A comparison of nutrient- and light-limited photosynthesis in psammophytic versus epilithic forms of Halimeda (Caulerpales,Halimedaceae) from the Bahamas

The relative nutritional status, with respect to phosphorus (P _i ) vs. nitrogen (N) limitation, and light-limited photosynthesis (P _s ) was examined over a broad range of quantum fluxes (I) for four Halimeda species, Halimeda tuna (Ellis and Solander) Lamouroux, H. simulans Howe, H. lacrimosa Howe and H. copiosa Goreau and Graham, taken from clear, shallow, Bahamian waters. The results support the hypothesis that psammophytic forms (i.e., sand dwellers anchored by a bulbous rhizoidal system) differ in nutrient status from epilithic forms (i.e., attached to rock by inconspicuous rhizoids). Maximum photosynthetic rates (P _max) for the epilithic species H. lacrimos and H. copiosa decreased (P<0.05) following P _i enrichment, but increased (P<0.05) following N pulses. Conversely, following brief exposures to P _i , P _max in the sand-dwelling forms H. tuna and H. simulans was elevated (P<0.05). These findings suggest that shallow species of Halimeda are adapted to take advantage of episodic nutrient pulses, and that partitioning of limiting resources may occur between the various life forms. Shallow water Halimeda species appear well adapted to variable light regimes, including low light conditions. In all cases, light-saturated photosyntheses (I _k ) occurred at irradiances much lower than the ambient levels available on typical sunny days. Associated with low saturation irradiances were low light requirements for photosynthetic compensation (I _c ) and reasonably efficient use of low photon flux densities as indicated by relatively steep slopes () of the P _s vs. I curves. Of the four species, H. copiosa was the most shade adapted, with considerably higher values and considerably lower I _c , I _k and photoinhibition values.     

Limitation to Carbon Assimilation of Two Perennial Species in Semi-Arid South-East Spain

Diurnal and seasonal changes of net photosynthetic rate (P_n) and the efficiency of photosystem 2 (F_v/F_m) were measured on two perennial species growing on a soil catena in semi-arid south-east Spain. Stipa tenacissima, a tussock grass, grows on shallow soil at the top of the catena and Retama sphaerocarpa, a leguminous shrub, grows in the valley bottom. A linear relationship was found between light saturated photosynthetic rates (P_max) and diffusive leaf conductance (g_l) in both Retama and Stipa indicating that the intercellular CO₂ concentration (c_i) was maintained constant in both species diurnally. Relatively high values of calculated c_i in Retama cladodes suggested that was not the primary limitation to carbon assimilation. F_v/F_m for the two species when well watered was around 0.8. Although Retama cladodes maintained this value throughout the year, F_v/F_m decreased to a minimum of 0.43 in Stipa leaves, at the end of the dry season. Our data suggest that plants in the Rambla Honda can substantially reduce transpiration without reducing photosynthetic rates to the same extent by closing their stomata, because P_n is reduced primarily by high respiration, decreased mesophyll conductance and by photoinhibition or permanent damage of photosystem 2.     

草甘膦对入侵植物加拿大一枝黄花和伴生植物白茅光合特性的影响

入侵植物加拿大一枝黄花(Solidago canadensis)给许多地区带来了较大危害,目前常采用化学防除法进行防除,但除草剂防治入侵植物的同时难免会影响土著植物的生长。为探讨草甘膦对入侵植物与本地植物光合特性的影响,以加拿大一枝黄花及其伴生种白茅(Imperata cylindrica)为研究对象,采用盆栽控制试验方法,研究不同浓度草甘膦处理21后单种、混种加拿大一枝黄花和白茅的生长特征及光响应过程。结果表明:1)草甘膦显著抑制两种植物的生长。随处理浓度升高,加拿大一枝黄花的株高增长量不断减小、叶片枯萎率不断增加;白茅的分蘖死亡率、叶片枯萎率不断升高。白茅对草甘膦较敏感,0.6mL/L浓度下白茅先失绿,1.2mL/L下其分蘖死亡率、叶片枯萎率均超50%;1.8mL/L下加拿大一枝黄花叶片枯萎率超50%。施药后与单种相比,混种加拿大一枝黄花株高增长略快、叶片枯萎率略低,混种白茅分蘖死亡率及叶片枯萎率均较低,但单、混种之间差异不显著。种间关系显著影响白茅的分蘖数。2)随处理浓度递增,加拿大一枝黄花和白茅叶片净光合速率(P_n)、气孔导度(G_s)、蒸腾速率(T_r)均不断降低,白茅下降更快。两个物种胞间CO_2浓度(C_i)的变化不同,随着浓度升高,单种加拿大一枝黄花C_i先下降而后上升,而混种时的C_i则不断下降;单、混种白茅C_i均上升。3)草甘膦显著影响加拿大一枝黄花和白茅最大净光合速率(P_(nmax))、光饱和点(LSP)和光补偿点(LCP);对两个物种暗呼吸速率(R_d)的影响不显著,对加拿大一枝黄花表观量子效率(AQY)的影响同样不显著,但显著影响白茅AQY。种植方式显著影响两个物种P_(nmax)、LSP以及白茅R_d和AQY。0.6mL/L草甘膦对混种加拿大一枝黄花和白茅P_(nmax)的影响要大于对单种植株的影响,随处理浓度上升,对不同种植方式下两种植物P_(nmax)的影响趋近。与本地种白茅相比,入侵植物加拿大一枝黄花具有更高的光合速率和生长速率;草甘膦显著降低两个物种的生长和光合作用,白茅对草甘膦处理更敏感。     

Effects of plant species on phosphorus availability in a range of grassland soils

Vegetative conversion from grass to forest may influence soil nutrient dynamics and availability. A short-term (40 weeks) glasshouse experiment was carried out to investigate the impacts of ryegrass (Lolium perenne) and radiata pine (Pinus radiata) on soil phosphorus (P) availability in 15 grassland soils collected across New Zealand using ³³P isotopic exchange kinetics (IEK) and chemical extraction methods. Results from this study showed that radiata pine took up more P (4.5–33.5 mg P pot^–1) than ryegrass (1.1–15.6 mg pot^–1) from the soil except in the Temuka soil in which the level of available P (e.g., E _1min P_i, bicarbonate extractable P_i) was very high. Radiata pine tended to be better able to access different forms of soil P, compared with ryegrass. There were no significant differences in the level of water soluble P (Cp, intensity factor) between soils under ryegrass and radiata pine, but the levels of Cp were generally lower compared with original soils due to plant uptake. The growth of both ryegrass and radiata pine resulted in the redistribution of soil P from the slowly exchangeable P_i pool (E _{> 10m} P_i, reduced by 31.8% on the average) to the rapidly exchangeable P_i (E _1min-1d P_i, E _1d-10m P_i) pools in most soils. The values of R/r ₁ (the capacity factor) were also generally greater in most soils under radiata pine compared with ryegrass. Specific P mineralisation rates were significantly greater for soils under radiata pine (8.4–21.9%) compared with ryegrass (0.5–10.8%), indicating that the growth of radiata pine enhanced mineralisation of soil organic P. This may partly be ascribed to greater root phosphatase activity for radiata pine than for ryegrass. Plant species × soil type interactions for most soil variables measured indicate that the impacts of plant species on soil P dynamics was strongly influenced by soil properties.     

天竺桂凋落叶对凤仙花生长和光合特性的影响

采用盆栽实验,通过向土壤(每盆8kg)中添加0g·pot-1(CK)、20g·pot-1(L)、40g·pot-1(M)和80g·pot-1(H)天竺桂(Cinnamomum japonicum)凋落叶,模拟其自然分解对凤仙花(Impatiens balsamina)生长和光合特性的影响。结果显示:(1)添加天竺桂凋落叶M和H处理下,凤仙花生物量和地径均显著降低,而株高无明显变化;其叶绿素含量受到显著抑制,净光合速率(Pn)和水分利用效率(WUE)显著低于CK,而气孔导度(Gs)、胞间二氧化碳浓度(Ci)和蒸腾速率(Tr)3个气体交换参数显著高于CK。(2)Pn-PAR曲线和Pn-Ci曲线拟合表明,凤仙花在光饱和以及CO2饱和状态下的最大净光合速率(Pn max)、表观量子效率(AQY)、暗呼吸速率(Rd)、RuBP羧化效率(CE)和光呼吸速率(Rp)均随添加天竺桂凋落叶处理量的增加而呈下降趋势。(3)添加天竺桂凋落叶36d和67d时对凤仙花生长影响不明显,而处理58d时有明显抑制作用。研究表明,在模拟天竺桂凋落叶自然分解的土壤环境中,凤仙花的光合色素含量降低,抑制了其光合能力,对环境适应能力降低,导致凤仙花的生长受到抑制。     

A 31P NMR study of the internal pH of yeast peroxisomes

The internal pH of peroxisomes in the yeasts Hansenula polymorpha, Candida utilis and Trichosporon cutaneum X4 was estimated by ³¹P nuclear magnetic resonance (NMR) spectroscopy. ³¹P NMR spectra of suspensions of intact cells of these yeasts, grown under conditions of extensive peroxisomal proliferation, displayed two prominent P_i-peaks at different chemical shift positions. In control cells grown on glucose, which contain very few peroxisomes, only a single peak was observed. This latter peak, which was detected under all growth conditions, was assigned to cytosolic P_i at pH 7.1. The additional peak present in spectra of peroxisome-containing cells, reflected P_i at a considerably lower pH of approximately 5.8–6.0. Experiments with the protonophore carbonyl cyanide m-chlorophenylhydrazon (CCCP) and the ionophores valinomycin and nigericin revealed that separation of the two P_i-peaks was caused by a pH-gradient across a membrane separating the two pools. Experiments with chloroquine confirmed the acidic nature of one of these pools. In a number of transfer experiments with the yeast H. polymorpha it was shown that the relative intensity of the P_i-signal at the low pH-position was correlated to the peroxisomal volume fraction. These results strongly suggest that this peak has to be assigned to P_i in peroxisomes, which therefore are acidic in nature. The presence of peroxisome-associated P_i was confirmed cytochemically.Abbreviations CCCP Carbonyl cyanide m-chlorophenylhydrazon - DCCD N,N-dicyclohexylcarbodiimide     

Effects of low and elevated CO2 on C3 and C4 annuals

Abutilon theophrasti (C₃) and Amaranthus retroflexus (C₄), were grown from seed at four partial pressures of CO₂: 15 Pa (below Pleistocene minimum), 27 Pa (pre-industrial), 35 Pa (current), and 70 Pa (future) in the Duke Phytotron under high light, high nutrient, and wellwatered conditions to evaluate their photosynthetic response to historic and future levels of CO₂. Net photosynthesis at growth CO₂ partial pressures increased with increasing CO₂ for C₃ plants, but not C₄ plants. Net photosynthesis of Abutilon at 15 Pa CO₂ was 70% less than that of plants grown at 35 Pa CO₂, due to greater stomatal and biochemical limitations at 15 Pa CO₂. Relative stomatal limitation (RSL) of Abutilon at 15 Pa CO₂ was nearly 3 times greater than at 35 Pa CO₂. A photosynthesis model was used to estimate ribulose-1,5-bisphosphate carboxylase (rubisco) activity (Vc_max), electron transport mediated RuBP regeneration capacity (J _max), and phosphate regeneration capacity (PiRC) in Abutilon from net photosynthesis versus intercellular CO₂ (A–C _i) curves. All three component processes decreased by approximately 25% in Abutilon grown at 15 Pa compared with 35 Pa CO₂. Abutilon grown at 15 Pa CO₂ had significant reductions in total rubisco activity (25%), rubisco content (30%), activation state (29%), chlorophyll content (39%), N content (32%), and starch content (68%) compared with plants grown at 35 Pa CO₂. Greater allocation to rubisco relative to light reaction components and concomitant decreases in J _max and PiRC suggest co-regulation of biochemical processes occurred in Abutilon grown at 15 Pa CO₂. There were no significant differences in photosynthesis or leaf properties in Abutilon grown at 27 Pa CO₂ compared with 35 Pa CO₂, suggesting that the rise in CO₂ since the beginning of the industrial age has had little effect on the photosynthetic performance of Abutilon. For Amaranthus, limitations of photosynthesis were balanced between stomatal and biochemical factors such that net photosynthesis was similar in all CO₂ treatments. Differences in photosynthetic response to growth over a wide range of CO₂ partial pressures suggest changes in the relative performance of C₃ and C₄ annuals as atmospheric CO₂ has fluctuated over geologic time.     

Uptake kinetics and assimilation of phosphorus byCatenella nipaeandUlva lactucacan be used to indicate ambient phosphate availability

Uptake, assimilation and compartmentation of phosphate were studied in the opportunist green macroalgaUlva lactucaand the estuarine red algal epiphyteCatenella nipae. The Michaelis–Menten model was used to describe uptake rates of inorganic phosphate (P_i) at different concentrations. Maximum uptake rates (V _max) of P-starved material exceededV _maxof P-enriched material; this difference was greater forC. nipae. Uptake and allocation of phosphorus (P) to internal pools was measured using trichloroacetic acid (TCA) extracts and³²P. Both species demonstrated similar assimilation paths: when P-enriched, most³²P accumulated as free phosphate. When unenriched,³²P was rapidly assimilated into the TCA-insoluble pool.C. nipaeconsistently assimilated more³²P into this pool thanU. lactuca, indicatingC. nipaehas a greater P-storage capacity. In both species,³²P release data showed two internal compartments with very different biological half-lives. The rapidly exchanging compartment had a short half-life of 2 to 12 min, while the slowly exchanging compartment had a much longer half-life of 12 days in P-starvedC. nipaeor 4 days in P-starvedU. lactuca. In both species, the slowly exchanging compartment accounted for more than 90% of total tissue.U. lactucaandC. nipaeresponded differently to high external P_i.U. lactucarapidly took up P_i, transferring this P_iinto tissue phosphate and TCA-soluble P in a few hours (90% of total P).C. nipaetook up P_iat lower rates and stored much of this P in less mobile TCA-insoluble forms. Long-term storage of refractory forms of P makesC. nipaea useful bioindicator of the prevailing conditions of P_iavailability over at least the previous 7 days, whereas the P-status ofU.lactucamay reflect conditions over no more than the previous few hours or days.C. nipaeis a more useful bioindicator for P status of estuarine and marine waters thanU. lactuca.     

Comparison of Photosynthetic Traits Between Two Typical Shrubs: Legume and Non-Legume in Hunshandak Sandland

In Huanshandak Sandland, China, net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), intercellular CO₂ concentration (C _i), water use efficiency (WUE), photochemical efficiency of photosystem 2 (F_v/F_m), and leaf nitrogen content were compared for Hedysarum fruticosum var. mongolicum (H.f.m.), a nitrogen fixing shrub, and Salix gordejevii (S.g.), a nitrogen non-fixing shrub. P _N, E, and g _s of the two shrubs were similar in trends, i.e. two peaks were observed in diurnal courses. However, except C _i, other parameters of H.f.m. were higher during the measured days than those of S.g. The midday depression of P _N was mainly due to decrease in stomata conductance and to reduction of F_v/F_m at midday. The higher P _N of H.f.m. was consistent with the higher leaf N content and there was a positive relation between them. In addition, several C₄ traits were found in H.f.m., i.e. high saturation irradiance and WUE, low dark respiration rate, and C _i, which partly resulted in higher P _N. This seems to indicate that the C₃ plant H.f.m. may have C₄ photosynthesis pathway or C₄ enzymes.     

Stomatal and Mesophyll Limitations to Photosynthesis in One Evergreen and One Deciduous Mediterranean Oak Species

In the evergreen Quercus rotundifolia and the co-existing deciduous Q. faginea we studied the diurnal variations in photosynthetic capacity (P _max), measured as the rate of O₂ evolution at photon and CO₂ saturation, and in the rate of net CO₂ assimilation (P _N) in the field during the period of maximum photosynthetic activity. Our aim was to check the contribution of stomatal and non-stomatal limitations to the diurnal variation in photosynthesis, and to study the differences between both species. Q. faginea leaves displayed lower mass per unit area and higher nitrogen content than Q. rotundifolia leaves. The maximum stomatal conductance and P _N in the field were higher in Q. faginea than in Q rotundifolia. Also P _max of Q. faginea was higher than that of Q. rotundifolia. Both species attained in the field a high percentage of the P _max (around 82 % for Q. faginea and 73 % for Q. rotundifolia). This indicates reduced stomatal limitation of photosynthesis under favourable conditions, especially in Q. faginea. P _N underwent a sharp decrease towards mid-day in association with increase in the atmospheric vapour pressure deficit and decrease in the leaf water potential. P _max was also reduced during mid-day. This demonstrated the contribution of mesophyll limitations to the P _N in the two species under stress. The mesophyll limitation of photosynthesis seemed to be similar for both species, independently from the differences in leaf traits between them.     

HIGH PHOTOSYNTHETIC RATES UNDER A COLIMITATION FOR INORGANIC PHOSPHORUS AND CARBON DIOXIDE1

Inorganic phosphorus (P_i) and carbon (here, CO₂) potentially limit the photosynthesis of phytoplankton simultaneously (colimitation). A single P_i limitation generally reduces photosynthesis, but the effect of a colimitation is not known. Therefore, photosynthesis was measured under P_i‐limited conditions and high and low CO₂, and osmo‐mixotrophic (i.e., growth in the presence of glucose) conditions that result in colimiting conditions in some cases. The green alga Chlamydomonas acidophila Negoro was used as a model organism because low P_i and CO₂ concentrations likely influence its photosynthetic rates in its natural environment. Results showed a decreasing maximum photosynthetic rate (P_max) and maximum quantum yield (Φ_II) with increasing P_i limitation. In addition, a P_i limitation enhanced the relative contribution of dark respiration to P_max (R_d:P_max) but did not influence the compensation light intensity. P_max positively correlated with the cellular RUBISCO content. Osmo‐mixotrophic conditions resulted in similar P_max, Φ_II, and RUBISCO content as in high‐CO₂ cultures. The low‐CO₂ cultures were colimited by P_i and CO₂ and had the highest P_max, Φ_II, and RUBISCO content. Colimiting conditions for P_i and CO₂ in C. acidophila resulted in an enhanced mismatch between photosynthesis and growth rates compared to the effect of a single P_i limitation. Primary productivity of colimited phytoplankton could thus be misinterpreted.     

Nitrogen and carbon fixation byAnabaena sp. isolated from a rice paddy and grown under P and light limitations

Anabaena sp., isolated from a rice paddy, was investigated for its nitrogen fixation as measured by acetylene reduction activity (ARA) in P-limited continuous and light-limited semi-continuous cultures. Growth rate (μ) under P limitation was a function of cell P content (q _p). Both the photosynthetic capacity (P_max) and photosynthetic efficiency (α) increased with μ when expressed per cell, but not per unit chla. The ARA of steady-state cells under P limitation increased with μ and was linearly related to C-fixation rate. This was apparently a consequence of the control of C-fixation by P limitation. In light-limited cells, steady state ARA, both at the culture light intensity and in the dark, increased asymptotically with μ, but the activity in the dark was only about 51% of that in the light. When the light level of steady-state cells grown at a high in intensity was switched to a low level, ARA decreased exponentially with time. Dark ARA activity also showed a similar decline, but at much lower levels. Thus, ARA depended not only on light history, but also immediate photosynthesis. Steady-state ARA at the ambient intensity or in the dark showed a strong correlation with¹⁴C-fixation rate. ARA of light-limited cells showed the same light-saturation characteristics as their¹⁴C-fixation, with the same initial saturation intensity,I _k. The ratios of P_max to the maximum ARA (ARA_max), and α to the slope of ARA (α_ara) were identical. A comparison of gross to net photosynthesis and N₂ fixation suggested that there was little leakage or excretion of fixed C or N.     

Properties of the phosphate and phosphite transport systems of Phytophthora palmivora

Germlings of Phytophthora palmivora possess at least two systems for the uptake of inorganic phosphate (P_i). The first is synthesized on germination in medium containing 50 M P_i and has a K_m of approx. 30 M (V_max=7–9 nmol P_i/h·10⁶ cells). The second is synthesized under conditions of P_i-deprivation and has a higher affinity for P_i (K_m=1–2 M), but a lower V_max (0.5–2 nmol P_i/h·10⁶ cells). The fungicide phosphite likewise enters the germlings via two different transport systems, the synthesis of which also depends on the concentration of P_i in the medium. The K_m of the lower affinity system is 3 mM (V_max=20 nmol phosphite/h·10⁶ cells) and that of the higher affinity system is 0.6 mM (V_max=12 nmol/h·10⁶ cells). P_i and phosphite are competitive inhibitors for each other's transport in both systems. However, whereas mM concentrations of phosphite are necessary to inhibit P_i transport, only M concentrations of P_i are required to inhibit phosphite transport. A third system of uptake for P_i also exists, since when phosphate-deprived cells are presented with mM concentrations of P_i, they transport the anion at a very high rate (around 100 nmol/h·10⁶ cells). High rates of transport of phosphite are also observed when these cells are presented with mM concentrations of this anion.     

Diurnal Oscillation in the Intercellular CO2 Concentration of Spring Wheat Under the Semiarid Conditions

Yields of wheat in semiarid and arid zones are limited by drought, and water condition is very important at each stage of development. Studies carried out at Loess Plateau in the northwestern part of China indicated that yield of spring wheat (Triticum aestivum L.) cv. Dingxi 81-392 was reduced by 41% when subjected to water stress. The effects of two water regimens on net photosynthetic rate (P _N), stomatal conductance (g _s), and intercellular CO₂ concentration (C _i) were investigated at the jointing, booting, anthesis, and grain filling stages. Low soil moisture in comparison to adequate one had invariably reduced P _N during the diurnal variations at the four growth stages. P _N and g _s in both soil moisture regimes was maximally reduced at midday. C _i and the stomatal limitation fluctuated remarkably during photosynthesis midday depression processes, especially at the grain filling stage. Hence atmospheric drought at midday was one of the direct causes inducing stomata closure and the g _s depression, but it was beneficial for maintaining stable intrinsic water use efficiency. Fluctuation in C _i implicated that non-stomatal limitation also plays an important role during the period of photosynthesis midday depression. Consequently stomatal and/or non-stomatal limitation are the possible cause of the midday photosynthesis decline.     

Effect of ectomycorrhizal infection on growth and photosynthetic characteristics of <Emphasis Type="Italic">Pinus densiflora</Emphasis> seedlings grown under elevated CO<Subscript>2</Subscript> concentrations

The effect of ectomycorrhizal Pisolithus tinctorius (Pt) infection was studied on the growth and photosynthetic characteristics of Pinus densiflora seedlings grown at ambient (360 µmol mol⁻¹, AC) and elevated (720 µmol mol⁻¹, EC) CO₂ concentrations. After 18 weeks, Pt inoculation had led to significantly increased dry mass and stem diameter of P. densiflora at both CO₂ concentrations, relative to non-inoculated seedlings. Moreover, EC significantly increased the ectomycorrhizal development. The phosphate content in needles inoculated with Pt was about three times higher than without inoculation at both CO₂ concentrations. The PAR saturated net photosynthetic rates (P _sat) of P. densiflora inoculated with Pt were clearly higher than for control seedlings at both CO₂ concentrations, and the maximum net photosynthetic rate (P _N) at saturated CO₂ concentration (P _max) was higher than in controls. Moreover, the carboxylation efficiency (CE) and RuBP regeneration rate of the P _N/C _i curve for P. densiflora inoculated with Pt were significantly higher than for non-inoculated seedlings at both CO₂ concentrations, especially at EC. The water use efficiency (WUE) of seedlings inoculated with Pt grown at EC was significantly raised. Allocation of photosynthates to roots was greater in Pt inoculated pine seedlings, because of the enhanced activity of ectomycorrhiza associated with seedlings at EC. Moreover, P _N of non-inoculated seedlings grown for 18 weeks at EC tended to be down regulated; in contrast, Pt inoculated seedlings showed no down-regulation at EC. The activity of ectomycorrhiza may therefore be enhanced physiological function related to water and phosphate absorption in P. densiflora seedlings at EC.This study was partly sponsored by the Ministry of Education, Sport, Culture, Science and Technology of Japan (RR2002, Basic Research B and Sprout study).     

遮光对香果树幼苗光合特性及叶片解剖结构的影响

设置了全光照(L0)、透光率60%(L1)、透光率25%(L2)、透光率10%(L3)4种光环境,研究了不同遮光处理对香果树幼苗光合作用及叶片解剖结构的影响。结果表明:(1)遮光对香果树幼苗净光合速率(P_n)、蒸腾速率(T_r)、气孔导度(G_s)、气孔限制值(L_s)、胞间CO_2浓度(C_i)、蒸汽压亏缺(V_(pdl))均产生了显著影响。全光和L1处理下Pn和Tr日变化呈明显的双峰曲线,中午出现了明显的"午休",其余处理的变化相对缓和,双峰和"午休"现象不明显;G_s的日变化规律与Pn基本一致;C_i的日变化整体呈先期缓慢降低后期抬升的趋势,与对照相比遮光显著提高了叶片胞间CO_2浓度;Ls和V_(pdl)呈现出先升后降的弧形变化,并且遮光下L_s和V_(pdl)日均值显著低于全光照。一天中P_n和T_r的日均值大小排序为L1L0L2L3。(2)不同光环境下香果树幼苗的光合响应特征明显不同。全光下,香果树幼苗能够维持较高的净光合作用速率而未呈现明显的光抑制,表现出一定的光忍耐和喜光性。L1处理下的最大净光合速率(P_(max))、暗呼吸速率(R_d)和表观量子效率(AQY)显著增加,说明香果树对光强具有一定的可塑性,轻度遮光有利于光合效率提高;强度遮光(L2、L3)导致光补偿点(LCP)和光饱和点(LSP)显著降低,有利于充分利用弱光,但P_(max)、R_d明显下降,光合效率受到抑制。(3)遮光显著影响了香果树叶片的气孔分布。遮光条件下香果树叶片的气孔密度显著减小,气孔器长度、宽度与单个气孔器面积也呈现类似的变化。(4)遮光条件下香果树叶片厚度减小,上表皮增厚,下表/ST增加;维管束内单列导管数目和导管直径均有明显增大,促进了叶片水分传导运输。(5)在育苗生产中,为了适应香果树幼苗生长发育的需求,建议有效辐射强度保持在自然光强的60%以上。     

Cytological peculiarities of some extremely halophilic soil archaeobacteria

Ultrastructure and morphogenesis of extremely halophilic neutrophilic (Halobacteriam distributum, Halococcus turkmenicus) and alkaliphilic (Natronobacterium pharaonis, Natronococcus occultus) archaeobacteria were studied. The H. distributum culture was rather polymorphous and produced cells of four types. Due to the irregular cell fission in different planes packages of various numbers of cells surrounded by a common capsule were formed. Resting forms (halocysts) with multilayer covers were present in the population. The N. pharaonis culture consisted of rod-like cells and cyst-like forms. Besides, under conditions of carbon limitation, multicellular aggregated forms were found in the culture. Encapsulated single cells and aggregated forms with a common capsule were observed in H. turkmenicus and N. occultus cultures.     

Photosynthetic Characteristics,Dark Respiration,and Leaf Mass Per Unit Area in Seedlings of Four Tropical Tree Species Grown Under Three Irradiances

We investigated the effect of growth irradiance (I) on photon-saturated photosynthetic rate (P _max), dark respiration rate (R _D), carboxylation efficiency (CE), and leaf mass per unit area (LMA) in seedlings of the following four tropical tree species with contrasting shade-tolerance. Anthocephalus chinensis (Rubiaceae) and Linociera insignis (Oleaceae) are light-demanding, Barringtonia macrostachya (Lecythidaceae) and Calophyllum polyanthum (Clusiaceae) are shade-tolerant. Their seedlings were pot-planted under shading nets with 8, 25, and 50 % daylight for five months. With increase of I, all species displayed the trends of increases of LMA, photosynthetic saturation irradiance, and chlorophyll-based P _max, and decreases of chlorophyll (Chl) content on both area and mass bases, and mass-based P _max, R _D, and CE. The area-based P _max and CE increased with I for the light-demanders only. Three of the four species significantly increased Chl-based CE with I. This indicated the increase of nitrogen (N) allocation to carboxylation enzyme relative to Chl with I. Compared to the two shade-tolerants, under the same I, the two light-demanders had greater area- and Chl-based P _max, photosynthetic saturation irradiance, lower Chl content per unit area, and greater plasticity in LMA and area- or Chl-based P _max. Our results support the hypothesis that light-demanding species is more plastic in leaf morphology and physiology than shade-tolerant species, and acclimation to I of tropical seedlings is more associated with leaf morphological adjustment relative to physiology. Leaf nitrogen partitioning between photosynthetic enzymes and Chl also play a role in the acclimation to I.     

Sagebrush carbon allocation patterns and grasshopper nutrition: the influence of CO2 enrichment and soil mineral limitation

Summary Artemisia tridentata seedlings were grown under carbon dioxide concentrations of 350 and 650 l l^–1 and two levels of soil nutrition. In the high nutrient treatment, increasing CO₂ led to a doubling of shoot mass, whereas nutrient limitation completely constrained the response to elevated CO₂. Root biomass was unaffected by any treatment. Plant root/shoot ratios declined under carbon dioxide enrichment but increased under low nutrient availability, thus the ratio was apparently controlled by changes in carbon allocation to shoot mass alone. Growth under CO₂ enrichment increased the starch concentrations of leaves grown under both nutrient regimes, while increased CO₂ and low nutrient availability acted in concert to reduce leaf nitrogen concentration and water content. Carbon dioxide enrichment and soil nutrient limitation both acted to increase the balance of leaf storage carbohydrate versus nitrogen (C/N). The two treatment effects were significantly interactive in that nutrient limitation slightly reduced the C/N balance among the high-CO₂ plants. Leaf volatile terpene concentration increased only in the nutrient limited plants and did not follow the overall increase in leaf C/N ratio. Grasshopper consumption was significantly greater on host leaves grown under CO₂ enrichment but was reduced on leaves grown under low nutrient availability. An overall negative relationship of consumption versus leaf volatile concentration suggests that terpenes may have been one of several important leaf characteristics limiting consumption of the low nutrient hosts. Digestibility of host leaves grown under the high CO₂ treatment was significantly increased and was related to high leaf starch content. Grasshopper growth efficiency (ECI) was significantly reduced by the nutrient limitation treatment but co-varied with leaf water content.     

Ribulose-1, 5-bisphosphate carboxylase activity,chlorophyll and protein concentrations in differentPopulus clones

Ribulose-1,5-bisphosphate carboxylase activity (RuBPC), chlorophyll (chl) and protein (prot) concentrations and chlorophyll/protein (chl/prot) ratios were determined in five differentPopulus clones together with their maximal net CO₂ uptake rates (P_max). A classic reference clone (Populus ×euramericana “Robusta” (Dode) Guinier) was compared with four recently selected euramerican and interamerican crossings. Chl/prot ratio and RuBPC activity varied among the different clones, while chl a/chlb ratio showed only a very low coefficient of variation (1.7%) for the five clones. Poplar clone “Robusta” could be distinguished from the recent faster growing clones based on the different biochemical characteristics. A significant correlation was found between both total chl concentration and chl/prot ratio with P_max for the five clones.