Cold resistance in Antarctic angiosperms

Deschampsia antarctica Desv. (Poaceae) and Colobanthus quitensis (Kunth) Bartl. (Cariophyllaceae) are the only two vascular plants that have colonized the Maritime Antarctic. The primary purpose of the present work was to determine cold resistance mechanisms in these two Antarctic plants. This was achieved by comparing thermal properties of leaves and the lethal freezing temperature to 50% of the tissue (LT50). The grass D. antarctica was able to tolerate freezing to a lower temperature than C. quitensis. The main freezing resistance mechanism for C. quitensis is supercooling. Thus, the grass is mainly a freezing‐tolerant species, while C. quitensis avoids freezing. D. antarctica cold acclimated; thus, reducing its LT50. C. quitensis showed little cold‐acclimation capacity. Because day length is highly variable in the Antarctic, the effect of day length on freezing tolerance, growth, various soluble carbohydrates, starch, and proline contents in leaves of D. antarctica growing in the laboratory under cold‐acclimation conditions was studied. During the cold‐acclimation treatment, the LT50 was lowered more effectively under long day (21/3 h light/dark) and medium day (16/8) light periods than under a short day period (8/16). The longer the day length treatment, the faster the growth rate for both acclimated and non‐acclimated plants. Similarly, the longer the day treatment during cold acclimation, the higher the sucrose content (up to 7‐fold with respect to non‐acclimated control values). Oligo and polyfructans accumulated significantly during cold acclimation only with the medium day length treatment. Oligofructans accounted for more than 80% of total fructans. The degrees of polymerization were mostly between 3 and 10. C. quitensis under cold acclimation accumulated a similar amount of sucrose than D. antarctica, but no fructans were detected. The suggestion that survival of Antarctic plants in the Antarctic could be at least partially explained by accumulation of these substances is discussed.     

Biochemical and photochemical changes in response to triacontanol in rice (Oryza sativa L.)

Triacontanol (TRIA) increased the contents of total chlorophyll (Chl), Chl a and Chl b by 25.1%, 26.1% and 22.4% respectively 4 h after treatment in rice seedlings. The minimal fluorescence (F₀), the maximal fluorescence (Fm) and Fv/Fm were also higher in TRIA-treated plants. In actinic light, other Chl fluorescence parameters were measured at different photon flux densities (PFD) to construct light response curves of the quantum yield of PSII electron transport (_PSII), light response curves of photochemical quenching (qp), and light response curves of non-photochemical quenching (qN), respectively. The _PSII and qp declined with the increasing PFD with a higher level present in TRIA-treated plants. The qN increased with the increasing PFD with a lower level present in TRIA-treated plants. Two-dimensional gel electrophoresis indicated a protein expression difference between TRIA-treated materials and the controls at the total-soluble-protein level. Rubisco was 30% higher in TRIA-treated plants than in controls. The quantity of other proteins was unchanged in response to TRIA. These data provide biochemical and photochemical evidence for the effects of TRIA on photosynthesis.     

Low temperature regulates sucrose-phosphate synthase activity in Colobanthus quitensis (Kunth) Bartl. by decreasing its sensitivity to Pi and increased activation by glucose-6-phosphate

Colobanthus quitensis (Kunth) Bartl. is widely distributed from Mexico to the Antarctic. C. quitensis is a freezing resistant species that accumulates sucrose in response to cold. We tested the hypothesis that low temperature modifies the kinetic properties of C. quitensis sucrose phosphate synthase (SPS) to increase its activity and ability to synthesize sucrose during cold acclimation. Cold acclimation caused a fourfold increment in sucrose concentration and a 100% increase in SPS activity, without changes in the level of SPS protein. Cold acclimation did not affect the optimal temperature and pH for SPS activity. However, it caused a tenfold increase in the inhibition constant (K _i) for inorganic phosphate (Pi) calculated as a function of fructose-6-phosphate (Fruc-6-P). SPS from cold acclimated plants also exhibited a higher reduction of its Michaelis constant (K _m) for glucose-6-phosphate (Gluc-6-P) with respect to non-acclimated plants. We suggest that the increase in C. quitensis SPS K _i for Pi and the increase in activation by Gluc-6-P in response to cold keep SPS activated, leading to high sucrose accumulation. This may be an important adaptation that allows efficient accumulation of sucrose during the harsh Antarctic summer.     

Robustness of activity of Calvin cycle enzymes after high light and low temperature conditions in Antarctic vascular plants

Only two vascular plants have naturally colonized the Maritime Antarctic: Colobanthus quitensis and Deschampsia antarctica. We propose that one of the reasons of their success in this environment is the robustness of their CO₂ assimilation machinery. In order to understand the mechanisms involved in the positive photosynthetic rates under stressful conditions, we analyzed changes in the activity of two key Calvin cycle enzymes: Ribulose bisphosphate carboxylase oxygenase (Rubisco) and stromal Fructose-1,6-bisphosphatase after high light and low temperature treatments. Our results show that the activity of both enzymes does not decrease after 48 h high light/low temperature treatments, a feature usually observed in plants adapted to harsh environments. The activation state of both enzymes remained high throughout the treatments. This feature has been related to the redox state of the chloroplast, suggesting that both plants maintain their redox balance under high light and/or low temperature conditions assayed. Both plants differed in their responsiveness to cold acclimation as observed by gas exchange and enzymatic measurements. We propose that these differences may be related to microclimate adaptations to the environment they naturally develop in the Maritime Antarctic.     

Thermostability and photostability of photosystem II of the resurrection plant Haberlea rhodopensis studied by chlorophyll fluorescence

The stability of PSII in leaves of the resurrection plant Haberlea rhodopensis to high temperature and high light intensities was studied by means of chlorophyll fluorescence measurements. The photochemical efficiency of PSII in well-hydrated Haberlea leaves was not significantly influenced by temperatures up to 40 degrees C. Fo reached a maximum at 50 degrees C, which is connected with blocking of electron transport in reaction center II. The intrinsic efficiency of PSII photochemistry, monitored as Fv/Fm was less vulnerable to heat stress than the quantum yield of PSII electron transport under illumination (phiPSII). The reduction of phiPSII values was mainly due to a decrease in the proportion of open PSII centers (qP). Haberlea rhodopensis was very sensitive to photoinhibition. The light intensity of 120 micromol m(-2) s(-1) sharply decreased the quantum yield of PSII photochemistry and it was almost fully inhibited at 350 micromol m(-2) s(-1). As could be expected decreased photochemical efficiency of PSII was accompanied by increased proportion of thermal energy dissipation, which is considered as a protective effect regulating the light energy distribution in PSII. When differentiating between the three components of qN it was evident that the energy-dependent quenching, qE, was prevailing over photoinhibitory quenching, qI, and the quenching related to state 1-state 2 transitions, qT, at all light intensities at 25 degrees C. However, the qE values declined with increasing temperature and light intensities. The qI was higher than qE at 40 degrees C and it was the major part of qN at 45 degrees C, indicating a progressing photoinhibition of the photosynthetic apparatus.     

Effects of water stress on photosystem II photochemistry and its thermostability in wheat plants

Modulated chlorophyll fluorescence, rapid fluorescence induction kinetics and the polyphasic fluorescence transients (OJIP) were used to evaluate PSII photochemistry in wheat plants exposed to water stress and/or heat stress (25-45C). Water stress showed no effects on the maximal quantum yield of PSII photochemistry (Fv/Fm), the rapid fluorescence induction kinetics, and the polyphasic fluorescence transients in dark-adapted leaves, indicating that water stress had no effects on the primary photochemistry of PSII. However, in light-adapted leaves, water stress reduced the efficiency of excitation energycapture by open PSII reaction centres (F'v/F'm) and the quantum yield of PSII electron transport (PSII), increased the non-photochemical quenching (qN) and showed no effects on the photochemical quenching (qP). This suggests that water stress modified the PSII photochemistry in the light-adapted leaves and such modifications may be a mechanism to down-regulate the photosynthetic electron transport to match a decreased CO2 assimilation. In addition, water stress also modified the responses of PSII to heat stress. When temperature was above 35C, thermostability of PSII was strongly enhanced in water-stressed leaves, which was reflected in a less decrease in Fv/Fm, qP, F'v/F'm, and PSII in water-stressed leaves than in well-watered leaves. There were no significant variations in the above fluorescence parameters between moderately and severely water-stressed plants, indicating that the moderate water-stressed plants, indicating that the moderate water stress treatment caused the same effects on thermostability of PSII as the severe treatment. It was found that increased thermostability of PSII may be associated with an improvement of resistance of the O2-evolving complex and the reaction centres in water-stressed plants to high temperature.Key words: Chlorophyll fluorescence, heat stress, photosystem II photochemistry, water stress, wheat (Tritium aestivum L.).      

The long-term response to fluctuating light quality is an important and distinct light acclimation mechanism that supports survival of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> under low light conditions

The long-term response (LTR) of higher plants to varying light qualities increases the photosynthetic yield; however, the benefit of this improvement for physiology and survival of plants is largely unknown, and its functional relation to other light acclimation responses has never been investigated. To unravel positive effects of the LTR we acclimated Arabidopsis thaliana for several days to light sources, which preferentially excite photosystem I (PSI) or photosystem II (PSII). After acclimation, plants revealed characteristic differences in chlorophyll fluorescence, thylakoid membrane stacking, phosphorylation state of PSII subunits and photosynthetic yield of PSII and PSI. These LTR-induced changes in the structure, function and efficiency of the photosynthetic machinery are true effects by light quality acclimation, which could not be induced by light intensity variations in the low light range. In addition, high light stress experiments indicated that the LTR is not involved in photoinhibition; however, it lowers non-photochemical quenching (NPQ) by directing more absorbed light energy into photochemical work. NPQ in turn is not essential for the LTR, since npq mutants performed a normal acclimation. We quantified the beneficial potential of the LTR by comparing wild-type plants with the LTR-deficient mutant stn7. The mutant exhibited a decreased effective quantum yield and produced only half of seeds when grown under fluctuating light quality conditions. Thus, the LTR represents a distinct acclimation response in addition to other already known responses that clearly improves plant physiology under low light conditions resulting in a pronounced positive effect on plant fitness.     

Differences in light energy utilisation and dissipation between dipterocarp rain forest tree seedlings

 The light environment within tropical rain forests varies considerably both spatially and temporally, and photon flux density (PFD) is considered to be an important factor determining the growth and survival of rain forest tree seedlings. In this paper we examine the ability of four ecologically contrasting dipterocarps (Dryobalanops lanceolata, Shorea leprosula, Hopea nervosa and Vatica oblongifolia) to utilise and dissipate light energy when grown in different light environments in lowland dipterocarp rain forest in the Danum Valley Conservation Area, Sabah, East Malaysia. Specifically we report (i) photosynthetic light response curves and associated fluorescence characteristics, including quantum yield (ΦPSII) and non-photochemical quenching (qN) and (ii) the extent to which photoinhibition occurs when plants grown in either high or low light are exposed to short bursts of high PFD. When grown in low light (artificial or forest shade) all four species had low light saturated rates of photosynthesis which were achieved at low PFDs. In addition, values of ΦPSII and qN were similar over a range of measurement PFDs. D. lanceolata and S. leprosula were also grown at high PFD and showed marked differences in their responses. S. leprosula demonstrated an ability to increase its rate of photosynthesis and there was a small increase in capacity to dissipate excess light energy non-photochemically at high PFDs. Partitioning of this qN into its fast, photo-protective (qE) and slow, photoinhibitory (qI) components indicated that there was an increase in qE quenching. In contrast, although D. lanceolata survived in the high light environment, greater rates of photosynthesis were not observed and the plants showed a greater capacity to dissipate energy non-photochemically. Partitioning of qN revealed that the majority of this increase was attributable to the slower relaxing phases. Received: 10 February 1996 / Accepted: 14 June 1996     

Photosynthetic Acclimation of Erythrophleum guineense and Dalbergia odorifera to Winter Low Temperature in a Marginal Tropical Area

In marginal tropical areas, air temperature in winter usually decreases by 10℃ compared with summer at night/day. Although tropical plants are sensitive to low temperature, the mechanism underlying photosynthetic acclimation of tropical trees to winter low temperature is unclear. To address this question, the photosystem I (PSI) and photosystem II (PSII) activities, and energy distribution in PSI and PSII were examined in summer and winter in two tropical high quality timber tree species Erythrophleum guineense and Dalbergia odorifera grown in a marginal tropical area (21°54′N, 101°46′E). Our results indicated that the photosynthetic apparatus of Eguineense and Dodorifera was maintained stable in winter. The effective quantum yield of PSII decreased significantly in winter, but non photochemical quenching (NPQ) significantly increased. In winter, cyclic electron flow (CEF) was significantly stimulated in both species, which was significantly and positively correlated with NPQ. Meanwhile, the stimulation of CEF led to an increase in P700 oxidation ratio and the over reduction of PSI acceptor side was prevented. Antimycin A (a specific inhibitor of PGR5 dependent CEF) significantly aggravated PSII photoinhibition under high light in both species. These results suggested that stimulation of CEF is an important mechanism for photosynthetic acclimation to winter low temperature in a marginal tropical area in the two tropical tree species.     

CaCl_2对UV-B辐射下菠菜叶片光合特性的影响

以"丹麦旺盛菠菜"为材料,通过UV-B和CaCl2复合处理,测定光合色素含量、Hill反应活力、叶绿素荧光、MDA含量和抗氧化酶活性等参数,探讨了CaCl2对UV-B辐射下菠菜叶片电子传递链和光合膜酶保护系统的影响。结果表明,UV-B处理下,光合色素含量、chl/car、类囊体膜上PSII潜在活性(Fv/Fo)、光化学淬灭系数(qP)、非光化学淬灭系数(qN)、PSII光量子产量(ΦPSⅡ)、原初光能转化效率(Fv/Fm),以及Hill反应活力等降低,chla/chlb和MDA含量升高;喷洒CaCl2可不同程度缓解UV-B的伤害。不同处理下,POD、SOD和CAT活性的变化呈现补偿效应。UV-B强度与菠菜叶片PSII功能受损程度呈正相关,CaCl2则主要通过提高chlb含量、类囊体膜上的光量子产量和POD活性,以缓解伤害。重度UV-B辐射下,CaCl2使chlb含量显著提高可能是导致PSII捕光效率提高的重要因素。     

Functioning of the photosynthetic apparatus under low and high light conditions in chlorotic spruce needles as evaluated by <Emphasis Type="Italic">in vivo</Emphasis> chlorophyll fluorescence

The photosynthetic apparatus rapidly responds to the environmental influences. In vivo chlorophyll fluorescence was applied for the evaluation of photosystem II (PSII) and electron-transport chain functioning and for determination of photochemical and nonphotochemical quenching in chlorotic spruce needles exposed to urban pollution. More injured needles had lower content of chloroplast pigments and changed chloroplast ultrastructure, in comparison with less injured needles. The maximum PSII efficiency was measured in dark-adapted samples, whereas other parameters were measured under low and high light conditions (125 and 1400 µmol photons/(m²s), respectively). The PSII efficiency and relative electron transport rate (rel. ETR) were lowered at both irradiance levels while the photochemical quenching was significantly lower only in high light. Nonphotochemical quenching coefficients (qN) values were higher at both light levels in more injured needles, however, the difference was insignificant. High nonphotochemical quenching in both needle groups probably made possible the photosynthetic apparatus to function at the high light level. Our results suggested that the lowering of the chlorophyll content could be considered as a protecting event rather than just the consequence of the stress.From Fiziologiya Rastenii, Vol. 52, No. 2, 2005, pp. 191–197.Original English Text Copyright © 2005 by Lepedu, Viljevac, Cesar, Ljubei.This article was submitted by the authors in English.This revised version was published online in April 2005 with a corrected cover date.     

The change of chlorophyll fluorescence parameters in winter barley during recovery after freezing shock and as affected by cold acclimation and irradiance

The change of chlorophyll fluorescence parameters in froze leaves of 3 leaf-age seedlings were examined using two winter barley cultivars (Chumai 1 and Mo 103) differing in cold tolerance to investigate physiological response to low temperature as affected by cold acclimation (under 3/1 degrees C, day/night for 5 days before freezing treatment) and irradiation size (high irradiance: 380+/-25 micromol m(-2)s(-1) and low irradiance: 60+/-25 micromol m(-2)s(-1)) during recovery. The results showed that non-lethal freezing shock (exposed to -8 degrees C for 18 h) did not obviously affect maximum quantum efficiency in photosystem II (PSII), but dramatically increased non-photochemical quenching and reduced effective quantum yield in PSII. Cold acclimation significantly improved stability of photosynthetic function of leaves after freezing stress through buffering excessive energy and alleviating photoinhibition during recovery, indicating it increased recovery ability of barley plants from freezing injury. High irradiance was quite harmful to the stability of PSII in barley plants during recovery from freezing injury. The electron transport rate of PSII varied with cold-acclimation, irradiance and genotype. Cold acclimation caused significant increase in electron transport rate of PSII for relatively tolerant cultivar Mo 103, but not for relatively sensitive cultivar Chumai 1. It can be concluded that some chlorophyll fluorescence parameters during recovery from freezing shock may be used as the indicators in identification and evaluation of cold tolerance in barley.     

Effect of cold acclimation on the photosynthetic performance of two ecotypes of Colobanthus quitensis (Kunth) Bartl

The effects of cold acclimation of two ecotypes (Antarctic and Andes) of Colobanthus quitensis (Kunth) Bartl. Caryophyllaceae on their photosynthetic characteristics and performance under high light (HL) were compared. Non-acclimated plants of the Antarctic ecotype exhibited a higher (34%) maximal rate of photosynthesis than the Andes ecotype. In cold-acclimated plants the light compensation point was increased. Dark respiration was significantly increased during the exposure to 4 degrees C in both ecotypes. Cold-acclimated Antarctic plants showed higher Phi(PSII) and qP compared with the Andes ecotype. In addition, the Antarctic ecotype exhibited higher heat dissipation (NPQ), especially in the cold-acclimated state, which was mainly associated with the fast relaxing component of non-photochemical quenching (NPQ(F)). By contrast, the Andes ecotype exhibited a lower NPQ(F) and a significant increase in the slowly relaxing component (NPQ(s)) at low temperature and HL, indicating higher sensitivity to low temperature-induced photoinhibition. Although the xanthophyll cycle was fully operational in both ecotypes, cold-acclimated Antarctic plants exposed to HL exhibited higher epoxidation state of the xanthophyll cycle pigments (EPS) compared with the cold-acclimated Andes ecotype. Thus, the photosynthetic apparatus of the Antarctic ecotype operates more efficiently than that of the Andes one, under a combination of low temperature and HL. The ecotype differences are discussed in relation to the different climatic conditions of the two Colobanthus.     

Photochemical response to drought acclimation in two sunflower genotypes

The effects of drought acclimation on CO₂ assimilation and light utilization were investigated in two sunflower genotypes ( Helianthus annuus L., T32 and Viki) in relation to water deficit and/or high light conditions. Drought interaction with PSII efficiency was observed in the genotype T32 with a sustained decrease in the potential photochemical efficiency of PSII, F_n/F_m. In response to drought acclimation, T32 displayed some tendency to accumulate closed PSII traps (higher value of 1-qp) without an enhancement of thermal deactivation (Stem-Volmer non-photochemical quenching, NPQ). Irrespective of the growth conditions (growth chamber or greenhouse), only Viki was responsive to drought acclimation, with (1) increased net photosynthesis in well-watered plants, (2) higher maintenance of photochemical electron transfer under water deficit and/or high light, (3) limited PSII inactivation (lower value of 1-qp) through increased non-photochemical energy dissipation (Stern-Volmer NPQ) which was readily reversible even at low leaf water potentials, and (4) higher F_v/F_m recovery after high light treatment. Additionally, drought acclimation delayed turgor loss during subsequent water stress in Viki. Thus, the response to drought acclimation, with an adjustment of water relations and of energy utilization by PSII, was observed under both growth conditions and was mainly genotype dependent.     

Low-temperature modulation of the redox properties of the acceptor side of photosystem II: photoprotection through reaction centre quenching of excess energy

Although it has been well established that acclimation to low growth temperatures is strongly correlated with an increased proportion of reduced Q_A in all photosynthetic groups, the precise mechanism controlling the redox state of Q_A and its physiological significance in developing cold tolerance in photoautotrophs has not been fully elucidated. Our recent thermoluminescence (TL) measurements of the acceptor site of PSII have revealed that short‐term exposure of the cyanobacterium Synechococcus sp. PCC 7942 to cold stress, overwintering of Scots pine (Pinus sylvestris L.), and acclimation of Arabidopsis plants to low growth temperatures, all caused a substantial shift in the characteristic T_M of S₂Q_B^– recombination to lower temperatures. These changes were accompanied by much lower overall TL emission, restricted electron transfer between Q_A and Q_B, and in Arabidopsis by a shift of the S₂Q_A^–‐related peak to higher temperatures. The shifts in recombination temperatures are indicative of a lower activation energy for the S₂Q_B^– redox pair and a higher activation energy for the S₂Q_A^– redox pair. This results in an increase in the free‐energy gap between P680⁺Q_A^– and P680⁺Pheo^– and a narrowing of the free energy gap between Q_A and Q_B electron acceptors. We propose that these effects result in an increased population of reduced Q_A (Q_A^–), facilitating non‐radiative P680⁺Q_A^– radical pair recombination within the PSII reaction centre. The proposed reaction centre quenching could be an important protective mechanism in cyanobacteria in which antenna and zeaxanthin cycle‐dependent quenching are not present. In herbaceous plants, the enhanced capacity for dissipation of excess light energy via PSII reaction centre quenching following cold acclimation may complement their capacity for increased utilization of absorbed light through CO₂ assimilation and carbon metabolism. During overwintering of evergreens, when photosynthesis is inhibited, PSII reaction centre quenching may complement non‐photochemical quenching within the light‐harvesting antenna when zeaxanthin cycle‐dependent energy quenching is thermodynamically restricted by low temperatures. We suggest that PSII reaction centre quenching is a significant mechanism enabling cold‐acclimated organisms to acquire increased resistance to high light.     

Cold-acclimation limits low temperature induced photoinhibition by promoting a higher photochemical quantum yield and a more effective PSII restoration in darkness in the Antarctic rather than the Andean ecotype of Colobanthus quitensis Kunt Bartl (Cariophyllaceae)

ABSTRACT: BACKGROUND: Ecotypes of Colobanthus qutensis Kunt Bartl (Cariophyllaceae) from Andes Mountains andMaritime Antarctic grow under contrasting photoinhibitory conditions, reaching differentialcold tolerance upon cold acclimation. Photoinhibition depends on the extent of photodamageand recovery capability. We propose that cold acclimation increases resistance to lowtemperature-induced photoinhibition, limiting photodamage and promoting recovery undercold. Therefore, the Antarctic ecotype (cold hardiest) should be less photoinhibited and havebetter recovery from low-temperature-induced photoinhibition than the Andean ecotype. Bothecotypes were exposed to cold induced photoinhibitory treatment (PhT). Photoinhibition andrecovery of photosystem II (PSII) was followed by fluorescence, CO2 exchange, andimmunoblotting analyses. RESULTS: The same reduction (25%) in maximum PSII efficiency (Fv/Fm) was observed in both coldacclimated(CA) and non-acclimated (NA) plants under PhT. A full recovery was observed inCA plants of both ecotypes under dark conditions, but CA Antarctic plants recover faster thanthe Andean ecotype.Under PhT, CA plants maintain its quantum yield of PSII, while NA plants reduced itstrongly (50% and 73% for Andean and Antarctic plants respectively). Cold acclimationinduced the maintenance of PsaA and Cyt b6/f and reduced a 41% the excitation pressure inAntarctic plants, exhibiting the lowest level under PhT. Cold acclimation decreasessignificantly NPQs in both ecotypes, and reduce chlorophylls and D1 degradation in Andeanplants under PhT.NA and CA plants were able to fully restore their normal photosynthesis, while CA Antarcticplants reached 50% higher photosynthetic rates after recovery, which was associated toelectron fluxes maintenance under photoinhibitory conditions. CONCLUSIONS: Cold acclimation has a greater importance on the recovery process than on limitingphotodamage. Cold acclimation determined the kinetic and extent of recovery process underdarkness in both C. quitensis ecotypes. The greater recovery of PSII at low temperature in theAntarctic ecotype was related with its ability to maintain PsaA, Cyt b6/f and D1 protein afterphotoinhibitory conditions. This is probably due to either a higher stability of thesepolypeptides or to the maintenance of their turnover upon cold acclimation. In both cases, itis associated to the maintenance of electron drainage from the intersystem pool, whichmaintains QA more oxidized and may allow the synthesis of ATP and NADPH necessariesfor the regeneration of ribulose 1,5-bisphosphate in the Calvin Cycle. This could be a keyfactor for C. quitensis success under the harsh conditions and the short growing period in theMaritime Antarctic.     

Differential response of photosynthesis in greenhouse- and field-ecotypes of tomato to long-term chilling under low light

Three greenhouse- and four field-ecotype varieties of domestic tomato (Lycopersicon esculentum) were compared for the sensitivity of their photosynthetic apparatus to chilling under low light intensity. After chilling at 12/7 degrees C under 100 micromolm(-2)s(-1) of light for 10 days, they were allowed to recover at 25/18 degrees C and 600 micromolm(-2)s(-1) of light for 10 days. For both pre-chilling and recovered plants, greenhouse-ecotype varieties did not necessarily show higher net CO(2) assimilation rate (A), quantum yield of electron transport at PSII (Phi(PSII)) and photochemical quenching (q(P)) than field-ecotype varieties. However for the post-chilling period, greenhouse-ecotype varieties, exhibited higher A, and Phi(PSII) values than field-ecotype varieties. The difference in Phi(PSII) was found to be largely due to q(P). The absence of ecotypic differences in pre-chilling plants indicates that the trait was not expressed constitutively, but relied mainly on adaptation/acclimation mechanisms. Greenhouse-ecotype varieties were able to adapt to low temperature and low light more quickly, and then exhibited higher A, Phi(PSII), q(P) values and greater re-growth capacity after chilling than field-ecotype varieties. Plant re-growth capacity after chilling was highly correlated with Phi(PSII) and q(P) measured in chilled plants, suggesting the usefulness of Phi(PSII) and q(P) measured at low temperature after defined chilling stresses as screening indexes for chilling tolerance in breeding programs.     

Limitations to photosynthesis inCoffea canephoraas a result of nitrogen and water availability

Plants of C. canephora grown in pots under low nitrogen (LN) or high nitrogen (HN) applications were submitted to either cyclic water stress or daily irrigation. Water deficit led to marked decreases in net carbon assimilation rate (A) and, to a lesser extent, in stomatal conductance (g_s), regardless of the N treatments. In well-watered plants, A appreciably increased in HN plants relative to LN plants without significant changes in g_s. As a whole, changes in internal CO₂ concentration predominantly reflected changes in A rather than in g_s. Under irrigated conditions, A, but not g_s, correlated with leaf N concentration in a curvilinear way. Photosynthetic nitrogen-use efficiency was considerably low, and decreased with increasing leaf N concentration. Limited N, but not water, slightly decreased the maximum photochemical efficiency of photosystem II (PSII). Under continuous irrigation, LN plants had a smaller quantum yield of electron transport (_PSII) through slight decreases of photochemical quenching (q_p) and capture efficiency of excitation energy by open PSII reaction centres, and increases in Stern-Volmer non-photochemical quenching. Under water-stressed conditions, changes in PSII photochemistry were apparent only in HN plants, with a 25 % decrease in _PSII, due mainly to variations in q_p. Biochemical constraints, rather than stomatal or photochemical limitations, provoked the decreases in A under limited supply of either N or water.     

Leaf functional and micro-morphological photoprotective attributes in two ecotypes of Colobanthus quitensis from the Andes and Maritime Antarctic

Colobanthus quitensis (Kunth) Bartl. (Cariophyllaceae) is distributed from Mexico to the Maritime Antarctic. It grows forming inconspicuous populations in humid and cold sites along high elevations in the Andes Mountains. Mediterranean Andes is characterized by a wider oscillation of diurnal and seasonal temperature, while the Maritime Antarctic is characterized by permanent low temperatures. Both places may experience high irradiance during sunny days (reaching up to 2,000 μmol photons m⁻² s⁻¹); however, the frequency of sunny days in the Maritime Antarctica is significantly lower (less than 20% of the whole growing season). We study whether acclimation to each environment relies on different photoprotective mechanisms. The Andean ecotype that has a longer growing season and a higher light integral reduces light absorption by the development of smaller chloroplasts with lower stacking granum area and down-regulation of Lhcb2. It also enhances the dissipation of the excess of absorbed energy by higher level of de-epoxidation of xanthophylls pool. On the other hand, the Antarctic ecotype which has developed under a shorter growing season, with lower total irradiance and continuous low temperatures, maximizes photochemical process even at low temperatures and it has a lower light-harvesting/core complex ratio and higher level of photoprotection supplied by an unusually high β-carotene and xanthophylls cycle pool. It resembles a well full light acclimated plant, probably due to higher excitation pressure imposed by lower temperature even at moderate irradiance. It is suggested that the biochemical plasticity of this species, highlighted by the development of these different strategies, is essential to cope successfully with these particular environments.     

Cold acclimation and photoinhibition of photosynthesis in Scots pine

Cold acclimation of Scots pine did not affect the susceptibility of photosynthesis to photoinhibition. Cold acclimation did however cause a suppression of the rate of CO₂ uptake, and at given light and temperature conditions a larger fraction of the photosystem II reaction centres were closed in cold-acclimated than in nonacclimated pine. Therefore, when assayed at the level of photosystem II reaction centres, i.e. in relation to the degree of photosystem closure, cold acclimation caused a significant increase in resistance to photoinhibition; at given levels of photosystem II closure the resistance to photoinhibition was higher after cold acclimation. This was particularly evident in measurements at 20° C. The amounts and activities of the majority of analyzed active oxygen scavengers were higher after cold acclimation. We suggest that this increase in protective enzymes and compounds, particularly Superoxide dismutase, ascorbate peroxidase, glutathione reductase and ascorbate of the chloroplasts, enables Scots pine to avoid excessive photoinhibition of photosynthesis despite partial suppression of photosynthesis upon cold acclimation. An increased capacity for light-induced de-epoxidation of violaxanthin to zeaxanthin upon cold acclimation may also be of significance.Abbreviations APX ascorbate peroxidase - DHA dehydroascorbate - DHAR dehydroascorbate reductase - Fm maximal fluorescence when all reaction centres are closed - Fv/Fm maximum photochemical yield of PSII - GR glutathione reductase - GSH reduced glutathione - Je rate of photosynthetic electron transport - MDAR monodehydroascorbate reductase - q_N nonphotochemical quenching of fluorescence - q_P photochemical quenching of fluorescence - SOD superoxide dismutase This work was supported by the Swedish Natural Science Research Council and the National Natural Science Foundation of China.