Measurements of 14C Incorporation by Illuminated Intact Leaves of Coffee Plants from Gas Mixtures Containing 14CO2

A study was made of the incorporation of ¹⁴C by intact leavesof Coffea arabica (cultivars Mundo Novo, Catuai, 1130–13,and H 6586–2) and Coffea canephora (cultivar Guarini)supplied with gas mixtures containing ¹⁴CO₂ under controlledconditions. Samples of the leaves were combusted and the ¹⁴Cin the CO₂ produced measured using a liquid scintillation counter.The results were used to estimate photosynthetic rates. Theeffects of changing the partial pressures of O₂ and CO₂ on thephotosynthetic rate were studied and estimates made of the CO₂compensation point and photorespiration. The data obtained show differences between the mean net photosyntheticrates of the C. arabica cultivars (6·14 mg CO₂ dm^–2h^–1) and the mean rate for the C. canephora cultivar (3·96mg CO₂ dm^–2 h^–1). The cultivar of the latter speciesphotorespired more rapidly than the cultivar Catuai of C. arabica.Rates of photosynthesis in coffee measured using the ¹⁴CO₂ methodwere similar to rates obtained by others using an infrared gasanalyser. The ¹⁴CO₂ method proved to be reliable for photosyntheticmeasurements and the apparatus is suitable for use in fieldconditions.     

Reactions of birch leaves to changes in light during early ontogeny: comparison between in vivo and in vitro techniques to measure carbon uptake

Trends in several photosynthetic parameters and their responseto changed growth light were followed for 15 d in leaves ofyoung birch saplings using a rapid-response gas exchange measuringequipment. These in vivo measurements were compared to biochemicalassays that were made from the same leaves after the gas exchangestudies. The measurements were made on leaves that were selectedprior to the study and were at that time of similar age. Forthe first 7 d the photosynthetic parameters were followed fromthe growth conditions of moderate light (200 µmol m^–2s^–1; referred to as controls later in the text). On day7 some of the saplings were transferred to grow either underhigh (450 µmol m^–2 s^–1; referred to as highlight plants) or low (75 µmol m^–2 s^–1; referredto as low light plants) light and the capability of the preselectedleaves for acclimation was followed for 6 d. For comparison,at the end of the experiment the measurements were made on bothcontrols and on young leaves that had developed under high andlow light. Generally the in vivo measured rate of CO₂ uptake (gross photosynthesis)both at 310 ppm CO₂ and 2000 ppm CO₂ corresponded very wellto the biochemically determined CO₂ fixation capacity in vitroafter rapid extraction (measured as the initial and total activityof Rubisco, respectively). However, if the flux of CO₂ intothe chloroplasts was limited by the closure of the stomata,as was the case of the high light plants, then the in vitromeasured Rubisco activity was greater than the in vivo measuredCO₂ uptake. V_max, calculated from the mesophyll conductanceat 1% O₂, exceeded the initial activity of Rubisco (assayedat saturating RuBP and CO₂) constantly by 60%. The catalyticactivity of Rubisco in birch leaves was overall very low, evenwhen calculated from the total activity of Rubisco (K_cat 0.63–1.18 s^–1), when compared to herbaceous C₃ species. Signs of light acclimation were not observed in most of thephotosynthetic parameters and in chloroplast structure whenmature birch leaves were subjected to changes in growth lightfor 6 d. However, the change of the growth light either to highor low light caused day-to-day fluctuations in most of the measuredphotosynthetic parameters and in the case of the high lightplants signs of photoinhibition and photodestruction were alsoobserved (decrease in the amount of chlorophyll and increasein chlorophyll a/b ratio). As a result of these fluctuationsthese plants achieved a new and lower steady-state conditionbetween the light and dark reactions, as judged from the molarratio of RuBP to Rubisco binding site. Key words: Acclimation, photosynthesis, light, Rubisco, birch     

Non-Stomatal Inhibition Associated with Inactivation of Rubisco in Dehydrated Coffee Leaves under Unshaded and Shaded Conditions

The effect of rapid dehydration due to withholding of irrigationon leaf photosynthesis in coffee (Coffea arabica L.) was studiedby comparing the CO₂-dependent rate of photosynthesis in intactleaves (A/Ci curve), the amounts of Chl, total soluble proteinand ribulose-l,5-bisphosphate carboxylase (Rubisco) in leaves,and the activities of photochemical reactions of isolated chloroplastsand of Rubisco under unshaded and shaded conditions. The CO₂-saturatedrate of photosynthesis and the carboxylation efficiency (theinitial slope of A/Ci curve) decreased with decreasing leafwater potential (     

Photoinhibition and the diurnal variation of phytoplankton photosynthesis--I. Development of a photosynthesis--irradiance model from studies of in situ responses

Diurnal series of fluorescence and photosynthesis assays wereconducted in high altitude (3803 m), tropical (16°), LakeTiticaca (Peru/Bolivia). Near-surface diurnal thermoclines formedon typical days of high photon flux density (PFD, {small tilde}2000 µE m^–2 s^–1). In the depth range of diurnalstratification profiles of in vivo fluorescence, both without(F_a and with (F_b DCMU, exhibited a mean decrease of 64% frommorning to mid-day, but little change (mean increase of 1.5%)through the afternoon. Three times during the day surface, mid-depth(3–5 m) and deep (15–20 m) phytoplankton sampleswere incubated with H¹⁴CO₃^– under short (<2 h) exposuresto a range of in situ PFDs. Comparison of phytoplankton in differentsamples (ANOVA) showed identical photosynthetic response insunrise (isothermal) samples but a significant drop in surfaceand mid-depth photosynthesis at all PFDs during times of diurnalstratification. Similarly, both low-light () and light-saturated(P² _max photosynthetic parameters were lower in mid-day surfacesamples compared to deep samples. In addition, previously photoinhibitedsamples had a higher threshold intensity for photoinhibition,I_T. These results, together with diurnal time series of fluorescencefrom in situ incubations, demonstrate that recovery from extendedepisodes of photoinhibition during diurnal stratification isslower than suggested by previous observations in vitro. Photosynthesisby near-surface phytoplankton is different in light increasingup to I_T than light decreasing from I_T. This effect can be modeledby reducing and P_max as a function of the maximum photoinhibitingPFD in the diurnal light history. ¹Present address: Division of Molecular Plant Biology, Universityof California, Berkeley, Berkeley, CA 94720, USA     

Nitrogen supply as a factor influencing photoinhibition and photosynthetic acclimation after transfer of shade-grown Solanum dulcamara to bright light

We have compared the ability of shadegrown clones of Solamum dulcamara L. from shade and sun habitats to acclimate to bright light, as a function of nitrogen nutrition before and after transfer to bright light. Leaves of S. dulcamara grown in the shade with 0.6 mM NO ₃ ^- have similar photosynthetic properties as leaves of plants grown with 12.0 mM NO ₃ ^- . When transferred to bright light for 1–2 d the leaves of these plants show substantial photoinhibition which is characterized by about 50% decrease in apparent quantum yield and a reduction in the rate of photosynthesis in air at light saturation. Photoinhibition of leaf photosynthesis is associated with reduction in the variable component of low-temperature fluorescence emission, and with loss of in-vitro electron transport, especially of photosystem II-dependent processes.We find no evidence for ecotypic differentiation in the potential for photosynthetic acclimation among shade and sun clones of S. dulcamara, or of differentiation with respect to nitrogen requirements for acclimation. Recovery from photoinhibition and subsequent acclimation of photosynthesis to bright light only occurs in leaves of plants provided with 12.0 mM NO ₃ ^- . In these, apparent quantum yield is fully restored after 14 d, and photosynthetic acclimation is shown by an increase in light-saturated photosynthesis in air, of light-and CO₂-saturated photosynthesis, and of the initial slope of the CO₂-response curve. The latter changes are highly correlated with changes in ribulose-bisphosphate-carboxylase activity in vitro. Plants supplied with 0.6 mM NO ₃ ^- show incomplete recovery of apparent quantum yield after 14 d, but CO₂-dependent leaf photosynthetic parameters return to control levels.Symbols and abbreviations F_o initial level of fluorescence at 77 K - F_m maximum level of fluorescence at 77 K - F_v variable components of fluorescence at 77 K (F_v=F_m-F_o) - PSI, PSII photosystem I and II, respectively - RuBP ribulose-1,5-bisphosphate - RuBPCase ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39)     

Effects of Nitrogen Nutrition on Photosynthesis in Cd-treated Sunflower Plants

Increased nitrogen supply stimulates plant growth and photosynthesis.Since it was shown that heavy metals may cause deficienciesof essential nutrients in plants the potential reversal of cadmiumtoxicity by increased N nutrition was investigated. The effectson photosynthesis of low Cd (0, 0.5, 2 or 5 mmol m^-3) combinedwith three N treatments (2, 7.5 or 10 mol m^-3) were examinedin young sunflower plants. Chlorophyll fluorescence quenchingparameters were determined at ambient CO₂and at 100 or 800 µmolquanta m^-2 s^-1. The vitality index (R_fd) decreased approx. three-timesin response to 5 mmol m^-3Cd, at 2 and 10 mol m^-3N. The maximumphotochemical efficiency of PSII reaction centres (F_v/ F_m) wasnot influenced by Cd or N treatment. The highest Cd concentrationdecreased quantum efficiency of PSII electron transport (_II)by 30%, at 2 and 10 mol m^-3N, mostly due to increased closureof PSII reaction centres (q_P). Photosynthetic oxygen evolutionrates at saturating CO₂were decreased in plants treated with5 mmol m^-3Cd, at all N concentrations. The results indicatethat Cd treatment affected the ribulose-1,5-bisphosphate (RuBP)regeneration capacity of the Calvin cycle more than other processes.At the same time, the amounts of soluble and ribulose-1,5-bisphosphatecarboxylase/oxygenase (Rubisco) protein increased with Cd treatment.Decreased photosynthesis, but substantially increased Rubiscocontent, in sunflower leaves under Cd stress indicate that asignificant amount of Rubisco protein is not active in photosynthesisand could have another function. It is shown that optimal nitrogennutrition decreases the inhibitory effects of Cd in young sunflowerplants. Copyright 2000 Annals of Botany Company Helianthus annuus L., cadmium, nitrogen, photosynthesis, Rubisco, sunflower     

Cold-Tolerant Crop Species Have Greater Temperature Homeostasis of Leaf Respiration and Photosynthesis Than Cold-Sensitive Species

Some plant species show constant rates of respiration and photosynthesismeasured at their respective growth temperatures (temperaturehomeostasis), whereas others do not. However, it is unclearwhat species show such temperature homeostasis and what factorsaffect the temperature homeostasis. To analyze the inherentability of plants to acclimate respiration and photosynthesisto different growth temperatures, we examined 11 herbace-ouscrops with different cold tolerance. Leaf respiration (R_area)and photosynthetic rate (P_area) under high light at 360 µll^–1 CO₂ concentrations were measured in plants grown at15 and 30°C. Cold-tolerant species showed a greater extentof temperature homeostasis of both R_area and P_area than cold-sensitivespecies. The underlying mechanisms which caused differencesin the extent of temperature homeostasis were examined. Theextent of temperature homeostasis of P_area was not determinedby differences in leaf mass and nitrogen content per leaf area,but by differences in photosynthetic nitrogen use efficiency(PNUE). Moreover, differences in PNUE were due to differencesin the maximum catalytic rate of Rubisco, Rubisco contents andamounts of nitrogen invested in Rubisco. These findings indicatedthat the temperature homeostasis of photosynthesis was regulatedby various parameters. On the other hand, the extent of temperaturehomeostasis of R_area was unrelated to the maximum activity ofthe respiratory enzyme (NAD-malic enzyme). The R_area/P_area ratiowas maintained irrespective of the growth temperatures in allthe species, suggesting that the extent of temperature homeostasisof R_area interacted with the photosynthetic rate and/or thehomeostasis of photosynthesis.     

光强在低温弱光胁迫后番茄叶片光合作用恢复中的作用

为了研究光强在低温弱光胁迫后番茄叶片光合作用恢复中的作用,以番茄品种浙粉202为材料,研究了低温弱光后恢复期全光照与遮荫对光合作用和叶绿素荧光参数的影响。结果表明：低温弱光（8℃/12℃,PFD 80 μmol·m^-2·s^-1）导致番茄叶片P_n、Φ_PSⅡ、qP和F_v′/F_m′的下降,但诱导了NPQ的上升,未引起F_v/F_m的变化;全光照（100%光照）下恢复1 使得植株叶片P_n、F_v/F_m、Φ_PSⅡ、qP、NPQ和F_v′/F_m′均大幅下降,随后光合和荧光参数可缓慢恢复至对照水平;遮荫（40%光照）恢复植株F_v/F_m、Φ_PSⅡ和F_v′/F_m′仅在第一天稍有下降,而P_n和qP还略有上升,NPQ虽有所降低但仍显著高于对照水平,随后光合和荧光参数均可迅速恢复到对照水平。说明低温弱光虽抑制了光合作用的进行,但并未引起光抑制的发生;全光照恢复加剧了叶片光抑制的发生,而遮荫恢复可通过叶片PSⅡ光化学活性的快速恢复和天线色素热耗散能力的增强以保护光合机构免受伤害,有利于光合作用的迅速恢复。     

The Relation Between the Nitrogen Concentration and Photosynthetic Capacity of Potato (Solanum tuberosum L.) Leaves

Measurements of the rate of light-saturated photosynthesis (P_max)were made on terminal leaflets of potato plants growing in cropssupplied with 0, 3, 6, 12, 24 and 36 g N m^–2. Measurementswere made between 100 and 154 d after planting. Two types ofleaf were selected—the fourth leaf on the second-levelbranch (L₄, _B1) and the youngest terminal leaflet that was measurable(L_YM). Later, the total nitrogen concentration of each leaflet(N_L) was measured. A linear regression between P_max and N_L,common to both leaf positions, explained 68.5% of the totalvariation. With L₄, _B1 leaves there was a significant improvementin the proportion of variation explained when regressions withseparate intercepts and a common slope were fitted to individualfertilizer treatments. These results suggest that an increasingproportion of leaf nitrogen was not associated with the performanceof the photosynthetic system with increasing nitrogen supply.This separation between nitrogen treatments was not as clearfor L_YM leaves. Stomatal conductance to transfer of water vapourwas neither influenced by leaf position nor directly by nitrogensupply. Rather conductance declined in parallel with the declinein photosynthetic capacity. Solanum tuberosum, potato, nitrogen, photosynthesis, stomatal conductance, leaf     

An Analysis of the Photosynthesis and Productivity of Vegetative Crops in the United Kingdom

The rate of total dry matter production of a vegetative crop,under optimal water and nutrient regimens is related to someleaf and canopy photosynthetic characteristics. Three leaf photosyntheticcharacteristics are examined in detail: the light utilizationefficiency at normal ambient CO₂ and O₂ concentrations, a, therate of light saturated photosynthesis per unit leaf area, F_max,and the ratio of the rates of photorespira tion and gross photosynthesis.The genetic variability in each of these characteristics issought from published data on a wide range of C₃ and C₄ planttypes. Within C₃ and C₄ plant types there are significant genetic differencesonly in F_max,, although differences exist between C₃ and C₄plants in the other two characteristics. The effects of thesedifferences on the rate total dry matter production are estimated,and it is concluded that there is no compelling evidence toindicate that improvements in total dry matter production rates,in the U.K., are likely to result from genetic manipulationof these characteristics in the existing range of plant material.     

Phytotoxicity of Phthalate Plasticisers: 2. SITE AND MODE OF ACTION

The effects of phthalate esters on chlorophyll a₂ fluorescencein radish plants (Raphanus sativus L. cv. Cherry Belle) wereexamined Fluorescence yield was increased in those plants exposedto an aerial concentration of 120 ng dm^–3 dibutyl phthaiatc(DBP) at a rate of 3.0 dm³ min^–1 for 13 d. Comparisonof fluorescence enhancement ratios and F_red/F_ox, suggests thatDBP inhibits photosynthesis in radish plants at a site afterQ_A. Both DBP and diisobutyl phthalate (DIBP) strongly inhibiteduncoupled (PS2+PS1) electron transport rates in thylakoids isolatedfrom spinach. At a chlorophyll concentration of 10 µgcm^–3 the concentrations of DBP and DIBP exhibiting 50%inhibition were 44 mmol m^–3 and 42 mmol m^–3 respectively.Basal electron transport rates were also inhibited, with 87mmol m^–3 of DBP or DIBP producing 50% inhibition. Measurementof photosystcm 1 activity suggested that the main site of actionof these phthalates was localized at a site near the reducingside of photosystem 2. Key words: Phthalate, plasticiser, chlorophyll, fluorescence, photosynthesis, inhibition     

Influence of Drought on Mitochondrial Activity, Photosynthesis, Nocturnal Acid Accumulation and Water Relations in the CAM Plants Prenia sladeniana(ME-type) and Crassula lycopodioides(PEPCK-type)

The activity of photosynthesis and mitochondrial respiration,nocturnal organic acid accumulation and water relations wereinvestigated in Prenia sladeniana L. Bol. [malic enzyme (ME)-type]andCrassula lycopodioides Lam. [phosphoenolpyruvate carboxykinase(PEPCK)-type] to compare the physiological responses to waterdeficit in crassulacean acid metabolism (CAM) plants differingin their decarboxylating enzyme systems. Withholding water inhibiteddaytime gas exchange within 2 d while night time CO₂gain andmalic acid accumulation remained relatively unchanged in bothspecies. In P. sladeniana, maximum photochemical efficiency(F_v/F_m) and photosynthetic electron transport declined to nearlythe same degree as CO₂supply was restricted during drought.Despite limited CO₂availability, photosynthetic activity waslargely unaffected in C. lycopodioides, as were mitochondrialproperties. There is no indication of a drought-induced increasein the capability to totally oxidize malate, yielding 4 CO₂, in either species. Nevertheless, the enhanced ratio of malateto glycine oxidation may have increased the in vivo capabilityfor malate oxidation in P. sladeniana. Although pressure potentialwas maintained throughout the experiment in both species, activeosmotic adaptation occurred only inP. sladeniana. The observeddecrease in photosynthetic and mitochondrial activity may haveresulted from the large increase in osmotic concentration inthis species. Copyright 2000 Annals of Botany Company Chlorophyll fluorescence analysis, Crassula lycopodioides Lam., crassulacean acid metabolism, citric acid, gas exchange, malic acid, mitochondria, photosynthetic electron transport, Prenia sladeniana L. Bol., water relations     

Growth at elevated CO2 leads to down-regulation of photosynthesis and altered response to high temperature in Quercus suber L. seedlings

The effects of growth at elevated CO₂ on the response to hightemperatures in terms of carbon assimilation (net photosynthesis,stomatal conductance, amount and activity of Rubisco, and concentrationsof total soluble sugars and starch) and of photochemistry (forexample, the efficiency of excitation energy captured by openphotosystem II reaction centres) were studied in cork oak (Quercussuber L.). Plants grown in elevated CO₂ (700 ppm) showed a down-regulationof photosynthesis and had lower amounts and activity of Rubiscothan plants grown at ambient CO₂ (350 ppm), after 14 monthsin the greenhouse. At that time plants were subjected to a heat-shocktreatment (4 h at 45C in a chamber with 80% relative humidityand 800–1000 mol m^–2 s^–1 photon flux density).Growth in a CO₂-enriched atmosphere seems to protect cork oakleaves from the short-term effects of high temperature. ElevatedCO₂ plants had positive net carbon uptake rates during the heatshock treatment whereas plants grown at ambient CO₂ showed negativerates. Moreover, recovery was faster in high CO₂-grown plantswhich, after 30 min at 25C, exhibited higher net carbon uptakerates and lower decreases in photosynthetic capacity (A_max aswell as in the efficiency of excitation energy captured by openphotosystem II reaction centres (F_vJF_m than plants grown atambient CO₂. The stomata of elevated CO₂ plants were also lessresponsive when exposed to high temperature. Key words: Elevated CO₂, temperature, acclimation, photosynthesis, Quercus suber L.     

Effects of Different Nitrogen Sources and Concentrations on CAM Photosynthesis in Kalanchoe blossfeldiana

When Kalanchoë blossfeldiana Poelln. cv. Hikan plants werecultured in solutions containing 0.2, 1.0, 5.0 or 10 mM of nitrateor ammonium under a long-day photoperiod, some criteria of CAM(Crassulacean acid metabolism) photosynthesis (diurnal changesof CO₂ uptake, titratable acidity and malate content in leaves)were examined. The plants absorbed 90 to 100% of CO₂ duringthe light phase regardless of the supplied nitrogen. Nitrate-grownplants absorbed about 10% of CO₂ during the dark phase regardlessof the supplied concentration, whereas in ammonium-grown plantsthe nocturnal CO₂ uptake occurred at 0.2 mM, at which the plantsdepleted nitrogen and no uptake was observed at the higher concentrations.Changes of nocturnal increase in titratable acidity and malatecontent almost corresponded with the changes in the amount ofnocturnal CO₂ uptake. Also K. daigremontiana plants suppliedwith 10 mM of ammonium had a less CAM-like pattern of diurnalCO₂ uptake than the plants supplied with 10 mM of nitrate. Theseresults suggest that a sufficient supply of ammonium depressesCAM photosynthesis.     

Influence of nitrogen supply and growth irradiance on photoinhibition and recovery in Heuchera americana (Saxifragaceae)

Prior work demonstrated that Heuchera americana, an evergreen herb inhabiting the deciduous forest understory in the southeastern United States, has a 3-4-fold greater photosynthetic capacity under the low-temperature, strong-light, open canopies of winter compared to the high-temperature, weak-light, closed canopies of summer. Moreover, despite the reductions in soil nitrogen, the chilling temperatures, and the increased quantum flux associated with winter, chronic photoinhibition was not observed in this species at this time of the year. We were interested in the photosynthetic acclimation and photoinhibition characteristics of this species when grown under contrasting light and nitrogen regimes. Newly expanded shade-acclimated leaves of forest-grown plants exposed to strong light varying in intensity and duration at 25°C showed a reduction in F_v/F_m (the ratio of variable to maximum room temperature chlorophyll fluorescence measured after dark adaptation), which was correlated with a decline in ø_a (the intrinsic quantum yield of CO₂-saturated O₂ evolution on an absorbed light basis). Plants grown in the glasshouse under contrasting light (high and low light; HL and LL, respectively) and nitrogen supply (high and low nitrogen; HN and LN, respectively) regimes showed that photosynthetic acclimation to HL was impaired in LN regimes. The HL-LN plants also had the lowest values of F_v/F_m and of ø on both incident and absorbed light bases and had 50% less chlorophyll (per unit area) compared to plants from other growth regimes. Controlled exposure to bright light at low temperatures (2-3°C) for 3 h resulted in a sharp decrease in F_v/F_m (and rise in F_o, the minimum fluorescence yield) in all plants. Shade-grown plants from both N regimes were highly susceptible to chronic photoinhibition, as indicated by a greater reduction in F_v/F_m and incomplete recovery after 18 h in weak light at 25°C. The HL-HN plants were the least susceptible to chronic photoinhibition, having the smallest decrease in F_v/F_m with near full recovery within 6 h. The decline in Fv/Fm in HL-LN plants was comparable to that of shade-acclimated plants, but recovered fully within 6 h. Low-N plants from both light regimes displayed greater increases in F_o which did not return to pretreatment levels after 18 h of recovery. These studies indicate that HL-LN plants were sensitive to chronic photoinhibition and, at the same time, had a high capacity for dynamic photoinhibition. Experimental garden studies showed that H. americana grown in an open field in summer were photoinhibited and did not fully recover overnight or during extended periods of weak light. These results are discussed in relation to the photosynthetic acclimation of H. americana under natural conditions.     

Estimation of the sensitivity to photoinhibition in Striga hermonthica-infected sorghum

Sensitivity to photoinhibition was assessed in sorghum infectedwith the angiosperm root parasite Striga her-monthica and inuninfected sorghum plants, at four times during the developmentof the host-parasite association. Photoinhibition was inducedby exposing either leaf discs or intact leaves to a photosyntheticphoton flux density of 2000 µmol m^–2 s^–1 for4 h. The inhibition of apparent quantum yield (_a) and photosynthesisin high light (A₁₅₀₀) were assessed in leaf discs using an oxygenelectrode and the recovery of these from photoinhibition wasfollowed in intact leaves using an infra-red gas analyser. Fromsoon after attachment of the parasite, infected sorghum plantshad a lower A₁₅₀₀. During the period when Striga induced a loweringof A₁₅₀₀, _a was more sensitive to photoinhibition in Striga-infectedplants. However, at the same time, the high-light-induced inhibitionof A₁₅₀₀ was similar in Striga-infected and uninfected plants.Recovery of both _a and A₁₅₀₀ was incomplete after 6 h and thetime-course of recovery was similar in Striga-infected and uninfectedplants. The results indicate that Striga-infected plants weremore sensitive to photoinhibition and that photoinhibition wasprimarily due to damage to electron transport/photo-phosphorylationand not disablement of the recovery processes. Key words: Photoinhibition, quantum yield, recovery from photoinhibition, parasitic plants     

Chlorophyll fluorescence and photosynthetic response to light in 1-year-old needles during spring and early summer in <Emphasis Type="Italic">Pinus leucodermis</Emphasis>

Ten-year-old trees from four Italian populations of Pinus leucodermis (populations A, B, C and D), which were collected from different sites at different altitudes, were grown near Florence, Italy. Needle CO₂ gas exchange and chlorophyll fluorescence response to increasing light intensities were evaluated; gas exchange and chlorophyll fluorescence variation between April and July were also monitored. Populations A, B and C showed a similar photosynthetic response to increasing photosynthetic photon flux density (PPFD) intensities, while at various light intensities population D, which originated from the highest altitude, showed the highest photosynthetic rates. In this population photosynthesis was saturated at PPFDs higher than 900 µmol m^-2s^-1 and a slow decrease of effective photosystem II quantum yield and F'_V/F'_M in response to increasing PPFDs were found. The same trees also showed a faster recovery in photosynthesis from limitations induced by winter temperatures than the other three populations. This work showed that photosynthetic response to light in population D was different from the other populations; trees from this population were probably naturally selected to prevent photoinhibition due to excess light.     

Responses of N2 Fixation-linked Respiration to Host-plant Energy Status in White Clover Acclimated to a Controlled Environment

Ryle, G. J. A., Powell, C. E. and Gordon, A. J. 1988. Responsesof N₂ fixation-linked respiration to host-plant energy statusin white clover acclimated to a controlled environment.—J.exp. Bot. 39: 879–887. Single plants of white clover, acclimated to a controlled environmentand dependent for nitrogen on N₂ fixation in their root nodules,were darkened, defoliated or exposed to enhanced CO₂ levelsto establish the quantitative relationships between the photosynthesisof the host plant and the N₂ fixation metabolism of root nodules. The nodule respiration associated with N₂ fixation (FLR) declinedrapidly to 10–15% of its normal rate following plant darkeningearly in the photoperiod. Darkening at progressively later intervalsduring the photoperiod demonstrated a positive, apparently linearrelationship between duration of illumination and total FLRduring the photoperiod and the following night period. Completeor partial defoliation reduced FLR according to the leaf arearemoved: again, there was a strong positive correlation betweencurrent rate of photosynthesis, whether of defoliated or undefoliatedplants and the FLR of root nodules. Doubling the current rateof photosynthesis, by enhancing CO₂ levels around the shoots,promoted FLR within 1–2 h when plants were stressed bylack of light. However, enhanced CO₂ levels increased FLR onlyslowly over a period of several hours in plants entrained tothe normal growing conditions. It is concluded that, in these plants acclimated to a uniformand favourable controlled environment, the supply and utilizationof photosynthetic assimilate in N₂ fixation was finely balancedand quantitatively linked during a single diurnal period andthat nodule functioning was not depressed by lack of energysubstrate. Key words: White clover, N₂ fixation, photosynthesis.     

Photosynthetic responses of Coffea arabica leaves to a short-term high light exposure in relation to N availability

It is known that the coffee (Coffea arabica L.) plant which is originally from shade habitats would have a limited ability to grow under full sun. Previous work has shown that nitrogen fertilisation can reduce the leaf damage when the plants are exposed to high light intensities during several days. In the present work we aimed to study the effects of the high irradiance during the first hours and evaluate the positive contribution of nitrogen fertilisation in the case of short-term exposure to strong light. Young plants (1.5–2 years old) grown in 1.5 kg of a mixed soil were supplemented with a nutrient solution containing 15 mM nitrogen in the form of NH₄NO₃, every 7 days (2N treatment), 15 days (1N treatment) and 45 days (0N treatment). Top mature leaves were exposed to a photosynthetic photon flux density of 1 500 μmol m^?2 s^?1 for a maximal period of 8 h, and changes in photosynthesis and pigment composition were monitored along the period of high light exposure. Photosynthetic capacity, leaf conductance to water vapour, electron transport capacity and maximum carboxylation activity, as well as some leaf fluorescence parameters (minimal fluorescence, photochemical efficiency of PSII and quantum yield of photosynthetic electron transport) were reduced by the stress, with a generally stronger impact observed in the 0N plants. The photochemical quenching was affected only in the 0N plants, while the non-photochemical quenching increased in 2N plants but decreased in the 0N ones. The results showed that 2N plants presented a better initial status of the photosynthetic parameters and of the content of photoprotective pigments. Those plants showed ability to trigger some protective mechanisms, as observed by the tendency to increase the xanthophyll pool content, specially in zeaxanthin and in non-photochemical quenching. Also, protein content presented a tendency to increase after 1.5 h, which was maintained until the end of the high light period. We conclude that nitrogen availability is a key factor in the acclimation process to high light.     

Effects of P deficiency on the uptake, flows and utilization of C, N and H2O within intact plants of Ricinus communis L.

The influence of P deficiency on the uptake, flow and utilizationof C, N and H₂0 by intact NO₃-fed castor bean plants {Ricinuscommunis L.) was studied over a 9 d period in the middle oftheir vegetative growth. The modelling techniques incorporateddata on net increments or losses of C, N and H₂O in plant parts,photosynthetic gains in and respiratory losses of C, molar C:Nratios of solutes in phloem and xylem sap and transpirationallosses of H₂0. Plant growth was inhibited within 3 d of withholdingP supply and dry matter production was less than one-third ofthe controls. Leaf growth was particularly depressed, whileroot growth was much less affected than that of the shoot. Shoot:rootratio of low-P plants was 1.5 compared with 2.6 under P supply.Over the 9 d study period total plant C and N increased by 560and 47 mmol, respectively, in the controls, but by only 113and 6.9 mmol in the low-P treatment. The particularly low incrementof N in P-deficient plants was due principally to decreasedN0₃^- uptake. Flows of C and N during the study period were markedlydifferent between control and P-deficient plants. The partitioningprofile for C in P-deficient plants showed a dramatic inhibitionof net photosynthesis and attendant photoassimilate flow. Proportionaldownward to upward allocation of carbon increased with increasein sink size of the root relative to shoot. This was reflectedin greater relative allocation of C to root dry matter and rootrespiration than in P-sufficient plants, and suppressed cyclingof C from root to shoot via xylem. Nitrogen intake and xylemtransport to the shoot of P-deficient plants were only 15% ofthe control and, as in the case of C, downward allocation ofN predominated over upward phloem translocation. Apart fromthese severe changes, however, the basic patterns of N flowsincluding xylem-to-phloem and xylem-to-xylem transfer of N werenot changed, a feature highlighting the vital nature of thesetransfer processes even under deficiency conditions. The alterationsin flows and partitioning of C, N and H₂O in response to low-Pconditions are discussed in relation to the corresponding effectsof moderate salt stress in Ricinus and the conclusion is reachedthat changes in nutrient flows under P deficiency were morehighly co-ordinated than when plants experience salt stress.Flow profiles under P deficiency which favour root growth andactivity are viewed as a means for increasing the potentialcapability of the plant to acquire P from the nutrient medium. Key words: Ricinus communis L., P deficiency, carbon, nitrogen, water, partitioning, xylem transport, phloem transport