Effects of phosphorus and chilling under low irradiance on photosynthesis and growth of tomato plants

To determine the effects of phosphorus nutrition on chilling tolerance of photosynthetic apparatus, tomato (Lycopersicon esculentum Mill. cv. Kenfengxin 2002) plants were raised under different P contents and subjected to 7 d of chilling at 9/7 °C. After chilling (2 h or 7 d) plant growth, P content in tissue, gas exchange and chlorophyll fluorescence were measured. Decreasing P concentration [P] in the nutrient solution markedly reduced plant growth and the chilled plants exhibiting higher optimum [P] than the unchilled plants. Decreasing [P] significantly decreased light saturated net photosynthetic rate (P_Nsat), maximum carboxylation velocity of Rubisco (V_cmax), maximum potential rate of electron transport contributed to Rubisco regeneration (J_max), quantum efficiency of photosystem (PS) 2 (ΠPS2) and O₂ sensitivity of P_Nsat (PS_O2) and this trend was especially apparent in chilled plants.     

Rice can acclimate to lethal level of salinity by pretreatment with sublethal level of salinity through osmotic adjustment

The physiological ability to adapt for various environmental changes is known as acclimation. When exposed to sublethal level of stress, plants develop the ability to withstand severe stress, as acquired tolerance. The present study was conducted to explicate the physiological basis of acquired tolerance in rice. Rice seedlings (variety IR 20) were grown in half strength Hoagland solution, and after 22nd day, they were kept in half strength Hoagland solution containing 50 mM NaCl (sublethal dose) for 7 days followed by half strength Hoagland solution containing 100 mM NaCl (lethal dose) for another 7 days. The non-pretreated 29 days old rice seedlings maintained in half strength Hoagland solution were directly transferred to half strength Hoagland solution containing 100 mM NaCl (lethal dose) solution for 7 days. The control plants were maintained in half strength Hoagland solution without NaCl. Various morphological and physiological parameters were recorded on 29th and 36th days old seedlings from control, pretreated and non-pretreated plants. The results revealed significant reduction in growth parameters (shoot length, root length, leaf area and total dry matter production) of non-pretreated plants below that of pretreated plants. The pretreated plants showed increased values to the extreme of 19.8 per cent in leaf water potential (ψ_w), 9 per cent in relative water content (RWC), 26 per cent in photosynthetic rate (P _N), 28 per cent in leaf stomatal conductance, and 47 per cent in chlorophyll a over non-pretreated plants. The same trend was also observed in chlorophyll a/b ratio (6.6%) and F _v/F _m ratio (19.3%). However, a reverse trend was seen in F _o value. The pretreated plants showed improved ionic regulation as evident from low Na⁺, Cl⁻ and high K⁺ contents, which is attributed to enhanced plant water status and photosynthesis. Both pretreated and non-pretreated plants had higher contents of osmolytes viz., sucrose, leaf soluble sugars and proline contents than control plants. However, starch content revealed an inverse trend. Therefore, the present study reveals that rice can acclimate to lethal dose of salinity stress by pretreatment with sublethal dose of NaCl. Section Editor: J. M. Cheeseman     

Effect of Xanthobacter,isolated and characterized from rice roots,on growth of wetland rice

With an autotrophic, N-free medium, Xanthobacter populations were isolated from the roots of wetland rice grown under field conditions. Xanthobacter populations ranged from 3.2×10⁴ to 5.1×10⁵ colony-forming units (cfu) g^-1 of root and averaged 47-fold higher on the root or rhizoplane than in the neighbouring nonrhizosphere. Characterization studies indicated dissimilarities in carbon utilization and motility among the isolated Xanthobacter strains and other recognized Xanthobacter species. Under gnotobiotic conditions, the population of one isolate, Xanthobacter sp. JW-KR1, increased from 10⁵ to 10⁷ cfu plant^-1 1 d after inoculation when a rice plant was present, but declined to numbers below the limit of detection (<10⁴ cfu assembly^-1) after 3 d in the absence of a plant. Scanning electron microscopy revealed Xanthobacter as pleomorphic forms on the rhizoplane. To assess the effect of Xanthobacter on plant growth, rice plants were grown under greenhouse conditions in plant assemblies containing sand and half-strength Hoagland's nutrient solution with and without nitrogen. Plants were either inoculated with 10⁵ cfu Xanthobacter g^-1 of sand or left uninoculated. After 40 d, plants without nitrogen showed no significant differences in top or root dry weight, plant height, root length, or number of tillers or leaves, whether the plants were inoculated or uninoculated. However, when nitrogen was added, inoculated plants had a significantly larger top dry weight (15%) and number of leaves (19%) than uninoculated plants. Under conditions of added and no added nitrogen, acetylene reduction assays showed Xanthobacter sp. JW-KR1 produced <0.1 (below detection limit) and 7 nmol C₂H₄ plant^-1 h^-1, respectively. Under the conditions studied, the results suggest that both Xanthobacter and wetland rice derive some benefits from their association.     

Alternative Path Mediated ATP Synthesis in Roots of Pisum sativum upon Nitrogen Supply

Changes in the efficiency of root respiration were examined on intact plants of Pisum sativum L. cv Rondo after addition of nitrate or ammonium to the culture solutions. Nitrate was absorbed immediately after addition and elicited a respiratory rise (O₂-uptake as well as CO₂-production) to 160% at most. This occurred both in roots of plants fixing N₂ and in those of non-nodulated plants pregrown for 1 or 2 weeks on a nitrogen-free culture solution. In older plants, used after 2 weeks of N-free growth, the full capacity of the cytochrome path was engaged in root respiration. This was demonstrated by the absence of an effect of the uncoupler carbonylcyanide m-chlorophenylhydrazone in the presence of 25 millimolar salicylhydroxamate, an inhibitor of the alternative path. In these plants more than 90% of the nitrate-induced stimulation of root respiration was salicylhydroxamate-sensitive. In young plants, used after 1 week of N-free growth, the cytochrome path was not saturated. Its activity increased instantaneously at the expense of alternative path activity, which initially dropped to zero and subsequently increased to 160% of the control 7 hours after nitrate supply. The rate of photosynthesis rose to 120% of the control, but not before 1 hour after nitrate supply, suggesting that the stimulation of root respiration was not due to a higher rate of photosynthesis. Experiments with plants grown with a split-root system showed that respiration rate and alternative path activity only increased in the root halves exposed to nitrogen. Ammonium was equally effective as nitrate in stimulating root respiration. These results lead to the conclusion that alternative-path mediated root respiration contributes to synthesis of ATP during at least the first 24 hours following nitrogen supply.     

Differential Effects of Ammonium and Nitrate on Growth Performance of <i>Glechoma longituba</i> under Heterogeneous Cd Stress

Water, minerals, nutrients, etc., can be shared by physiological integration among inter-connected ramets of clonal plants. Nitrogen plays an important role in alleviating cadmium (Cd) stress for clonal plants. But how different forms of nitrogen affect growth performance of clonal plants subjected to heterogeneous Cd stress still remains poorly understood. A pot experiment was conducted to investigate the differential effects of ammonium and nitrate on growth performance of Glechoma longituba under heterogeneous Cd stress. In the experiment, parent ramets of Glechoma longituba clonal fragments were respectively supplied with modified Hoagland solution containing 7.5 mM ammonium, 7.5 mM nitrate or the same volume of nutrient solution without nitrogen. Cd solution with different concentrations (0, 0.1 or 2.0 mM) was applied to offspring ramets of the clonal fragments. Compared with control (N-free), nitrogen addition to parent ramets, especially ammonium, significantly improved antioxidant capacity [glutathione (GSH), proline (Pro), peroxidase (POD,) superoxide dismutase (SOD) and catalase (CAT)], PSII activity [maximum quantum yield of PSII (F_v/F_m) and effective quantum yield of PSII (ΦPSII)], chlorophyll content and biomass accumulation of the offspring ramets suffering from Cd stress. In addition, negative effects of nitrate on growth performance of whole clonal fragments were observed under Cd stress with high concentration (2.0 mM). Transportation or sharing of nitrogen, especially ammonium, can improve growth performance of clonal plants under heterogeneous Cd stress. The experiment provides insight into transmission mechanism of nitrogen among ramets of clonal plants suffering from heterogeneous nutrient supply. Physiological integration might be an important ecological strategy for clonal plants adapting to heterogeneous environment stress conditions.     

Response to Chilling of Zea mays, Tripsacum dactyloides and their Hybrid

Maize (Zea mays ssp. mays) and eastern gamagrass (Tripsacum dactyloides) are known for their susceptibility to chilling injuries. Their hybrid (Z. mays × T. dactyloides) showed higher tolerance to low temperatures (–2 °C) in the field than its parents. Exposure to 5 °C for 2 or 3 d reduced the variable to maximal chlorophyll fluorescence ratio (F_V/F_M), an indicator of the maximum photochemical efficiency of the photosystem 2, and the variable to minimal fluorescence ratio (F_V/F₀) more in maize and eastern gamagrass than in hybrid plants. Chlorophyll contents for rewarming plants (25 °C for 3 d) were lower than before chilling in both parents while values for hybrid plants were similar. Electrolyte leakage was higher in chilled than control plants but it did not show significant differences among genotypes. Our data suggest that hybrid plants have higher capacity to recover from chilling injury in controlled conditions than their parents.     

Physiological and ultrastructural effects of lead on tobacco

The effects of lead toxicity on leaf gas exchange, chlorophyll content, chlorophyll fluorescence, chloroplast ultrastructure, and opening of stomata were examined in tobacco (Nicotiana tabacum L.) plants. Plants were grown in a growth chamber for 7 d in Hoagland nutrient solution supplemented with 0.0 (control), 5, 10, 25, 50, 100, 300 and 500 μM Pb(NO₃)₂. Plants treated with 5, 10, and 25 μM Pb(NO₃)₂ exhibited no abnormalities. Root and shoot growth, net photosynthetic rate and stomatal conductance were significantly reduced in plants treated with 100, 300 and 500 μM Pb(NO₃)₂. In plants treated with 500 μM Pb(NO₃)₂, the majority of stomata were closed. The effect of Pb(NO₃)₂ on chlorophyll content and chlorophyll fluorescence parameters was negligible. However, in plants exposed to 100, 300, and 500 μM Pb(NO₃)₂, the mesophyll cells showed altered chloroplasts with disrupted thylakoid membranes.     

Hg and Cd Induced Changes in Proline Content and Activities of Proline Biosynthesizing Enzymes in Phaseolus Aureus and Triticum Aestivum

The effect of mercury and cadmium, in the form of HgCl₂ and CdCl₂ respectively, on proline accumulation and two key proline biosynthesizing enzymes, ¹-pyrroline-5-carboxylate synthetase (P5CS) and ¹-pyrroline-5-carboxylate reductase (P5CR), was investigated in Phaseolus aureus Roxb. and Triticum aestivum L. The 5-d-old seedlings were exposed to 0.05, 0.1, 0.2 or 0.4 mM concentrations of the metals in Hoagland solution for 12 and 36 h. T. aestivum exhibited considerably greater accumulation of proline than P. aureus in response to the metal treatment. Among the two metals, Hg induced greater accumulation of proline than Cd. The activity of P5CS increased significantly in response to the metal treatment, particularly in T. aestivum in which the activity of the enzyme in the control was much higher than that was in P. aureus. The activity of P5CR on the other hand mostly decreased in response to the metal treatment. The study indicated a strong dependence of the metal induced proline accumulation on the constitutive P5CS content of the plants.     

The influence of chilling on photosynthesis and activities of some enzymes of sucrose metabolism in Lycopersicon esculentum Mill

The effects of chilling stress on leaf photosynthesis and sucrose metabolism were investigated in tomato plants (Lycopersicon esculentum Mill. cultivar Marmande). Twenty-one-day-old seedlings were grown in a growth chamber at 25/23 °C (day/night) (control) and at 10/8 °C (day/night) (chilled) for 7 days. The most evident effect of chilling was the marked reduction of plant growth and of CO₂ assimilation as measured after 7 days, the latter being associated with a decrease in stomatal closure and an increase in Ci. The inhibition in photosynthetic rate was also related to an impairment of photochemistry of photosystem II (PSII), as seen from the slight, but significant change in the ratio of F_v/F_m. The capacity of chilled leaves to maintain higher q_P values with respect to the controls suggests that some protection mechanism prevented excess reduction of PSII acceptors. The results of the determination of starch and soluble sugar content could show that chilling impaired sucrose translocation. The activity of leaf invertase increased significantly in chilled plants, while that of other sucrose-metabolizing enzymes was not affected by growing temperature. Furthermore, the increase in invertase (neutral and acid) activity, which is typical of senescent tissue characterized by reduced growth, seems to confirm that tomato is a plant which is not a plant genetically adapted to low temperatures.     

Growth and nitrogen fixation inAlnus glutinosa (L.) Gaertn. under carbon dioxide enrichment of the root atmosphere

The effects of aeration of the N-free rooting medium with elevated CO₂ on (a) acetylene reduction by perlite-grown plants and (b) N₂-fixation and long-term growth of nutrient solution-grown plants were determined for nodulatedAlnus glutinosa (L.) Gaertn. In the former experiments, roots of intact plants were incubated in acetylene in air in darkened glass jars for 3 hr, followed by a further 3 hr incubation period in air enriched with CO₂ (0–5%). During incubation, the CO₂ content of the jars increased by 0.17% per hour due to respiration of the root system, so that the CO₂ content at 3 hr was 0.5%. Additional enrichment of the rooting medium gas-phase with CO₂ equivalent to 1.1% and 1.75% CO₂ of the gas volume significantly increased nitrogenase activity (ethylene production) by 55% and 50% respectively, while enrichment with greater than 2.5% CO₂ decreased activity. In contrast, ethylene production by control plants, where CO₂ was not added to the assay jars, decreased by 8% over the assay period. In long-term growth experiments, nodulated roots of intactAlnus glutinosa plants were sealed into jars containing N-free nutrient solution (pH 6.3) and aerated with air, or air containing elevated levels of CO₂ (1.5% and 5%). Comparison of the appearance of CO₂-treated with air treated plants suggested that 1.5% CO₂ stimulated plant growth. However, at harvest after 5 or 6 weeks variability between plants masked the significance of differences in plant dry weight. A significant increase of 33% in total nitrogen of plants aerated with 1.5% CO₂, compared with air-treated plants, was demonstrated, broadly in line with the short-term increase in acetylene reducing activity observed following incubations with similar CO₂ concentrations. Shoot dry weight was not affected significantly by long-term exposure to 5% CO₂, the main effect on growth being a 20% reduction in dry weight of the root system, possibly through inhibition of root system respiration. However, in contrast to the inhibitory effects of high CO₂ on acetylene reduction there was no significant effect on the amounts of N₂ fixed.     

Adaptation to chilling: photosynthetic characteristics of the cultivated tomato and a high altitude wild species

Abstract When tomato plants of the high-altitude species Lycopersicon hirsutum and of the cultivated Lycopersicon esculentum were grown at 24/18°C (day/night), the effects of temperature, photon flux density, and intercellular CO₂ concentration up to about 600 μl l^?1 on net CO₂ uptake were similar in the two species. Acclimation of these plants at 12/6°C (day/night) resulted, after 4 d or longer, in a similar downward shift of about 5°C in the optimum temperature for CO₂ uptake. However, in comparison with the cultivated species, the high-altitude plants achieved a higher rate of CO₂ uptake at saturating concentrations of intercellular CO₂, maintained a higher level of saturating-light CO₂ uptake rate at 10°C after exposure to chilling stress (10°C and photon flux density of 400 μmol m^?2s^?1 d and 5°C night) for 7–18 d, and displayed a better capacity for rapid recovery after prolonged stress. The greater capacity for CO₂ uptake observed in the high-altitude species during and after exposure to chilling stress was also reflected in its higher growth rate under those conditions compared with plants of L. esculentum. These advantages of the high-altitude species may partly explain its ability to survive and complete its life cycle under the environmental conditions prevailing in its natural habitat.     

The effects of phosphate supply on growth of plants from the Brasilian Cerrado: experiments with seedlings of the annual weed,Bidens gardneri Baker (Compositeae) and the tree,Qualea grandiflora (Mart.) (Vochysiaceae)

Summary Plants of the cerrado tree species Qualea grandiflora and the annual herb Bidens gardneri were grown from seed in controlled environment rooms at 30/20° C and 12 hour photoperiod. Seedlings were grown in pots or small tubes containing sand and provided with various amounts of mineral solutions based on the formulation of Hoagland and Arnon but with the phosphate content modified in some cases. In a long-term experiment lasting 213 days, plants supplied with full strength Hoagland's solution all died but plants of Qualea given 1/10 strength solution survived, although they grew very slowly. Low relative growth rates (0.008–0.036 d^–1) were also a feature of other experiments with Qualea and calculated rates of net assimilation rate gave values of 3–7 mg CO₂ dm^–2 h^–1. Expansion of the photosynthetic surface proceeded slowly and the cotyledons were the main site of photosynthesis for more than 40 days. The low rates of growth occurred despite significant uptake of phosphorus by young plants and in shortterm experiments growth was independent of the amount of phosphate supplied and accumulated. In contrast, the values of R found for plants of Bidens reached 0.24 d^–1. Growth of young plants was dependent on the external supply of phosphorus, being reduced when this was low and also when it was very high. Growth of the photosynthetic surface was also much more rapid than for Qualea and also varied with supply of phosphorus. The results are discussed in the context of the occurrence of these species in the Cerrado.     

Effects of chilling on the biochemical and functional properties of thylakoid membranes

The mechanism of chilling resistance was investigated in 4-week-old plants of the chilling-sensitive cultivated tomato, Lycopersicon esculentum Mill. cv H722, and rooted cuttings of its chilling-resistant wild relative, L. hirsutum Humb. and Bonpl., which were chilled for 3 days at 2°C with a 14-hour photoperiod and light intensity of 250 micromoles per square meter per second. This chilling stress reduced the chlorophyll fluorescence ratio, stomatal conductance, and dry matter accumulation more in the sensitive L. esculentum than in the resistant L. hirsutum. Photosynthetic CO₂ uptake at the end of the chilling treatment was reduced more in the resistant L. hirsutum than in L. esculentum, but recovered at a faster rate when the plants were returned to 25°C. The reduction of the spin trap, Tiron, by isolated thylakoids at 750 micromoles per square meter per second light intensity was taken as a relative indication of the tendency for the thylakoids to produce activated oxygen. Thylakoids isolated from the resistant L. hirsutum with or without chilling treatment were essentially similar, whereas those from chilled leaves of L. esculentum reduced more Tiron than the nonchilled controls. Whole chain photosynthetic electron transport was measured on thylakoids isolated from chilled and control leaves of the two species at a range of assay temperatures from 5 to 25°C. In both species, electron transport of the thylakoids from chilled leaves was lower than the controls when measured at 25°C, and electron transport declined as the assay temperature was reduced. However, the temperature sensitivity of thylakoids from chilled L. esculentum was altered such that at all temperatures below 20°C, the rate of electron transport exceeded the control values. In contrast, the thylakoids from chilled L. hirsutum maintained their temperature sensitivity, and the electron transport rates were proportionately reduced at all temperatures. This sublethal chilling stress caused no significant changes in thylakoid galactolipid, phospholipid, or protein levels in either species. Nonchilled thylakoid membranes from L. hirsutum had fourfold higher levels of the fatty acid 16:1, than those from L. esculentum. Chilling caused retailoring of the acyl chains in L. hirsutum but not in L. esculentum. The chilling resistance of L. hirsutum may be related to an ability to reduce the potential for free radical production by close regulation of electron transport within the chloroplast.     

Effects of increasing inorganic carbon supply to roots on net nitrate uptake and assimilation in tomato seedlings

We investigated the influence of an increased inorganic carbon supply in the root medium on NO^?₃ uptake and assimilation in seedlings of Lycopersicon esculentum (L.) Mill. cv. F144. The seedlings were pre-grown for 4 to 7 days with 0 or 100 mM NaCl in hydroponic culture using 0.2 mM NO^?₃ (group A) or 0.2 mM NH⁺₄ (group B) as nitrogen source. The nutrient solution for group A plants was aerated with air or with air containing 4 800 μumol mol^?1 CO₂. Nitrate uptake rate and root and leaf malate contents in these plants were determined. The plants of group B were subdivided into two sets. Plants of one set were transferred either to N-free solution containing 0 or 5 mM NaHCO₃, or to a medium containing 2 mM NO^?₃ and 5 mM NaHCO₃. Both sets of group B plants were grown for 12 h in darkness prior to 2 h of illumination, and were assayed for malate content and NO^?₃ uptake rate (only for plants grown in N-free solution). The second set of group B plants was labeled with ¹⁴C by a 1-h pulse of H¹⁴CO^?₃ which was added to a 5 mM NaHCO₃ solution containing 0 or 100 mM NaCl and 0 or 2 mM NO^?₃, and ¹⁴C-assimilates were extracted and fractionated. The roots of group B plants growing in carbonated medium accumulated twice as much malate as did control plants. This malate was accumulated only when NO^?₃ was absent from the root medium. Both a high level of root malate and aeration with CO₂-enriched air stimulated NO^?₃ uptake. Analysis of ¹⁴C-assimilates indicated that with no NO^?₃ in the medium, the ¹⁴C was present mainly in organic acids, whereas with NO^?₃, a large proportion of ¹⁴C was incorporated into amino acids. Transport of root-incorporated ¹⁴C to the shoot was enhanced by NO^?₃, while the amino acid fraction was the major ¹⁴C-assimilates in the shoot. It is concluded that inorganic carbon fixed through phosphoenolpyruvate carboxylase (EC 4.1.1.31) in roots of tomato plants may have two fates: (a) as a carbon skeleton for amino acid synthesis; and (b) to accumulate, mainly as malate, in the roots, in the absence of a demand for the carbon skeleton. Inorganic carbon fixation in the root provides carbon skeletons for the assimilation of the NH⁺₄ resulting from NO₃ reduction, and the subsequent removal of amino acids through the xylem. This ‘removal’ of NO^?₃ from the cytoplasm of the root cells may in turn increase NO^?₃ uptake.     

Leaf Gas Exchange and Chlorophyll Fluorescence Parameters in Phaseolus Vulgaris as Affected by Nitrogen and Phosphorus Deficiency

The effects of N and P deficiency, isolated or in combination, on leaf gas exchange and fast chlorophyll (Chl) fluorescence emission were studied in common bean cv. Negrito. 10-d-old plants grown in aerated nutrient solution were supplied with high N (HN, 7.5 mol m⁻³) or low N (LN, 0.5 mol m⁻³), and also with high P (HP, 0.5 mol m⁻³) or low P (LP, 0.005 mol m⁻³). Regardless of the external P supply, in LN plants the initial fluorescence (F₀) increased 12 % in parallel to a quenching of about 14 % in maximum fluorescence (F_m). As a consequence, the variable to maximum fluorescence ratio (F_v/F_m) decreased by about 7 %, and the variable to initial fluorescence ratio (F_v/F₀) was lowered by 25 % in relation to control plants. In LP plants, F_v/F_m remained unchanged whilst F_v/F₀ decreased slightly as a result of 5 % decline in F_m. Under N deficiency, the net photosynthetic rate (P _N) halved at 6 d after imposition of treatment and so remained afterwards. As compared to LN plants, P _N declined in LP plants latter and to a less extent. From 12 d of P deprivation onwards. P _N fell down progressively to display rates similar to those of LN plants only at the end of the experiment. The greater P _N in LP plants was not reflected in larger biomass accumulation in relation to LN beans. In general, P and N limitation affected photosynthesis parameters and growth without showing any synergistic or additive effect between deficiency of both nutrients. This revised version was published online in June 2006 with corrections to the Cover Date.     

Bacterial inoculation of maize affects carbon allocation to roots and carbon turnover in the rhizosphere

To assess the influence of bacteria inoculation on carbon flow through maize plant and rhizosphere,¹⁴C allocation after¹⁴CO₂ application to shoots over a 5-day period was determined. Plants were grown on C- and N-free quartz sand in two-compartment pots, separating root and shoot space. While one treatment remained uninoculated, treatments two and three were inoculated withPantoea agglomerans (D5/23) andPseudomonas fluorescens (Ps I A12), respectively, five days after planting. Bacterial inoculation had profound impacts on carbon distribution within the system. Root/rhizosphere respiration was increased and more carbon was allocated to roots of plants being inoculated. After five days of¹⁴CO₂ application, more ethanol-soluble substances were found in roots of inoculated treatments and lower rhizodeposition indicated intensive C turnover in the rhizosphere. In both inoculated treatments the intensity of photosynthesis measured as net-CO₂-assimilation rates were increased when compared to the uninoculated plants. However, high C turnover in the rhizosphere reduced shoot growth of D5/23 inoculated plants, with no effect on shoot growth of Ps I A12 inoculated plants. A separation of labeled compounds in roots and rhizodeposition revealed that neutral substances (sugars) constituted the largest fraction. The relative fractions of sugars, amino acids and organic acids in roots and rhizodeposition suggest that amino acid exudation was particularly stimulated by bacterial inoculation and that turnover of this substance group is high in the rhizosphere.     

Effects of chilling on tomato fruit texture

The effects of chilling on tomato ( Lycopersicon esculentum Mill cv. Caruso) texture were investigated using fruit stored at 22°C (nonchilled) or 5°C (chilled) for 28 days. or at 5°C for 15 days before transfer to 22°C to facilitate ripening during and additional 13 days (prechilled). Prechilled fruit exhibited symptoms of slight chilling injury, i.e. development of mealiness, accelerated softening relative to that of nonchilled fruit and nonuniform surface colour development. The firmness of all fruit decreased during ripening and chilled storage when measured by flat plate compression and puncture, especially during the early stages of ripening of nonchilled and prechilled fruit. The compression firmness of pericarp tissue similarly decreased during ripening of nonchilled and prechilled fruit, but was maintained during chilling. Total moisture content (ca 94%) of tissue, uronide content (32-35% w/w) and extracted β-galactosidase activity did not differ significantly ( P > 0.05) among fruit during ripening and chilled storage. The degree of uronide methyl esterification in ethanol-insoluble solids prepared from pericarp tissue (EIS) was relatively low for all fruit. i.e. <40%. EIS from which greater levels of pectinesterase were extracted (i.e. nonchilled>chilled>prechilled) exhibited decreased levels of uronide methyl esterification. Markedly elevated levels of β-glucosidase activity were extracted from prechilled EIS. Total polygalacturonase activity (mainly as PGI) and autolysis of enzyme-extracted EIS were inversely correlated ( P ≤ 0.05) only with the loss of nonchilled fruit and tissue firmness and prechilled fruit firmness. Results suggest a possible role for β-glucosidase in textural changes of prechilled fruit and tissue (e.g. loss of firmness, development of mealiness) and also implicate loss of skin strength in the softening of whole fruit during chilling.     

Effects of elevated CO<Subscript>2</Subscript> and nitrogen on wheat growth and photosynthesis

The effects of nitrogen [75 and 150 kg (N) ha⁻¹] and elevated CO₂ on growth, photosynthetic rate, contents of soluble leaf proteins and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and nitrate reductase (NR) were studied on wheat (Triticum aestivum L. cv. HD-2285) grown in open top chambers under either ambient (AC) or elevated (EC) CO₂ concentration (350 ± 50, 600 ± 50 μmol mol⁻¹) and analyzed at 40, 60 and 90 d after sowing. Plants grown under EC showed greater photosynthetic rate and were taller and attained greater leaf area along with higher total plant dry mass at all growth stages than those grown under AC. Total soluble and Rubisco protein contents decreased under EC but the activation of Rubisco was higher at EC with higher N supply. Nitrogen increased the NR activity whereas EC reduced it. Thus, EC causes increased growth and P_N ability per unit uptake of N in wheat plants, even if N is limiting.     

Hydrogen peroxide-scavenging enzymes and antioxidants in Echinochloa frumentacea as affected by triazole growth regulators

Paclobutrazol (PBZ)- and uniconazole (UCZ)-treated plants of Echinochloa frumentacea were shorter but had much wider leaves than untreated controls 10 days after treatment. Leaves of treated plants had a slightly higher concentration of soluble protein than the controls and exhibited enhanced activities of ascorbate peroxidase, monodehydroascorbate (MDHA) reductase, and glutathione (GSH) reductase. The triazoles did not influence the activity of dehydroascorbate (DHA) reductase. The leaves of treated plants had increased concentrations of water-soluble sulfhydryls and ascorbic acid. In contrast, the concentration of malondialdehyde (MDA), a by-product of lipid peroxidation, was lower in the leaves of treated plants than in controls. These results suggest that triazole growth regulators increased the activity of the endogenous H₂O₂-scavenging system in E. frumentacea.