, respectively, while the highest values of dark respiration rate (R _D) and apparent quantum yield (α) were observed at 75% shade. For Passiflora suberosa litoralis the highest value of P _max was observed at full sunlight. The highest mean values for P _max, R _D, and LCP for Passiflora palmeri var. sublanceolata were obtained at 25% shade. The highest values of net photosynthetic rate (P _N) for P. morifolia, P. palmeri var. sublanceolata, and P. suberosa litoralis were 21.09, 16.15, and 12.36 μmol(CO₂) m⁻² s⁻¹, observed at 50 and 75% shade and full sunlight, respectively. The values of the minimal chlorophyll fluorescence (F₀) were significantly different in P. suberosa litoralis and P. palmeri var. sublanceolata, increasing with the increase of the irradiance. In contrast, the values of maximum photochemical efficiency of PSII (F_v/F_m) were significantly different only in P. suberosa litoralis, being higher at 75%, progressively reducing with the increase of PPFD levels. The total concentration of chlorophyll (Chl) was higher in shaded plants than in the ones cultivated in full sunlight. On the other hand, the values of Chl a/b ratio were reduced in shaded plants. A significant effect of shade levels on leaf area (LA) and specific leaf area (SLA) was found for the three species, whose highest mean values were observed at 75% shade. The thickness of foliar tissues was significantly higher for the three species at full sunlight and 25% shade. These results suggested that P. morifolia and P. palmeri var. sublanceolata appeared to be adapted to moderate shade conditions. P. suberosa litoralis presented higher plasticity to greater variation of the irradiance levels, while the photoinhibition was one of the limiting factors for this species at full sunlight.     

Photosynthetic characteristics and growth of Mosla hangchowensis and M. dianthera under different irradiances

The photosynthetic and growth characteristics of Mosla hangchowensis, an endangered species and M. dianthera, a weed, were compared under three irradiances (PPFD) similar to shaded forest understory, forest edge and open land. Both species grown at lower PPFD had lower PPFD-saturated photosynthetic rate (P_max), saturation PPFD, compensation PPFD, apparent quantum yield, total mass and root/shoot ratio and higher specific leaf area, leaf area ratio and height ratio. At the same PPFD treatment, however, specific leaf area and leaf area ratio of M. hangchowensis were higher than those of M. dianthera, other above parameters were lower than those of M. dianthera. Water use efficiency did not differ between M. hangchowensis and M. dianthera, but it reached its maximum at 70 % of full PPFD. These results suggested the optimum habitat of M. hangchowensis is the forest edge.     

Influence of Etherel and Gibberellic Acid on Carbon Metabolism,Growth, and Essential Oil Accumulation in Spearmint (Mentha Spicata)

Controlled environment chamber and glasshouse studies were conducted on six herbaceous annual species grown at 350 (AC) and 700 (EC) mol(CO₂) mol^-1 to determine whether growth at EC resulted in acclimation of the apparent quantum yield of photosynthesis (QY) measured at limiting photosynthetic photon flux density (PPFD), or in acclimation of net photosynthetic rate (P _N) measured at saturating PPFD. It was also determined whether acclimation in P _N at limiting PPFD was correlated with acclimation of carboxylation efficiency or ribulose-1,5-bisphosphate (RuBP) regeneration rate measured at saturating PPFD. Growth at EC reduced both the QY and P _N at limiting PPFD in three of the six species. The occurrence of photosynthetic acclimation measured at a rate limiting PPFD was independent of whether photosynthetic acclimation was apparent at saturating measurement PPFD. At saturating measurement PPFD, acclimation to EC in the apparent carboxylation efficiency and RuBP regeneration capacity also occurred independently. Thus at least three components of the photosynthetic system may adjust independently when leaves are grown at EC. Estimates of photosynthetic acclimation at both high and low PPFD are necessary to accurately predict photosynthesis at the whole plant or canopy level as [CO₂] increases.     

Effects of Irradiance on Photosynthesis and Activity of Protease Inhibitors in Amaranthus hypochondriacus

Amaranthus hypochondriacus plants were grown under three photosynthetic photon flux densities (PPFD). Mature plants grown at full sunlight (38.8 mol m^–2 d^–1) had higher maximum net photosynthetic rate (P_N) and significantly higher leaf trypsin inhibitor activity than plants that developed under lower PPFD (19.4 and 12.8 mol m^–2 d^–1). In contrast, seeds collected from plants fully exposed to sunlight showed the lowest activity of trypsin inhibitor, higher rate of germination and susceptibility to infection by Aspergillus niger.     

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.     

Effects of water stress on gas exchange of field grown <Emphasis Type="Italic">Zea mays</Emphasis> L. in Southern Italy: an analysis at canopy and leaf level

Zea mays is cultivated in the Mediterranean regions where summer drought may lead to photoinhibition when irrigation is not available. In this work the response of maize to water stress was evaluated by gas exchange measurements at the canopy and leaf level. Leaf gas exchange was assessed before, during and after water stress, while canopy turbulent fluxes of mass and energy were performed on a continuous basis. In the early growth period, a linear increment of net ecosystem photosynthetic rate (P _NE) to incoming of photosynthetic photon flux density (PPFD) was found and net leaf photosynthetic rate (P _NL) showed the tendency to saturate under high irradiance. During water stress, the relationship between P _NE and PPFD became curvilinear and both P _NE and P _NL saturated in a range between 1,000 and 1,500 μmol (photons) m⁻² s⁻¹. Leaf water potential (ψ_l) dropped from −1.50 to −1.88 MPa during water stress, indicating that leaf and canopy gas exchanges were limited by stomatal conductance. With the restoration of irrigation, P _NE, P _NL and ψ_l showed a recovery, and P _NE and P _NL reached the highest values of whole study period. Leaf area index (LAI) reached a value of 3.0 m² m⁻². The relationship between P _NE and PPFD remained curvilinear and P _NE values were lower than those of a typical well-irrigated maize crop. The recovery in P _NE and P _NL after stress, and ψ_l values during stress indicate that the photosynthetic apparatus was not damaged while soil moisture stress after-effects resulted in a sub-optimal LAI values, which in turn depressed P _NE.     

Diurnal changes in gas exchange and chlorophyll fluorescence parameters of <Emphasis Type="Italic">Fritillaria cirrhosa</Emphasis> and <Emphasis Type="Italic">F. delavayi</Emphasis> under field conditions

To determine what factors limit the growth of wild Fritillaria cirrhosa and Fritillaria delavayi in field conditions, we investigated diurnal changes of the net photosynthetic rate (P _N) and the correlation between P _N and various environmental factors. Parameters of chlorophyll (Chl) fluorescence were evaluated to test whether ecological fragility caused the extinction of wild F. cirrhosa and F. delavayi. Our study reveals for the first time that F. cirrhosa and F. delavayi did not encounter significant stress under field conditions. A small reduction in maximum photochemical efficiency was observed under high irradiance. The maximum P _N of F. cirrhosa was 30 % higher than F. delavayi (p<0.05), and a similar difference was observed for apparent quantum yield (27.3 %, p<0.01). F. delavayi was better adapted to a wide range of irradiances and high environmental temperature. Correlation between P _N and environmental factors (without considering the effects of interactions among environmental factors on P _N) using leaves of F. cirrhosa revealed that the three primary influencing factors were air pressure (p<0.01), relative humidity (p<0.01), and soil temperature (p<0.05). In F. delavayi, the influencing factors were relative humidity (p<0.01), soil temperature (p<0.05), CO₂ concentration (p<0.05), and air pressure (p<0.05). Path analysis (considering effects among environmental factors on P _N) showed that air temperature (negative correlation), photosynthetic photon flux density (PPFD) and relative humidity were the three primary limiting factors influencing the growth of F. cirrhosa. For this species, relative humidity reacted indirectly with air pressure, which was reported singularly in other species. Limiting growth factors for F. delavayi were PPFD, air pressure (negative correlation), soil temperature (negative correlation) and air temperature (negative correlation).     

Diurnal Gas Exchange and Superior Resources Use Efficiency of Typical C4 Species in Hunshandak Sandland,China

Net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), leaf water potential (ψ_leaf), leaf nitrogen content, and photosynthetic nitrogen use efficiency (PNUE) were compared between a typical C₄ plant, Agriophyllum squarrosum and a C₃ plant, Leymus chinensis, in Hunshandak Sandland, China. The plant species showed different diurnal gas exchange patterns on June 12–14 when photosynthetic photon flux density (PPFD), air temperature (T _air), and water potential were moderate. P _N, E, and g _s of A. squarrosum showed distinct single peak while those of L. chinensis were depressed at noon and had two peaks in their diurnal courses. Gas exchange traits of both species showed midday depression under higher photosynthetic photon flux density (PPFD) and T _air when Ψ_leaf was significantly low down on August 6–8. However, those of A. squarrosum were depressed less seriously. Moreover, A. squarrosum had higher P _N, Ψ_leaf, water use efficiency (WUE), and PNUE than L. chinensis. Thus A. squarrosum was much more tolerant to heat and high irradiance and could utilise the resources on sand area more efficiently than L. chinensis. Hence species like A. squarrosum may be introduced and protected to reconstruct the degraded sand dunes because of their higher tolerance to stress and higher resource use efficiency. This revised version was published online in August 2006 with corrections to the Cover Date.     

Relationship between allocation of absorbed light energy in PSII and photosynthetic rates of C3 and C4 plants

Two C₃ dicotyledonous crops and five C₄ monocotyledons treated with three levels of nitrogen were used to evaluate quantitatively the relationship between the allocation of absorbed light energy in PSII and photosynthetic rates (P _N) in a warm condition (25–26°C) at four to five levels [200, 400, 800, 1,200 (both C₃ and C₄) and 2,000 (C₄ only) μmol m⁻² s⁻¹] of photosynthetic photon flux density (PPFD). For plants of the same type (C₃ or C₄), there was a linear positive correlation between the fraction of absorbed light energy that was utilized in PSII photochemistry (P) and P _N, regardless of the broad range of their photosynthetic rates due to species-specific effect and/or nitrogen application; meanwhile, the fraction of absorbed light energy that was dissipated through non-photochemical quenching (D) showed a negative linear regression with P _N for each level of PPFD. The intercept of regression lines between P and P _N of C₃ and C₄ plants decreased, and that between D and P _N increased with increasing PPFD. With P and D as the main components of energy dissipation and complementary to each other, the fraction of excess absorbed light energy (E) was unchanged by P _N under the same level of PPFD. At the same level of P _N, C₄ plants had lower P and higher D than C₃ plants, due to the fact that C₄ plants with little or no photorespiration is considered a limited energy sink for electrons. Nevertheless there was a significant negative linear correlation between D and P when data from both C₃ and C₄ plants at varied PPFD levels was merged. The slope of regression lines between P and D was 0.85, indicating that in plants of both types, most of the unnecessary absorbed energy (ca. 85%) could dissipate through non-photochemical quenching, when P was inhibited by low P _N due to species-specific effect and nitrogen limitation at all levels of illumination used in the experiment.     

Diurnal changes in leaf gas exchange and chlorophyll fluorescence in tropical tree species with contrasting light requirements

Interspecific ecophysiological differences in response to different light environments are important to consider in regeneration behavior and forest dynamics. The diurnal changes in leaf gas exchange and chlorophyll fluorescence of two dipterocarps, Shorea leprosula (a high light-requiring) and Neobalanocarpus heimii (a low light-requiring), and a pioneer tree species (Macaranga gigantea) growing in open and gap sites were examined. In the open site, the maximum net photosynthetic rate (P_n), photosystem II (PSII) quantum yield (; F/Fm), and relative electron transport rate (r-ETR) through PSII at a given photosynthetic photon flux density (PPFD) was higher in S. leprosula and M. gigantea than in N. heimii, while non-photochemical quenching (NPQ) at a given PPFD was higher in N. heimii. The maximum values of net photosynthetic rate (P_n) in M. gigantea and S. leprosula was higher in the open site (8–11 mol m^–2 s^–1) than in the gap site (5 mol m^–2 s^–1), whereas that in N. heimii was lower in the open site (2 mol m^–2 s^–1) than in the gap site (4 mol m^–2 s^–1), indicating that N. heimii was less favorable to the open site. These data provide evidence to support the hypothesis that ecophysiological characteristics link with plants regeneration behavior and successional status. Although P_n and stomatal conductance decreased at midday in M. gigantea and S. leprosula in the open site, both r-ETR and leaf temperature remained unchanged. This indicates that stomatal closure rather than reduced photochemical capacity limited P_n in the daytime. Conversely, there was reduced r-ETR under high PPFD conditions in N. heimii in the open site, indicating reduced photochemical capacity. In the gap site, P_n increased in all leaves in the morning before exposure to direct sunlight, suggesting a relatively high use of diffuse light in the morning.     

The Effect of Photoautotrophy on Photosynthesis and Photoinhibition of Gardenia Plantlets during Micropropagation

We studied the relationships between the degree of photoautotrophy, photosynthetic capacity, and extent of photoinhibition of Gardenia jasminoides Ellis plantlets in vitro. Two successive micropropagation stages (shoot multiplication and root induction), and three culture conditions [tube cap closure, photosynthetic photon flux density (PPFD), and sucrose concentration] which may influence the development of photoautotrophy in vitro were assayed. The ratios of variable chlorophyll fluorescence to either maximal (F_v/F_m) or ground (F_v/F₀) values were low, irrespective of the culture stage or growing conditions. Incomplete development of the photosynthetic apparatus and permanent photoinhibition may be involved. However, F_v/F_m and F_v/F₀ increased from shoot multiplication to root induction owing to a decrease in F₀ and an increase in F_m. This suggests that photoinhibition decreases later during micropropagation, when the photoautotrophy of plantlets is more advanced. The low sucrose content and high PPFD increased the photoinhibition of plantlets, whereas growth in tubes with permeable caps showed the opposite effect. The only culture factor with a significant (positive) effect on maximum photosynthetic rate (P _max) was PPFD. At shoot multiplication net photosynthetic rate (P _N) was positively correlated with the half time of the increase from F₀ to F_m (t_1/2). Such association may be mainly due to a common response of both traits to higher PPFD in culture. Within each culture stage, no relationship was observed between P _N and the degree of photoautotrophy, which was positively correlated with F_v/F_m and F_v/F₀ during root induction. During shoot multiplication, these correlations were not significant, or were even negative. Hence during the last stage of micropropagation, plantlets with a higher degree of photoautotrophy are less photoinhibited, whereas they do not follow this pattern at the earlier stage.     

Cotton genotypic variation in the photosynthetic response to irradiance

The photosynthetic response of 8 cotton (Gossypium hirsutum L.) genotypes to changing irradiance was investigated under field conditions during the 1998 through 2000 growing seasons. Equations developed to describe the response of net photosynthetic rate (P_N) to photosynthetic photon flux density (PPFD) demonstrated that, across all irradiances, the two okra leaf-type genotypes photosynthesized at a greater rate per unit leaf area than all of the six normal leaf-type genotypes. This superior photosynthetic performance of the okra leaf-type genotypes can be partially explained by their 13 % greater leaf chlorophyll content relative to that of the normal leaf-type genotypes. The 37 % reduction in leaf size brought upon by the okra leaf trait may have concentrated the amount of photosynthetic machinery per unit leaf area. Nevertheless, the lack of sufficient canopy leaf surface area suppressed the potential yield development that could accompany the higher P_N per unit leaf area.This revised version was published online in March 2005 with corrections to the page numbers.     

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.     

Excess irradiance causes early symptoms of senescence during leaf expansion in photoautotrophically <Emphasis Type="Italic">in vitro</Emphasis> grown tobacco plants

Photosynthetic parameters, growth, and pigment contents were determined during expansion of the fourth leaf of in vitro photoautotrophically cultured Nicotiana tabacum L. plants at three irradiances [photosynthetically active radiation (400–700 nm): low, LI 60 μmol m⁻² s⁻¹; middle, MI 180 μmol m⁻² s⁻¹; and high, HI 270 μmol m⁻² s⁻¹]. During leaf expansion, several symptoms usually accompanying leaf senescence appeared very early in HI and then in MI plants. Symptoms of senescence in developing leaves were: decreasing chlorophyll (Chl) a+b content and Chl a/b ratio, decreasing both maximum (F_V/F_M) and actual (Φ_PS2) photochemical efficiency of photosystem 2, and increasing non-photochemical quenching. Nevertheless, net photosynthetic oxygen evolution rate (P _N) did not decrease consistently with decrease in Chl content, but exhibited a typical ontogenetic course with gradual increase. P _N reached its maximum before full leaf expansion and then tended to decline. Thus excess irradiance during in vitro cultivation did not cause early start of leaf senescence, but impaired photosynthetic performance and Chl content in leaves and changed their typical ontogenetic course.     

Seasonal changes in light and temperature affect the balance between light harvesting and light utilisation components of photosynthesis in an evergreen understory shrub

In a temperate climate, evergreen species in the understory are exposed to large changes in photosynthetic photon flux density (PPFD) and temperature over the year. We determined the photosynthetic traits of leaves of an evergreen understory shrub Aucuba japonica at three sites at monthly intervals: understorys of a deciduous forest; an evergreen forest; and a gap in a mixed forest. This set up enabled us to separate the effects of seasonal change in PPFD and temperature on photosynthetic acclimation under natural conditions. The effects of PPFD and temperature were analysed by simple and multiple regression analyses. The amounts of light utilisation components (LU), represented by nitrogen and rubisco contents per area, were higher in winter, when temperature was low and PPFD was high. The LU relative to the amount of light harvesting components (LH), represented by chlorophyll a/b and rubisco/chlorophyll ratios, and the inverse of chlorophyll/nitrogen ratio were also higher in winter. We quantified the effects of PPFD and temperature on the LU and LH components. Across sites PPFD had stronger effects than air temperature, while within a site temperature had stronger effects on photosynthetic acclimation. We concluded that the photosynthetic apparatus is strongly affected by the prevailing PPFD at the time of leaf development. Within a given light regime, however, plants acclimated by increasing LU relative to LH primarily in response to temperature and to a lesser extent to PPFD.     

Gas Exchange of Spring Barley and Wheat Grown under Mild Water Shortage

During mild water stress (decrease of full water capacity from 60 to 35 %) net photosynthetic rate (P _N) of four spring barley and wheat genotypes was about twice lower than that for unstressed plants and was mainly limited by non-stomatal factors. Availability of CO₂ from intercellular spaces did not change significantly when stomatal conductance (g _s) decreased from 0.25-0.35 to 0.15-0.20 mol(H₂O) m⁻² s⁻¹. There may be two main processes leading to similar intercellular CO₂ concentration (c _i) in stressed and unstressed seedlings despite of twice lower P _N under mild water stress: (a) lower diffusion of CO₂ through stomata represented by lower g _s, (b) lower consumption of CO₂ by photosynthetic apparatus of stressed plants. Last factor is partially pronounced by lower response of P _N to c _i observed for stressed than for control plants. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.