Photosynthetic response to precipitation/rainfall in predominant tree (<Emphasis Type="Italic">Ulmus pumila</Emphasis>) seedlings in Hunshandak Sandland,China

The responses of gas exchange and chlorophyll fluorescence of field-growing Ulmus pumila seedlings to changes in simulated precipitation were studied in Hunshandak Sandland, China. Leaf water potential (Ψ_wp), net photosynthetic rate (P _N), stomatal conductance (g _s), and transpiration rate (E) were significantly increased with enhancement of precipitation from 0 to 20 mm (p<0.01), indicating stomatal limitation of U. pumila seedlings that could be avoided when soil water was abundant. However, P _N changed slightly when precipitation exceeded 20 mm (p>0.05), indicating more precipitation than 20 mm had no significant effects on photosynthesis. Maximum photochemical efficiency of photosystem 2, PS 2 (F_v/F_m) increased from 0.53 to 0.78 when rainfall increased from 0 to 10 mm, and F_v/F_m maintained a steady state level when rainfall was more than 10 mm. Water use efficiency (WUE) decreased significantly (from 78–95 to 23–27 μmol mol⁻¹) with enhancement of rainfalls. P _N showed significant linear correlations with both g _s and Ψ_wp (p<0.0001), which implied that leaf water status influenced gas exchange of U. pumila seedlings. The 20-mm precipitation (soil water content at about 15 %, v/v) might be enough for the growth of elm seedlings. When soil water content (SWC) reached 10 %, down regulation of PS2 photochemical efficiency could be avoided, but stomatal limitation to photosynthesis remained. When SWC exceeded 15 %, stomatal limitation to photosynthesis could be avoided, indicating elm seedlings might tolerate moderate drought.     

Leaf anatomical structures of <Emphasis Type="Italic">Paphiopedilum</Emphasis> and <Emphasis Type="Italic">Cypripedium</Emphasis> and their adaptive significance

Paphiopedilum and Cypripedium are closely related in phylogeny, but have contrasting leaf traits and habitats. To understand the divergence in leaf traits of Paphiopedilum and Cypripedium and their adaptive significance, we analyzed the leaf anatomical structures, leaf dry mass per area (LMA), leaf lifespan (LL), leaf nitrogen concentration (N _mass), leaf phosphorus concentration (P _mass), mass-based light-saturated photosynthetic rate (A _mass), water use efficiency (WUE), photosynthetic nitrogen use efficiency (PNUE) and leaf construction cost (CC) for six species. Compared with Cypripedium, Paphiopedilum was characterized by drought tolerance derived from its leaf anatomical structures, including fleshy leaves, thick surface cuticles, huge adaxial epidermis cells, lower total stoma area, and sunken stomata. The special leaf structures of Paphiopedilum were accompanied by longer LL; higher LMA, WUE, and CC; and lower N _mass, P _mass, A _mass, and PNUE compared with Cypripedium. Leaf traits in Paphiopedilum helped it adapt to arid and nutrient-poor karst habitats. However, the leaf traits of Cypripedium reflect adaptations to an environment characterized by rich soil, abundant soil water, and significant seasonal fluctuations in temperature and precipitation. The present results contribute to our understanding of the divergent adaptation of leaf traits in slipper orchids, which is beneficial for the conservation of endangered orchids.     

Photosynthetic characterization of Australian pitcher plant <Emphasis Type="Italic">Cephalotus follicularis</Emphasis>

Australian carnivorous pitcher plant Cephalotus follicularis Labill. produces two types of leaves. During the spring time, the plant produces a foliage type of noncarnivorous leaf called lamina. Later, the second type of leaf is produced — carnivorous pitcher. Using simultaneous measurements of gas exchange and chlorophyll (Chl) fluorescence photosynthetic efficiency of these two distinct forms of leaves were compared. In addition stomatal density, an important component of gas exchange, and Chl concentration were also determined. Pitcher trap had lower net photosynthetic rate (P _N) in comparison to noncarnivorous lamina, whereas the rate of respiration (R _D) was not significantly different. This was in accordance with lower stomatal density and Chl concentration in the pitcher trap. On the other hand maximum quantum yield of PSII (F_v/F_m) and effective quantum yield of photochemical energy conversion in PSII (Φ_PSII) was not significantly different. Nonphotochemical quenching (NPQ) was significantly higher in the lamina at higher irradiance. These data are in accordance with hypothesis that changing the leaf shape in carnivorous plants to make it a better trap generally makes it less efficient at photosynthesis. However, the pitcher of Cephalotus had much higher P _N than it was expected from the data set of the genus Nepenthes. Because it is not possible to optimize for contrasting function such as photosynthesis and carnivory, it is hypothesized that Cephalotus pitchers are less elaborated for carnivorous function than the pitchers of Nepenthes.     

Contrasting hydraulic strategies in <Emphasis Type="Italic">Salix psammophila</Emphasis> and <Emphasis Type="Italic">Caragana korshinskii</Emphasis> in the southern Mu Us Desert,China

Salix psammophila and Caragana korshinskii are two common shrubs in the southern Mu Us Desert, China. Their hydraulic strategies for adapting to this harsh, dry desert environment are not yet clear. This study examined the hydraulic transport efficiency, vulnerability to cavitation, and daily embolism refilling in the leaves and stems of these two shrubs during the dry season. In order to gain insight into water use strategies of whole plants, other related traits were also considered, including daily changes in stomatal conductance, leaf mass per area, leaf pressure–volume parameters, wood density and the Huber value. The leaves and stems of S. psammophila had greater hydraulic efficiency, but were more vulnerable to drought-induced hydraulic dysfunction than C. korshinskii. The difference between leaf and stem water potential at 50 % loss of conductivity was 0.12 MPa for S. psammophila and 0.81 MPa for C. korshinskii. Midday stomatal conductance decreased by 74 % compared to that at 8:30 in S. psammophila, whereas no change occurred in C. korshinskii. Daily embolism and refilling occurred in the stems of S. psammophila and leaves of C. korshinskii. These results suggest that a stricter stomatal regulation, daily embolism repair in stems, and a higher stem water capacitance could be partially compensating for the greater susceptibility to xylem embolism in S. psammophila, whereas higher leaf elastic modulus, greater embolism resistance in stems, larger difference between leaf and stem hydraulic safety, and drought-induced leaf shedding in C. korshinskii were largely responsible for its more extensive distribution in arid and desert steppes.     

Effect of gamma radiation on mutant induction of <Emphasis Type="Italic">Fagopyrum dibotrys</Emphasis> Hara

Agronomic traits, photosynthetic pigments, gas exchange, and chlorophyll (Chl) fluorescence parameters of red stem buckwheat (Fagopyrum dibotrys Hara) mutants induced by γ-radiation were compared with green control at seedling stage. Plant height, number of first-class branches, and rhizome biomass were inhibited significantly (p<0.01). Chl a, Chl b, and Chl a+b contents decreased with elevated dose of γ-rays, while increasing carotenoid content indicated that buckwheat was capable of adjusting to the radiation damage. Decrease in net photosynthetic rate was the result of both stomatal and non-stomatal limitations. Fluorescence parameters, such as F₀, F_m, F_v/F_m, F_v/F₀, Φ_PS2, electron transport rate, and photochemical quenching declined significantly (p<0.01) as compared with control due to photoinhibition, while non-photochemical quenching increased to enhance thermal dissipation. Lower parameters implied that leaf tissue was damaged significantly by high dose of γ-radiation and therefore leaf senescence was accelerated.     

Injured and uninjured leaf photosynthetic responses after mechanical injury on <Emphasis Type="Italic">Nerium oleander</Emphasis> leaves,and <Emphasis Type="Italic">Danaus plexippus</Emphasis> herbivory on <Emphasis Type="Italic">Asclepias curassavica</Emphasis> leaves

Insect herbivory has variable effects on plant physiology; so greater understanding is needed about how injury alters photosynthesis on individual injured and uninjured leaves. Gas exchange and light-adapted leaf chlorophyll fluorescence measurements were collected from uninjured and mechanical partial leaf defoliation in two experiments with Nerium oleander (Apocynaceae) leaves, and one experiment with Danaus plexippus herbivory on Asclepias curassavica (Asclepiadaceae) leaves. Gas exchange impairment (lower photosynthetic rate (P _n ), stomatal conductance (g _s)) indicates water stress in a leaf, suggests stomatal limitations causing injury P _n impairment. The same pattern of gas exchange impairment also occurred on uninjured leaves opposite from injured leaves in both N. oleander experiments. This is an interesting result because photosynthetic impairment is rarely reported on injured leaves near injured leaves. No photosynthetic changes occurred in uninjured A. curassavica leaves opposite from D. plexippus-fed leaves. Partially defoliated leaves that had P _n and g _s reductions lacked any significant changes in intercellular leaf [CO₂], C _i. These results neither support, nor are sufficient to reject, stomatal limitations to photosynthesis. Manually imposed midrib vein severance in N. oleander experiment #1 significantly increased leaf C _i, indicating mesophyll limitations to photosynthesis. Maximal light-adapted leaf photochemical efficiency () and also non-photochemical quenching (q _N) were reduced by mechanical or insect herbivory to both study species, suggesting leaf trouble handling excess light energy not used for photochemistry. Midrib injury on N. oleander leaves and D. plexippus herbivory on A. curassavica leaves also reduced effective quantum yield (ΦPSII) and photochemical quenching (q _P); so reduced plastoquinone pools could lead to additional PSII reaction center closure.     

Photosynthetic performance of <Emphasis Type="Italic">Lycoris radiata</Emphasis> var. <Emphasis Type="Italic">radiata</Emphasis> to shade treatments

The effects of shade on the growth, leaf photosynthetic characteristics, and chlorophyll (Chl) fluorescence parameters of Lycoris radiata var. radiata were determined under differing irradiances (15, 65, and 100% of full irradiance) within pots. The HI plants exhibited a typical decline in net photosynthetic rate (P _N) during midday, which was not observed in MI- and LI plants. This indicated a possible photoinhibition in HI plants as the ratio of variable to maximum fluorescence (F_v/F_m) value was higher and the minimal fluorescence (F₀) was lower in the, and LI plants. Diurnal patterns of stomatal conductance (g _s) and transpiration rate (E) were remarkably similar to those of P _N at each shade treatments, and the intercellular CO₂ concentration (C _i) had the opposite change trend. Under both shading conditions, the light saturation point, light compensation point and photon-saturated photosynthetic rate (P _max) became lower than those under full sunlight, and it was the opposite for the apparent quantum yield (AQY). The higher the level of shade, the lower the integrated daytime carbon gain, stomatal and epidermis cell densities, specific leaf mass (SLM), bulb mass ratio (BMR), leaf thickness, and Chl a/b ratio. In contrast, contents of Chls per dry mass (DM), leaf area ratio (LAR), leaf mass ratio (LMR), leaf length, leaf area and total leaf area per plant increased under the same shade levels to promote photon absorption and to compensate for the lower radiant energy. Therefore, when the integrated daytime carbon gain, leaf area and total leaf area per plant, which are the main factors determining the productivity of L. radiata var. radiata plant, were taken into account together, this species may be cultivated at about 60∼70% of ambient irradiance to promote its growth.     

Growth,biomass allocation,morphology, and photosynthesis of invasive <Emphasis Type="Italic">Eupatorium</Emphasis><Emphasis Type="Italic">adenophorum</Emphasis> and its native congeners grown at four irradiances

Eupatorium adenophorum is one of the more noxious invasive plants worldwide. However, the mechanisms underlying its invasiveness are still not well elucidated. In this study, we compared the invader with its two native congeners (E. heterophyllum and E. japonicum) at four irradiances in terms of growth, biomass allocation, morphology, and photosynthesis. The higher light-saturated photosynthetic rate (P _max) and total leaf area of the invader may contribute to its higher relative growth rate (RGR) and total biomass compared with its native congeners. Total biomass and RGR increased significantly with the increase of P _max and total leaf area. The higher support organ mass fraction and the lower root mass fraction of the invader may also contribute to its higher RGR and biomass through increasing carbon assimilation and reducing respiratory carbon loss, respectively. The higher growth rate of the invader increased its total leaf area, ramet number, and crown area. These traits may help the invader to form dense monoculture, outshading native plant species. However, consistently higher leaf area ratio, specific leaf area, and leaf mass fraction were not found across irradiances for the invader compared with its native congeners. Higher plasticity in response to irradiance was also not found for the invader. The invader retained advantages over the natives across irradiances, while its performance decreased with lower irradiance. The results indicate that the invader may be one of the few super invaders. Reducing irradiance may inhibit its invasions.     

Sensitivity of water relations and photosynthesis to summer precipitation pulses for <Emphasis Type="Italic">Artemisia tridentata</Emphasis> and <Emphasis Type="Italic">Purshia tridentata</Emphasis>

For much of the western USA, precipitation occurs in pulses, the nature of which determine soil water potential and plant physiological performance. This research utilized three experiments to examine the sensitivity of photosynthesis and water relations for two widespread Great Basin Desert shrub species, Artemisia tridentata (which has both deep and shallow roots) and Purshia tridentata (which reportedly has only deep roots), to (1) variation in pulse magnitude size, (2) the kinetics of responses to pulses, and (3) the relationship between pulse-size and antecedent soil water content. At the study site in the southwestern Great Basin Desert, USA, summer rainfall exhibits a greater frequency of larger-sized events, and longer inter-pulse intervals, compared to annual patterns. Compared to pre-watering values, stem water potential initially increased by about 2.00 MPa for A. tridentata and 1.00 MPa for P. tridentata following watering to simulate an 11.5 mm rainfall pulse. For the same water addition, stomatal conductance increased by 0.3 mol m⁻² s⁻¹ and photosynthetic CO₂ assimilation increased 8-fold for A. tridentata and 6-fold for P. tridentata. Water potential and photosynthetic gas exchange were maximal for both species 2–3 days following a pulse addition. In comparison to P. tridentata, the increase in photosynthesis for A. tridentata was more pronounced for plants treated incrementally with several small pulses compared to plants treated with one pulse of an equivalent total volume. The results indicate that both species can respond to a range of summer rainfall pulse magnitudes within about 2 days, with A. tridentata generally exhibiting larger responses in comparison to the co-dominant shrub species P. tridentata, which at this study site does indeed have shallow roots. In a future climate, the timing and magnitude of summer rainfall pulses will determine the extent to which these two species undergo changes in water status and photosynthetic carbon uptake, with implications for their fitness.     

Effects of soil and air drought on growth,plant water status and leaf gas exchange in three Mediterranean cedar species: <Emphasis Type="Italic">Cedrus atlantica</Emphasis>, <Emphasis Type="Italic">C. brevifolia</Emphasis> and <Emphasis Type="Italic">C. libani</Emphasis>

Three- and four-year-old potted, greenhouse-grown cedar seedlings were subjected to two different watering regimes: half received full water supply and the other half was submitted to moderate drought (50% of the full water supply). Height growth was the greatest for C. atlantica and the most-limited for C. brevifolia in the well-watered set. However, in the dry set, height growth was less affected by drought conditions for C. brevifolia than for C. atlantica. Cedrus libani gave intermediate results for both watering regimes. Moderate drought provoked a decrease in osmotic potential at full leaf turgor and a long-lasting osmotic adjustment. When irrigation was withheld completely to induce severe soil drying, gas exchange decreased and then stopped at predawn water potentials of −3.0 MPa for C. brevifolia, between −2.6 and −2.8 MPa for C. libani, and at −2.4 MPa for C. atlantica, irrespective of watering regime. For all species, the dry set showed lower net photosynthesis (A) and stomatal conductance (g _s) than the plants in the well-watered set. A and g _s responded to variations in atmospheric water-vapour pressure deficit (VPD). As VPD increased, A and g _s decreased, and this trend was proportionate to initial values at low VPD, but remained independent of previous watering treatments, plant water status or species. To conclude, C. brevifolia appears to be a species with limited growth potential but strong soil drought tolerance whereas C. atlantica has strong growth potential when an adequate water supply is available but is more sensitive to soil drought. C. libani shows an intermediate behaviour for growth and drought tolerance.     

Drought tolerance in the Mediterranean species <Emphasis Type="Italic">Quercus coccifera,Quercus ilex,Pinus halepensis</Emphasis>, and <Emphasis Type="Italic">Juniperus phoenicea</Emphasis>

We investigated the strategies of four co-occurring evergreen woody species Quercus ilex, Quercus coccifera, Pinus halepensis, and Juniperus phoenicea to cope with Mediterranean field conditions. For that purpose, stem water potential, gas exchange, chlorophyll (Chl) fluorescence, and Chl and carotenoid (Car) contents were examined. We recognized two stress periods along the year, winter with low precipitation and low temperatures that led to chronic photoinhibition, and summer, when drought coincided with high radiation, leading to an increase of dynamic photoinhibition and a decrease of pigment content. Summer photoprotection was related to non-photochemical energy dissipation, electron flow to alternative sinks other than photosynthesis, decrease of Chl content, and proportional increase of Car content. Water potential of trees with deep vertical roots (Q. coccifera, Q. ilex, and P. halepensis) mainly depended on precipitation, whereas water potential of trees with shallow roots (J. phoenicea) depended not only on precipitation but also on ambient temperature.     

Phenotypic plasticity of <Emphasis Type="Italic">Spartina alterniflora</Emphasis> and <Emphasis Type="Italic">Phragmites australis</Emphasis> in response to nitrogen addition and intraspecific competition

Phenotypic plasticity of the two salt marsh grasses Spartina alterniflora and Phragmites australis in salt marshes is crucial to their invasive ability, but the importance of phenotypic plasticity, nitrogen levels, and intraspecific competition to the success of the two species is unclear at present. Spartina alterniflora Loisel. is an extensively invasive species that has increased dramatically in distribution and abundance on the Chinese and European coasts, and has had considerable ecological impacts in the regions where it has established. Meanwhile, Phragmites australis Cav., a native salt marsh species on the east coast of China, has replaced the native S. alterniflora in many marshes along the Atlantic Coast of the US. This study determined the effects of nitrogen availability and culm density on the morphology, growth, and biomass allocation traits of Spartina alterniflora and Phragmites australis. A large number of morphological, growth, and biomass parameters were measured, and various derived values (culm: root ratio, specific leaf area, etc.) were calculated, along with an index of phenotypic plasticity. Nitrogen addition significantly affected growth performance and biomass allocation traits of Spartina alterniflora, and culm density significantly affected morphological characteristics in a negative way, especially for Spartina alterniflora. However, there were no significant interactions between nitrogen levels and culm density on the morphological parameters, growth performances parameters, and biomass allocation parameters of the two species. Spartina alterniflora appears to respond more strongly to nitrogen than to culm density and this pattern of phenotypic plasticity appears to offer an expedition for successful invasion and displacement of Phramites australias in China. The implication of this study is that, in response to the environmental changes that are increasing nitrogen levels, the range of Spartina alterniflora is expected to continue to expand on the east coast of China.     

Photosynthetic responses of radish (<Emphasis Type="Italic">Raphanus sativus</Emphasis> var. <Emphasis Type="Italic">longipinnatus</Emphasis>) plants to infection by turnip mosaic virus

Plant growth, chlorophyll (Chl) content, photosynthetic gas exchange, ribulose-1,5-bisphosphate carboxylase (RuBPCO) enzyme activity, and Chl fluorescence in radish (Raphanus sativus var. longipinnatus) plants were examined after turnip mosaic virus (TuMV) infection. Plant fresh mass, dry mass, Chl content, net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), and RuBPCO activity were significantly lower in infected plants after 5 weeks of virus infection as compared to healthy plants. The 5-week virus infection did not induce significant differences in intercellular CO₂ concentration (C _i, photochemical efficiency of photosystem 2, PS2 (F_v/F_m), excitation capture efficiency of open PS2 reaction centres (F_v'/F_m'), effective quantum efficiency of photosystem 2 (ΔF/F_m'), and photochemical quenching (q_P), but non-photochemical quenching (q_N) and alternative electron sink (AES) were significantly enhanced. Thus the decreased plant biomass of TuMV-infected plants might be associated with the decreased photosynthetic activity mainly due to reduced RuBPCO activity.     

Phenotypic responses of <Emphasis Type="Italic">Spartina anglica</Emphasis> to duration of tidal immersion

Although Spartina anglica C.E. Hubbard continues to be invasive in many countries, this species has experienced a drastic decline in coastal China over the last decade. We hypothesize that changes in the duration of tidal immersion were responsible for this decline because the elevation of the S. anglica-dominated area in coastal China has increased greatly over the last decade. We examined the effects of the duration of simulated tidal immersion and plant material provenance on growth, asexual reproduction, biomass accumulation, and allocation (percent of above-ground biomass to total biomass) of S. anglica in a greenhouse experiment. The provenance of S. anglica did not significantly affect any traits measured except for height, stalk diameter, and leaf area. However, all traits were affected by the duration of immersion. Plants grown under 6 h of immersion were taller and had more leaves, more roots, and larger leaf area than those under 2, 4, 8, and 10 h of immersion. Asexual traits and biomass of the plants grown under 6 h of immersion were significantly larger than those under other immersion durations. The results suggested that S. anglica benefits from tidal immersion and decreasing duration of tidal immersion may have resulted in the decline of the S. anglica populations in coastal China. Thus, controlling the duration of tidal immersion may be an effective way of controlling invasiveness of this species elsewhere in the world.     

Responses of photosynthetic parameters of <Emphasis Type="Italic">Mikania micrantha</Emphasis> and <Emphasis Type="Italic">Chromolaena odorata</Emphasis> to contrasting irradiance and soil moisture

Photosynthetic parameters were measured in two invasive weeds, Mikania micrantha and Chromolaena odorata, grown in soil under full, medium, and low irradiance and full, medium, and low water supply. Both species showed significantly higher net photosynthetic rate, quantum yield of PS 2 photochemistry and photochemical quenching coefficient under high than low irradiance. For M. micrantha, low irradiance caused decreased chlorophyll content (Chl), Chl a/b ratio and maximum photochemical efficiency of PS 2 (F_v/F_m), while drought decreased Chl content and F_v/F_m and increased nonphotochemical quenching (NPQ). However, these parameters were much less affected in C. odorata except that Chl content and NPQ slightly increased under drought and high irradiance. High irradiance increased xanthophyll pools in both species, especially M. micrantha under combination with drought.     

Effects of irradiance on photosynthetic characteristics and growth of <Emphasis Type="Italic">Mosla chinensis</Emphasis> and <Emphasis Type="Italic">M. scabra</Emphasis>

Photosynthetic and growth characteristics of Mosla chinensis and M. scabra were compared at three irradiances similar to shaded forest understory, forest edge, and open land. At 25 % full ambient irradiance, M. chinensis and M. scabra had similar photosynthetic characteristics, but saturation irradiance, compensation irradiance, and apparent quantum yield of M. chinensis were higher than those of M. scabra at full ambient irradiance and 70 % full ambient irradiance. At the same irradiance treatment, specific leaf area and leaf area ratio of M. chinensis were lower than those of M. scabra. Photon-saturated photosynthetic rate and water use efficiency of M. chinensis, however, were not significantly higher than those of M. scabra, and the leaf area and total biomass were lower than those of M. scabra. As a sun-acclimated plant, the not enough high photosynthetic capacity and lower biomass accumulation may cause that M. chinensis has weak capability to extend its population and hence be concomitant in the community.     

<Emphasis Type="Italic">Setaria Viridis</Emphasis> tolerance of high zinc concentrations

The effect of high zinc concentrations on the growth and photosynthetic apparatus of Setaria viridis (L.) Beauv. was investigated under laboratory conditions and a vegetation experiment. The experiments showed that zinc concentrations of 10⁻⁶ to 10⁻³ M did not influence seed germination. Moreover, zinc in concentrations of 40 and 80 mg kg substrate had no significant effect on most of the plant growth parameters. However, higher metal concentrations (160 and 320 mg kg substrate) inhibited shoot biomass, leaf area, inflorescence length, and biomass. At the same time, parameters such as the content of chlorophyll (a + b), F ₀, F_v, and the F_v/F_m rate were not reduced even in the presence of zinc in the highest concentrations (320 mg kg substrate). S. viridis tolerance of high zinc concentrations, as well as its capacity to accumulate zinc in roots and shoots, suggests the plant can be used for phytoremediation of zinc polluted soils.     

Improvement of growth conditions and gas exchange of<Emphasis Type="Italic"> Fagus sylvatica</Emphasis> L. seedlings planted below a recently thinned<Emphasis Type="Italic"> Pinus sylvestris</Emphasis> L. stand

Abstract Gas exchange and growth of beech seedlings planted in the understory of a recently thinned pinewood were recorded for 2 years. Relative irradiance was assessed by hemispherical photographs taken just after the thinning. Predawn water potential (_pd), daily gas exchange and chlorophyll fluorescence were measured several times during the two growing seasons. Maximum values of photosynthesis (A _max) and stomatal conductance to water vapour (g _wvmax) were established from daily data. Maximum quantum efficiency of PS II was recorded at dawn by taking the variable to maximum chlorophyll fluorescence ratio on dark adapted leaves (F _v/F _m). In the middle of each summer, leaf nitrogen content and leaf mass per area were evaluated, and height growth and basal area increment were recorded at the end of the season. The thinning treatment removed half the trees and generated around 10% more available relative irradiance (GLF). This was followed by an increase in net photosynthesis at saturating PPFD (A _sat) and in maximum stomatal conductance to water vapour (g _wvmax). Moreover, specific leaf mass (SLM) and mass based nitrogen content (N_m) showed higher values for seedlings in the thinned stand. In both years, a positive relationship was established between the area based nitrogen content (N_a) and maximum net photosynthesis (A _max). In 1998, a year with a dry summer, seedlings suffered a significant drop in daily A _max irrespective of the thinning regime. This was a response to an increase in stomatal limitation to net photosynthesis, g _wvmax reaching the lowest value on dates with the highest drought. A lack of decrease of Fv/Fm confirmed the absence of significant non-stomatal limitation to A as a consequence of photoinhibition after opening the pinewood. A higher maximum quantum efficiency of open PS II centres (Fv/Fm) was registered in seedlings in the thinned stand. The significance of the differences between the treatments was stronger in the second year after thinning. In 1999, a year with frequent summer storms, water availability increased for seedlings growing under the thinned pinewood. Overall, the reduced pine overstory had a positive effect on physiological responses of beech seedlings, which was translated into improved seedling growth.     

Comparative photosynthesis characteristics of <Emphasis Type="Italic">Calycanthus chinensis</Emphasis> and <Emphasis Type="Italic">Chimonanthus praecox</Emphasis>

Calycanthus chinensis is an endangered plant of the national second-grade protection of China restricted in a small area in Zhejiang Province. We studied parameters of photosynthesis, chlorophyll (Chl) contents, and Chl fluorescence (minimum fluorescence, F₀, maximum fluorescence, F_m, variable fluorescence, F_v, and F_v/F_m) of C. chinensis and Chimonanthus praecox. C. chinensis had lower compensation irradiance but higher saturation irradiance than C. praecox. Hence C. chinensis has more advantage in obtaining and utilizing photon energy and higher Chl content, and is more adaptive to higher temperature and propitious to thermal dissipation than C. praecox. In addition, C. chinensis produces abundant, well-preserved seed with a higher germination rate and a wider adaptability to temperature than C. praecox. Thus C. chinensis is prone to survival and viability, and gets rid of the endangered plant species of the national second-grade protection of China.     

Underestimated chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence measurements on <Emphasis Type="Italic">Buxus microphylla</Emphasis> red winter leaves

Leaves under stressful conditions usually show downregulated maximum quantum efficiency of photosystem II [inferred from variable to maximum chlorophyll (Chl) a fluorescence (F_v/F_m), usually lower than 0.8], indicating photoinhibition. The usual method to evaluate the degree of photoinhibition in winter red leaves is generally by measuring the F_v/F_m on the red adaxial surface. Two phenotypes of overwintering Buxus microphylla ‘Wintergreen’ red leaves, with different measuring site and leaf thickness, were investigated in order to elucidate how red pigments in the outer leaf layer affected the Chl a fluorescence (F_v/F_m) and photochemical reflectance index. Our results showed that the F_v/F_m measured on leaves with the same red surface, but different leaf thickness, exhibited a slightly lower value in half leaf (separated upper and lower layers of leaves by removing the leaf edge similarly as affected by winter freezing and thawing) than that in the intact leaf (without removing the leaf edge), and the F_v/F_m measured on the red surface was significantly lower than that on the inner or backlighted green surface of the same thickness. Our results suggest that the usual measurement of F_v/F_m on red adaxial surface overestimates the actual degree of photoinhibition compared with that of the whole leaf in the winter.