Effects of low pH and aluminum stresses on common beans (Phaseolus vulgaris) differing in low-phosphorus and photoperiod responses

Using common beans differing greatly in the response to photoperiod and low-phosphorus (P) stress, we investigated their responses to acidity and aluminum (Al)toxicity and the relationship between Al tolerance and organic acid exudation under Al or low P stress. A genotype Ginshi was found to be sensitive to low pH treatment. When exposed to pH 4.5, serious curvature in the root tips of cv. Ginshi was observed; however, it was completely corrected by the application of 5 or 10 μmol/L AlCl3; increasing calcium (Ca) could ameliorate Al toxicity, but it could not correct root curvature at pH 4.5. Common beans showed significant differences in both root growth and Al tolerance, and the varieties from the Andes were more tolerant to Al toxicity than those from the Mesoamerican origin. In the presence of 50 μmol/L AlCl3,all the common bean genotypes exuded citrate, and a significant difference in the amounts of citrate was observed among genotypes. The genotypes originated in the Mesoamerica tended to release more citrate than other origins in the presence of Al. The P-inefficient genotype DOR364 exuded more citrate than the P-efficient genotype G19833 in the presence of 50 μmol/L AlCl3, whereas no organic acids were detected in root exudates under low-P stress. A reduction of citrate exudation in the DOR364, but a slight increase of citrate exudation in the G19833, was observed under Al stress after they were exposed to 6-d P starvation. These results suggest that different low-P or Al tolerance in common beans might not be associated with organic acid exudation.     

Effects of low pH and aluminum stresses on common beans (Phaseolus vulgaris) differing in low-phosphorus and photoperiod responses

Using common beans differing greatly in the response to photoperiod and low-phosphorus (P) stress, we investigated their responses to acidity and aluminum (Al) toxicity and the relationship between Al tolerance and organic acid exudation under Al or low P stress. A genotype Ginshi was found to be sensitive to low pH treatment. When exposed to pH 4.5, serious curvature in the root tips of cv. Ginshi was observed; however, it was completely corrected by the application of 5 or 10 μmol/L AlCl₃; increasing calcium (Ca) could ameliorate Al toxicity, but it could not correct root curvature at pH 4.5. Common beans showed significant differences in both root growth and Al tolerance, and the varieties from the Andes were more tolerant to Al toxicity than those from the Mesoamerican origin. In the presence of 50 μmol/L AlCl₃, all the common bean genotypes exuded citrate, and a significant difference in the amounts of citrate was observed among genotypes. The genotypes originated in the Mesoamerica tended to release more citrate than other origins in the presence of Al. The P-inefficient genotype DOR364 exuded more citrate than the P-efficient genotype G19833 in the presence of 50 μmol/L AlCl₃, whereas no organic acids were detected in root exudates under low-P stress. A reduction of citrate exudation in the DOR364, but a slight increase of citrate exudation in the G19833, was observed under Al stress after they were exposed to 6-d P starvation. These results suggest that different low-P or Al tolerance in common beans might not be associated with organic acid exudation.     

Differential aluminum tolerance in soybean: An evaluation of the role of organic acids

The role of organic acids in aluminum (Al) tolerance has been the object of intensive research. In the present work, we evaluated the roles of organic acid exudation and concentrations at the root tip on Al tolerance of soybean. Exposing soybean seedlings to Al³⁺ activities up to 4.7 μ M in solution led to different degrees of restriction of primary root elongation. Al tolerance among genotypes was associated with citrate accumulation and excretion into the external media. Citrate and malate efflux increased in all genotypes during the first 6 h of Al exposure, but only citrate efflux in Al-tolerant genotypes was sustained for an extended period. Tolerance to Al was correlated with the concentration of citrate in root tips of 8 genotypes with a range of Al sensitivities (r²=0.75). The fluorescent stain lumogallion indicated that more Al accumulated in root tips of the Al-sensitive genotype Young than the Al-tolerant genotype PI 416937, suggesting that the sustained release of citrate from roots of the tolerant genotype was involved in Al exclusion. The initial stimulation of citrate and malate excretion and accumulation in the tip of all genotypes suggested the involvement of additional tolerance mechanisms. The experiments included an examination of Al effects on lateral root elongation. Extension of lateral roots was more sensitive to Al than that of tap roots, and lateral root tips accumulated more Al and had lower levels of citrate.     

Organic acid exudation induced by phosphorus deficiency and/or aluminium toxicity in two contrasting soybean genotypes

Experiments in nutrient solution were conducted to investigate the exudation of organic acids (OAs) induced by phosphorus deficiency (–P) and/or aluminium toxicity (+Al) in two contrasting soybean genotypes as related to internal OA concentration and related enzyme activities. Baxi 10 (BX10), a known P‐efficient soybean (Glycine max[L] Merr.) genotype, was shown to be more resistant to +Al than a P‐inefficient genotype Bendi 2 (BD2), indicating the potential of selecting soybean cultivars with dual resistance to –P and +Al. The two contrasting genotypes were further characterized for root exudation and formation of oxalate, malate and citrate and their related enzyme activities in response to –P, +Al or both combined. –P significantly induced malate and oxalate exudation from both soybean genotypes, although the P‐efficient BX10 tended to excrete much more oxalate than the P‐inefficient BD2. The +Al treatment triggered citrate efflux from both genotypes, with BX10 having a much greater efflux rate than BD2. Interestingly, –P did not appear to induce citrate exudation, whereas +Al had no obvious effect on malate or oxalate exudation from the two genotypes. The exudation of OAs was generally diminished under the coupled stress of –P and +Al in comparison with either single stress, implying a possible antagonistic effect of the two stresses on OA exudation. Root malate content was negatively correlated with its exudation in BX10 but positively in BD2. A similar tendency was observed for oxalate content and exudation only with less magnitude. Determination of six related enzymes, phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate phosphatase (PEPP), malate enzyme (ME), isocitrate dehydrogenase (ICDH), malate dehydrogenase (MDH), and pyruvate kinase (PK), in the root tips showed that their activities were not significantly altered during the early stage of treatments (2 and 4 days) whereas at 14 days after stress imposition, the activities of PEPC, PEPP, ME and ICDH were generally enhanced for both genotypes. However, the activity of these enzymes did not appear to be correlated with OA exudation or formation. This study clearly demonstrates that OA exudation is differentially induced by –P and +Al in soybean plants, with specific induction of oxalate and malate by –P and citrate by +Al. The lack of a close relationship between OA exudation and internal concentration or enzyme activities may suggest that the regulation of OA formation and exudation by –P and/or +Al could be imposed at different stages.     

Aluminum resistance of cowpea as affected by phosphorus-deficiency stress

Plants growing in acid soils suffer both phosphorus (P) deficiency and aluminum (Al) toxicity stresses. Selection of genotypes for adaptation to either P deficiency or Al toxicity has sometimes been unsuccessful because these two soil factors often interact. Two experiments were conducted to evaluate eight cowpea genotypes for Al resistance and to study the combined effect of P deficiency and Al toxicity stress on growth, P uptake, and organic acid anion exudation of two genotypes of contrasting Al resistance selected from the first experiment. Relative root inhibition by 30 μM Al ranged from 14% to 60% and differed significantly among the genotypes. Al significantly induced callose formation, particularly in Al-sensitive genotypes. P accumulation was significantly reduced (28% and 95%) by Al application for both the Al-resistant and the Al-sensitive genotypes. Al supply significantly enhanced malate release of root apices of both genotypes. However, the exudation rate was significantly higher in the Al-resistant genotype. P deprivation induced an enhanced malate exudation in the presence of Al only in the Al-resistant genotype IT89KD-391. Citrate exudation rate of the root apices was lower than malate exudation by a factor of about 10, and was primarily enhanced by P deficiency in both genotypes. Al treatment further enhanced citrate exudation in P-sufficient, but not in P-deficient plants. The level of citrate exudation was consistently higher in the Al-resistant genotype IT89KD-391 particularly in presence of Al.It is concluded that the Al-resistant genotype is better adapted to acid Al-toxic and P-deficient soils than the Al-sensitive genotype since both malate and citrate exudation were more enhanced by combined Al and P-deficiency stresses.     

Effect of phosphorus deficiency on aluminium-induced citrate exudation in soybean (Glycine max)

Aluminium (Al) toxicity or phosphorus (P) deficiency can induce exudation of organic acids from the roots of some plants, which is believed to be a tolerance mechanism against Al toxicity or P deficiency. In the present study, the effect of P deficiency on Al-induced citrate exudation was investigated in three soybean varieties differing in low-P tolerance. P starvation alone failed to induce secretion of organic acids from all three soybean varieties. However, P deficiency altered Al-induced citrate exudation over time, showing a complex interaction. Short × term P starvation (4 days) produced up to 50% increase in Al-induced citrate secretion, while longer-term (10 days) starvation reduced Al-induced citrate secretion to trace amounts. However, after a further 1 day in complete nutrient solution for recovery, Al-induced citrate exudation from the recovered roots was approximately 6 times higher than that from the continuously P-starved plants, but still approximately 3.6 times lower than that from the P-sufficient control. With increasing P or Al supply, Al-induced citrate exudation increased, while Al accumulation in soybean roots decreased in parallel with the decrease of P supply. The photosynthetic rate, stomatal conductance and transpiration were decreased by P deficiency, whereas the intracellular CO₂ concentration was increased. These findings indicate that P nutrition has a significant effect on Al-induced citrate exudation and Al accumulation in soybean root apices.     

Aluminium tolerance and high phosphorus efficiency helps Stylosanthes better adapt to low-P acid soils

Backgrond and Aims

Stylosanthes spp. (stylo) is one of the most important pasture legumes used in a wide range of agricultural systems on acid soils, where aluminium (Al) toxicity and phosphorus (P) deficiency are two major limiting factors for plant growth. However, physiological mechanisms of stylo adaptation to acid soils are not understood.

Methods

Twelve stylo genotypes were surveyed under field conditions, followed by sand and nutrient solution culture experiments to investigate possible physiological mechanisms of stylo adaptation to low-P acid soils.

Key Results

Stylo genotypes varied substantially in growth and P uptake in low P conditions in the field. Three genotypes contrasting in P efficiency were selected for experiments in nutrient solution and sand culture to examine their Al tolerance and ability to utilize different P sources, including Ca-P, K-P, Al-P, Fe-P and phytate-P. Among the three tested genotypes, the P-efficient genotype ‘TPRC2001-1’ had higher Al tolerance than the P-inefficient genotype ‘Fine-stem’ as indicated by relative tap root length and haematoxylin staining. The three genotypes differed in their ability to utilize different P sources. The P-efficient genotype, ‘TPRC2001-1’, had superior ability to utilize phytate-P.

Conclusions

The findings suggest that possible physiological mechanisms of stylo adaptation to low-P acid soils might involve superior ability of plant roots to tolerate Al toxicity and to utilize organic P and Al-P.Key words: Stylosanthes, phosphorus, P efficiency, organic P, Al toxicity, acid soil     

The role of phosphorus in aluminium-induced citrate and malate exudation from rape (Brassica napus)

Exudation of organic anions is believed to be a common tolerance mechanism for both aluminium toxicity and phosphorus deficiency. Nevertheless, which of these stresses that actually elicit the exudation of organic anions from rape ( Brassica napus L) remains unknown, and the combined effects of Al toxicity and P deficiency on rape have not been reported before. Therefore, in the current study, Brassica napus var. Natane nourin plants grown with or without 0.25 m M P were exposed to 0 or 50 µ M AlCl₃ and several parameters related to the exudation of organic anions from the roots were investigated. Eight days of P deficiency resulted in a significant growth reduction, but P deficiency alone did not induce exudation of organic anions. In contrast, Al strongly induced organic acid exudation, while simultaneously inhibiting root growth. Increased in-vitro activity of citrate synthase (CS, EC 4.1.3.7), malate dehydrogenase (MDH, EC 1.1.1.37) and phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), together with reduced root respiration, indicated that the Al-induced accumulation and subsequent exudation of citrate and malate were associated with both increased biosynthesis and reduced metabolism of citric and malic acid. Phosphorus-sufficient plants showed more pronounced aluminium-induced accumulation and exudation of organic anions than P-deficient plants. A divided root chamber experiment showed the necessity of direct contact between Al and roots to elicit exudation of organic anions. Prolonged exposure (10 days) to Al resulted in a decrease in the net exudation of citrate and malate, and the rate of decrease was much more rapid in P-deficient plants than in P-sufficient plants. It is concluded that P nutrition affects the level of Al-induced synthesis and exudation of organic anions. However, the mechanism needs further investigation.     

Low-P tolerance by maize (Zea mays L.) genotypes: Significance of root growth,and organic acids and acid phosphatase root exudation

We investigated some mechanisms, which allow maize genotypes to adapt to soils which are low in available P. Dry matter production, root/shoot-ratio, root length and root exudation of organic acids and acid phosphatase were investigated in four maize genotypes grown under P-deficient and P-sufficient conditions in sterile hydroponic culture. A low-P tolerant, an acid-tolerant and a low-P susceptible genotype of maize were compared with a Swiss commercial cultivar. The study found increased root development and increased exudation of acid phosphatase under P-deficient conditions in all maize genotypes, except for the Swiss cultivar. Effects on root formation and acid phosphatase were greater for the low-P tolerant than for the low-P susceptible, and the acid soil tolerant genotypes. Organic acid contents in root tissues were increased under P deficiency and related to increased PEPC activity. However, the increase in contents was associated with an increase in exudation for the low-P tolerant genotype only. The low-P susceptible genotype was characterized by high organic acid content in roots and low organic acid exudation. The organic acids content in the phloem exudates of shoots was related to root exudation under different P supply, to the difference between lines in organic acids root content, but not to the low-P tolerance or susceptibility of maize genotypes.     

Genotypic difference of potato in carbon budgeting as a mechanism of phosphorus utilization efficiency

Observed genotypic difference in P utilization efficiency in soil grown potatoes led to the present study to investigate possible mechanisms of P utilization efficiency in potato genotypes grown in nutrient solution under three P regimes (low, medium and high). For all genotypes relative growth rate (RGR), leaf P content, net assimilation rate (NAR) and leaf area ratio (LAR) increased while P utilization efficiency and leaf starch content decreased at the two higher P regimes compared to the low P regime. The P-efficient genotypes CGN 17903 and CIP 384321.3 had higher RGR compared to the P-inefficient genotypes CGN 22367 and CGN 18233, which resulted from enhanced NAR rather than from LAR. Net photosynthetic rate was similar for all genotypes. However, for P-inefficient genotype CGN 22367, the lower NAR could be explained by increased leaf dark respiration. For P-inefficient genotype CGN 18233 we speculate that increased carbon cost of root respiration or exudation or both, caused low NAR, since leaf dark respiration of this genotype was similar to that of P-efficient genotypes.     

A comparative study on the aluminium- and copper-induced organic acid exudation from wheat roots

It is well established that aluminium (Al) and some heavy metals can elicit organic acid exudation from a range of species. In the present research we found that copper (Cu) can also induce organic acid exudation from the roots of wheat, rye, triticale, maize and soybean. Using intact wheat plants, we made a comparative study of Al- and Cu- induced organic acid exudation. In 5-day-old wheat seedlings, severe Cu stress (40 µ M CuCl₂) mainly induced the exudation of malate and citrate, and Al-tolerant genotypes could release significantly greater amounts of malate than Al-sensitive genotypes. The time course of the exudation of malate and citrate from the roots of 5-day-old seedlings of wheat (cv. Atlas) in 200 µ M AlCl₃ was similar to that in 40 µ M CuCl₂. In older wheat plants (15-day-old), moderate Cu stress (12 µ M CuCl₂) induced the exudation of large amounts of citrate and addition of Al or La sharply reduced Cu-induced citrate exudation, while Cu or La did not affect Al-induced malate efflux. When half of the root system of Atlas wheat was immersed in Al- or Cu-containing solution and the remaining half in Al- or Cu-free solution, organic acids were only exuded into the solution containing Al or Cu. This suggests that no long distance signal transport is involved in organic acid exudation induced by Al or Cu, and that direct contact of Al or Cu with plant roots is a prerequisite for the induction of organic acid exudation. The anion-channel inhibitor niflumic acid (NIF) significantly stimulated the exudation of both citrate and malate from 5-day-old wheat seedlings under severe Al or Cu stress. Our results suggest that Cu-induced organic acid efflux may be a common response, which may play a role in alleviating Cu toxicity in plants.     

Magnesium ameliorates aluminum rhizotoxicity in soybean by increasing citric acid production and exudation by roots

Superior effectiveness of Mg over Ca in alleviating Al rhizotoxicity cannot be accounted for by predicted changes in plasma membrane Al3+ activity. The influence of Ca and Mg on the production and secretion of citrate and malate, and on Al accumulation by roots was investigated with soybean genotypes Young and PI 416937 which differ in Al tolerance. In the presence of a solution Al3+ activity of 4.6 microM, citrate and malate concentrations of tap root tips of both genotypes increased with additions of either Ca up to 3 mM or Mg up to 50 microM. Citrate efflux rate from roots exposed to Al was only enhanced with Mg additions and exceeded malate efflux rates by as much as 50-fold. Maximum citrate release occurred within 12 h after adding Mg to solution treatments. Adding 50 microM Mg to 0.8 mM CaSO4 solutions containing Al3+ activities up to 4.6 microM increased citrate concentration of tap root tips by 3- to 5-fold and root exudation of citrate by 6- to 9-fold. Plants treated with either 50 microM Mg or 3 mM Ca had similar reductions in Al accumulation at tap root tips, which coincided with the respective ability of these ions to relieve Al rhizotoxicity. Amelioration of Al inhibition of soybean root elongation by low concentrations of Mg in solution involved Mg-stimulated production and efflux of citrate by roots.     

Overexpression of Malate Dehydrogenase in Transgenic Alfalfa Enhances Organic Acid Synthesis and Confers Tolerance to Aluminum

Al toxicity is a severe impediment to production of many crops in acid soil. Toxicity can be reduced through lime application to raise soil pH, however this amendment does not remedy subsoil acidity, and liming may not always be practical or cost-effective. Addition of organic acids to plant nutrient solutions alleviates phytotoxic Al effects, presumably by chelating Al and rendering it less toxic. In an effort to increase organic acid secretion and thereby enhance Al tolerance in alfalfa (Medicago sativa), we produced transgenic plants using nodule-enhanced forms of malate dehydrogenase and phosphoenolpyruvate carboxylase cDNAs under the control of the constitutive cauliflower mosaic virus 35S promoter. We report that a 1.6-fold increase in malate dehydrogenase enzyme specific activity in root tips of selected transgenic alfalfa led to a 4.2-fold increase in root concentration as well as a 7.1-fold increase in root exudation of citrate, oxalate, malate, succinate, and acetate compared with untransformed control alfalfa plants. Overexpression of phosphoenolpyruvate carboxylase enzyme specific activity in transgenic alfalfa did not result in increased root exudation of organic acids. The degree of Al tolerance by transformed plants in hydroponic solutions and in naturally acid soil corresponded with their patterns of organic acid exudation and supports the concept that enhancing organic acid synthesis in plants may be an effective strategy to cope with soil acidity and Al toxicity.     

Dual effects of transgenic Brassica napus overexpressing CS gene on tolerances to aluminum toxicity and phosphorus deficiency

Background and aims

Low phosphorus (P) bioavailability and aluminum (Al) toxicity are two major constraints to plant growth in acid soil. To improve the tolerance of Brassica napus to Al toxicity and P deficiency, we generated transgenic canola (Brassica napus cv Westar) lines overexpressing a Pseudomonas aeruginosa citrate synthase (CS) gene and then investigated the effects of CS gene overexpressing in canola on enhancing tolerance to the two constraints.

Methods

The vector construction and plant transformation, molecular identification, estimation of extracellular and cellular citrate and malate concentrations, enzyme activity and gene expression analyse and Al tolerance and P acquisition assays were conducted using both hydroponics and soil culturing in the study.

Results

Both the root citrate and malate concentrations and their exudations in the two transgenic lines significantly increased compared with wild type (WT) following exposure to Al. These increases may be attributed to higher activities of the CS, malate dehydrogenase (MDH) and phosphoenolpyruvate carboxylase (PEPC) enzymes in the TCA cycle and the expression of BnALMT and BnMATE in the transgenic plants following Al exposure. The primary root elongation and prolonged Al treatment (10 days) experiments revealed that the transgenic lines displayed enhanced levels of Al tolerance. In addition, they showed enhanced citrate and malate exudation when grown in P-deficient conditions. Moreover, the enzyme activities of the transgenic lines were significantly higher compared with WT in response to P-deficient stress. The soil culture experiment showed that the transgenic lines possessed improved P uptake from the soil and accumulated more P in their shoots and seeds when FePO₄ was used as the sole P source.

Conclusions

These results indicate that the overexpression of the CS gene in B. napus not only leads to increased citrate synthesis and exudation but also changes malate metabolism, which confers improved tolerances to Al toxicity and P deficiency in the transgenic plants. These findings provide further insight into the dual effects of CS gene overexpression on Al toxicity and P deficiency in plants.     

A promoter-swap strategy between the AtALMT and AtMATE genes increased Arabidopsis aluminum resistance and improved carbon-use efficiency for aluminum resistance

The primary mechanism of Arabidopsis aluminum (Al) resistance is based on root Al exclusion, resulting from Al-activated root exudation of the Al(3+) -chelating organic acids, malate and citrate. Root malate exudation is the major contributor to Arabidopsis Al resistance, and is conferred by expression of AtALMT1, which encodes the root malate transporter. Root citrate exudation plays a smaller but still significant role in Arabidopsis Al resistance, and is conferred by expression of AtMATE, which encodes the root citrate transporter. In this study, we demonstrate that levels of Al-activated root organic acid exudation are closely correlated with expression of the organic acid transporter genes AtALMT1 and AtMATE. We also found that the AtALMT1 promoter confers a significantly higher level of gene expression than the AtMATE promoter. Analysis of AtALMT1 and AtMATE tissue- and cell-specific expression based on stable expression of promoter-reporter gene constructs showed that the two genes are expressed in complementary root regions: AtALMT1 is expressed in the root apices, while AtMATE is expressed in the mature portions of the roots. As citrate is a much more effective chelator of Al(3+) than malate, we used a promoter-swap strategy to test whether root tip-localized expression of the AtMATE coding region driven by the stronger AtALMT1 promoter (AtALMT1(P)::AtMATE) resulted in increased Arabidopsis Al resistance. Our results indicate that expression of AtALMT1(P)::AtMATE not only significantly increased Al resistance of the transgenic plants, but also enhanced carbon-use efficiency for Al resistance.     

Phosphorus benefits from grain-legume crops to subsequent maize grown on acid soils of southern Cameroon

We conducted field experiments over 2 years on two acid soils of southern Cameroon to test whether efficient uptake and use of phosphorus (P) from less available sources by grain legume genotypes could benefit subsequent rotational maize. We grew two crops each year. For the first crop we grew 4 genotypes of soybean and of cowpea, plus maize. For the second crop we grew maize. The first crops were fertilized with 0, 90 kg P ha⁻¹ as phosphate rock (PR) or 30 kg P ha⁻¹ as triple super phosphate (TSP). P application highly significantly increased shoot dry matter, P uptake, N₂ fixation and grain yields of the grain legumes with TSP generally more effective than PR. Two of the soybean and two of the cowpea genotypes were more efficient at using P. Only the P-efficient soybean and cowpea genotypes increased subsequent maize yields. Yields of the subsequent maize grown in rotation were significantly correlated with shoot P uptake for which the quantity of P applied with the crop residues of the pre-crop appeared to be a major factor. We also grew the grain legumes in nutrient solutions and measured organic acid-anion exudation from roots, root-surface phosphatase-activity, and root morphological characteristics. Enhanced exudation of organic acid anions from roots of P-deprived plants might have contributed to the P acquisition efficiency under field conditions of the P-efficient cowpea genotypes and one of the P-efficient soybean genotypes. A higher activity of root-surface acid phosphatase might have been important for the other P-efficient soybean genotype. The results show, that the potential positive rotational effect of cowpea and soybean on the acid, highly P-sorbing soils of southern Cameroon depends on breeding and using P-efficient genotypes when sparingly soluble and suboptimal rates of soluble P fertilizers are used. Section Editor: N. J. Barrow     

Wheat genotypes differing in aluminum tolerance differ in their growth response to CO2 enrichment in acid soils

Aluminum (Al) toxicity is a major factor limiting plant growth in acid soils. Elevated atmospheric CO₂ [CO₂] enhances plant growth. However, there is no report on the effect of elevated [CO₂] on growth of plant genotypes differing in Al tolerance grown in acid soils. We investigated the effect of short‐term elevated [CO₂] on growth of Al‐tolerant (ET8) and Al‐sensitive (ES8) wheat plants and malate exudation from root apices by growing them in acid soils under ambient [CO₂] and elevated [CO₂] using open‐top chambers. Exposure of ET8 plants to elevated [CO₂] enhanced root biomass only. In contrast, shoot biomass of ES8 was enhanced by elevated [CO₂]. Given that exudation of malate to detoxify apoplastic Al is a mechanism for Al tolerance in wheat plants, ET8 plants exuded greater amounts of malate from root apices than ES8 plants under both ambient and elevated [CO₂]. These results indicate that elevated [CO₂] has no effect on malate exudation in both ET8 and ES8 plants. These novel findings have important implications for our understanding how plants respond to elevated [CO₂] grown in unfavorable edaphic conditions in general and in acid soils in particular.     

Genotypic variation in phosphorus acquisition from sparingly soluble P sources is related to root morphology and root exudates in <Emphasis Type="Italic">Brassica napus</Emphasis>

Genotypic variations in the adaptive response to low-phosphorus (P) stress and P-uptake efficiency have been widely reported in many crops. We conducted a pot experiment to evaluate the P-acquisition ability of two rapeseed (Brassica napus) genotypes supplied with two sparingly soluble sources of P, Al-P and Fe-P. Then, the root morphology, proton concentrations, and carboxylate content were investigated in a solution experiment to examine the genotypic difference in P-acquisition efficiency. Both genotypes produced greater biomass and accumulated more P when supplied with Al-P than when supplied with Fe-P. The P-efficient genotype 102 showed a significantly greater ability to deplete sparingly soluble P from the rhizosphere soil because of its greater biomass and higher P uptake compared with those of the P-inefficient genotype 105. In the solution experiment, the P-efficient genotype under low-P conditions developed dominant root morphological traits, and it showed more intensive rhizosphere acidification because of greater H⁺ efflux, higher H⁺-ATPase activity, and greater exudation of carboxylates than the P-inefficient genotype. Thus, a combination of morphological and physiological mechanisms contributed to the genotypic variation in the utilization of different sparingly soluble P sources in B. napus.     

铁胁迫下大豆光合和磷/铁性状对磷素的响应

为了明确低铁胁迫下磷素用量对大豆光合和磷/铁性状的影响及基因型差异,为磷、铁肥的合理施用提供理论依据,以前期筛选的6个磷高效基因型和6个磷低效基因型大豆为供试材料,设4个P∶Fe配比处理,分别为0∶30、30∶30、150∶30和300∶30(μmol·L^-1),对大豆叶绿素荧光特性和磷、铁利用率进行了测定,利用单株粒重建立逐步回归方程并进行通径分析,通过因子得分综合评价磷高效和磷低效基因型对不同P∶Fe处理的响应。结果表明: 基因型效应、P∶Fe处理效应和两者互作效应对始花期(R1)光系统Ⅱ的相对电子传递速率(ETR)、光系统Ⅱ吸收的能量用于耗散为热量的比例(NPQ)、光系统Ⅱ吸收的能量用于进行光化学反应的比例(qL)的影响均达显著水平。典型相关分析表明,磷高效大豆基因型完熟期(R8)籽粒磷利用率与R1期光合速率呈负相关;磷低效大豆基因型R8期籽粒铁利用率与R1期NPQ呈正相关关系,而与R1期qL呈负相关关系;R1期PSⅡ实际光化学效率(Φ_PSⅡ)与磷高效基因型呈负相关,而与磷低效基因型呈正相关,这表明R1期Φ_PSⅡ可以作为鉴定低铁条件下不同磷效率大豆基因型的一个重要指标。利用因子得分综合评价发现,磷高效基因型表现为随着磷水平的上升而先降后升,而磷低效基因型则为先升后降,但两者拐点均出现在P∶Fe为30∶30处理下,这表明在低铁条件下P∶Fe为30∶30可以作为鉴定不同磷效率基因型的一个临界值。因而,在低铁地区种植磷高效大豆基因型时,磷肥的施用量至少要大于1∶1 (P∶Fe);而种植磷低效基因型时,磷肥的施用量不宜超过1∶1 (P∶Fe)。