Evaluation of some physiological traits as indicators of drought tolerance in bread wheat genotypes

The objective of this work was to evaluate the ability of some physiological traits to identify drought-tolerant bread wheat genotypes. To this end, twenty bread wheat genotypes were assessed under post anthesis drought stress (rain-fed) and irrigated conditions. The Stress Tolerance Index (STI) was used as a measure of drought tolerance. Relationships between STI and Cell Membrane Stability (CMS), Proline Concentration (PC), Relative Water Content (RWC), Chlorophyll a/b Ratio (Chl a/b), Relative Chlorophyll Content (RCC), Excised Leaf Water Retention (ELWR), and Relative Water Loss (RWL) were determined in order to find out whether these physiological traits could be used as the indicators of drought tolerance. The results showed that ELWR, RWL, and CMS could be considered as reliable indicators in screening wheat genotypes for drought tolerance.     

Bread wheat progenitors: <Emphasis Type="Italic">Aegilops tauschii</Emphasis> (DD genome) and <Emphasis Type="Italic">Triticum dicoccoides</Emphasis> (AABB genome) reveal differential antioxidative response under water stress

Antioxidant enzymes are known to play a significant role in scavenging reactive oxygen species and maintaining cellular homeostasis. Activity of four antioxidant enzymes viz., superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) was examined in the flag leaves of nine Aegilops tauschii and three Triticum dicoccoides accessions along with two bread wheat cultivars under irrigated and rain-fed conditions. These accessions were shortlisted from a larger set on the basis of field performance for a set of morpho-physiological traits. At anthesis, significant differences were observed in enzyme activities in two environments. A 45% elevation in average GR activity was observed under rain-fed conditions. Genotypic variation was evident within each environment as well as in terms of response to stress environment. Aegilops tauschii accession 3769 (86% increase in SOD, 41% in CAT, 72% in APX, 48% in GR activity) and acc. 14096 (37% increase in SOD, 32% CAT, 25% APX, 42% GR) showed up-regulation in the activity of all the four studied antioxidant enzymes. Aegilops tauschii accessions—9809, 14189 and 14113 also seemed to have strong induction mechanism as elevated activity of at least three enzymes was observed in them under rain-fed conditions. T. dicoccoides, on the other hand, maintained active antioxidative machinery under irrigated condition with relatively lower induction under stress. A significant positive correlation (r = 0.760) was identified between change in the activity of CAT and GR under stress. Changes in plant height, spike length and grain weight were recorded under stress and non-stress conditions on the basis of which a cumulative tolerance index was deduced and accessions were ranked for drought tolerance. Overall, Ae. tauschii accession 3769, 14096, 14113 (DD-genome) and T. dicoccoides accession 7054 (AABB-genome) may be used as donors to combine beneficial stress adaptive traits of all the three sub-genomes into a synthetic hexaploid for improving wheat for water stress conditions.     

Efficacy of putrescine and benzyladenine on photosynthesis and productivity in relation to drought tolerance in wheat (Triticum aestivum L.)

An experiment was conducted to find out the efficacy of putrescine and benzyladenine on photosynthesis and productivity in wheat. Seeds of wheat genotype HD 2329 (widely adapted under irrigated condition) were grown in ceramic pots under standard package and practices. Putrescine (0.1 mM) and benzyladenine (0.05 mM) were sprayed on the aerial portion of these plants at the time of anthesis. After spray, half of the plants were subjected to water stress by withholding irrigation. The non stressed plants were irrigated to keep the soil humidity at field capacity. Results showed that drought stress severly reduced the photosynthetic attributes, water status and chlorophyll content which were significantly improved by foliar application of putrescine/benzyladenine. The levels of free proline, amino acids and soluble sugars were higher under water stress conditions which were enhanced further by putrescine/benzyladenine. Memrane injury was also reduced by both the chemicals. Yield and yield attributes reduced under water stress conditions, but putrescine and benzyladenine treated plants exhibited significantly higher values over control. Most of these parameters were found significantly correlated with grain yield. It is suggested that both benyzladenine and putrescine were able to impart drought tolerance in wheat but the response of putrescine was more promising owing to better management of various physio-biochemical processes, particularly under water stress conditions.     

Molecular dissection of variation in carbohydrate metabolism related to water-soluble carbohydrate accumulation in stems of wheat

Water-soluble carbohydrates (WSCs; composed of mainly fructans, sucrose [Suc], glucose [Glc], and fructose) deposited in wheat (Triticum aestivum) stems are important carbon sources for grain filling. Variation in stem WSC concentrations among wheat genotypes is one of the genetic factors influencing grain weight and yield under water-limited environments. Here, we describe the molecular dissection of carbohydrate metabolism in stems, at the WSC accumulation phase, of recombinant inbred Seri/Babax lines of wheat differing in stem WSC concentrations. Affymetrix GeneChip analysis of carbohydrate metabolic enzymes revealed that the mRNA levels of two fructan synthetic enzyme families (Suc:Suc 1-fructosyltransferase and Suc:fructan 6-fructosyltransferase) in the stem were positively correlated with stem WSC and fructan concentrations, whereas the mRNA levels of enzyme families involved in Suc hydrolysis (Suc synthase and soluble acid invertase) were inversely correlated with WSC concentrations. Differential regulation of the mRNA levels of these Suc hydrolytic enzymes in Seri/Babax lines resulted in genotypic differences in these enzyme activities. Down-regulation of Suc synthase and soluble acid invertase in high WSC lines was accompanied by significant decreases in the mRNA levels of enzyme families related to sugar catabolic pathways (fructokinase and mitochondrion pyruvate dehydrogenase complex) and enzyme families involved in diverting UDP-Glc to cell wall synthesis (UDP-Glc 6-dehydrogenase, UDP-glucuronate decarboxylase, and cellulose synthase), resulting in a reduction in cell wall polysaccharide contents (mainly hemicellulose) in the stem of high WSC lines. These data suggest that differential carbon partitioning in the wheat stem is one mechanism that contributes to genotypic variation in WSC accumulation.     

Fructan metabolism in wheat in alternating warm and cold temperatures

The objective of this research was to develop a system in which the direction of fructan metabolism could be controlled. Three-week-old wheat seedlings (Triticum aestivum L. cv Caldwell) grown at 25°C were transferred to cold temperature (10°C) to induce fructan synthesis and then were transferred to continuous darkness at 25°C after defoliation and fructan degradation monitored. The total fructan content increased significantly 1 day after transferring from 25°C to 10°C in both leaf blades and the remainder of the shoot tissue, 90% of which was leaf sheath tissue. Leaf sheaths contained higher concentrations of fructan and greater portions of high molecular weight fructan than did leaf blades. Fructan content in leaf sheaths declined rapidly and was gone completely within 48 hours following transfer to 25°C in darkness. In leaf blades the invertase activity fluctuated during cold treatment. The activity of sucrose:sucrose fructosyl transferase increased markedly during cold treatment, while fructan hydrolase activity decreased slightly. In leaf sheaths, however, the activity of invertase decreased rapidly upon transfer to cold temperature and remained low. Trends in sucrose:sucrose fructosyl transferase and hydrolase activity in sheaths were the same as those of leaf blades. Sheath invertase and hydrolase activity increased when plants were transferred back to darkness at 25°C, while sucrose:sucrose fructosyl transferase activity decreased. These results indicate that changing leaf sheath temperature can be utilized to control the direction of fructan metabolism and thus provide a system in which the synthesis or degradation of fructan can be examined.     

Polymorphism in miR-31 and miR-584 binding site in the angiotensinogen gene differentially influences body fat distribution in both sexes

Angiotensinogen (AGT), its active fragments and microRNA-31 (miR-31) play an important role in adipocyte differentiation. AGT contains a miR-31 polymorphic binding site. We hypothesize that the rs7079 polymorphism in the miR-31/584 binding site of the AGT gene could influence body fat distribution. A total of 751 subjects (195 men, 556 women) were enrolled in the study. The rs7079 genotypes were determined by qRT-PCR. Anthropometric measurements were taken on all subjects, who were subsequently divided into two groups: obese (>30 kg m⁻²) and non-obese (<30 kg m⁻²). Linear regression models were created to determine the contributions of sex, obesity status and rs7079 to all measured parameters. Adding the rs7079 genotype significantly contributed to the linear regression model for waist circumference (p = 0.013), hip circumference (p = 0.018) and supraspinal skin-fold thickness (p = 1 × 10⁻³). Differences between sexes and between the obese and non-obese groups were observed. Waist circumference was lower in men carrying the A allele (p = 0.022); hip circumference was higher only in obese women carrying the A allele (p = 0.015). While men carrying the A allele had lower supraspinal skin-fold thickness (p = 0.022), this parameter was found to be higher in A allele carrying women (p = 3 × 10⁻³). The higher total sum of skin-fold thickness in A allele carrying women was restricted to obese individuals (p = 0.028). The presence of the A allele was associated with both lower tricipital skin-fold thickness in non-obese women (p = 0.023) and a trend of higher thickness in non-obese men (p = 0.065). Significant associations of rs7079 in the AGT gene and body fat distribution were observed. The distribution followed opposing patterns in both sexes.     

Genome-Wide Association Mapping of Yield and Grain Quality Traits in Winter Wheat Genotypes

The main goal of this study was to investigate the genetic basis of yield and grain quality traits in winter wheat genotypes using association mapping approach, and identify linked molecular markers for marker assisted selection. A total of 120 elite facultative/winter wheat genotypes were evaluated for yield, quality and other agronomic traits under rain-fed and irrigated conditions for two years (2011–2012) at the Tel Hadya station of ICARDA, Syria. The same genotypes were genotyped using 3,051 Diversity Array Technologies (DArT) markers, of which 1,586 were of known chromosome positions. The grain yield performance of the genotypes was highly significant both in rain-fed and irrigated sites. Average yield of the genotypes ranged from 2295 to 4038 kg/ha and 4268 to 7102 kg/ha under rain-fed and irrigated conditions, respectively. Protein content and alveograph strength (W) ranged from 13.6–16.1% and 217.6–375 Jx10-4, respectively. DArT markers wPt731910 (3B), wPt4680 (4A), wPt3509 (5A), wPt8183 (6B), and wPt0298 (2D) were significantly associated with yield under rain-fed conditions. Under irrigated condition, tPt4125 on chromosome 2B was significantly associated with yield explaining about 13% of the variation. Markers wPt2607 and wPt1482 on 5B were highly associated with protein content and alveograph strength explaining 16 and 14% of the variations, respectively. The elite genotypes have been distributed to many countries using ICARDA’s International system for potential direct release and/or use as parents after local adaptation trials by the NARSs of respective countries. The QTLs identified in this study are recommended to be used for marker assisted selection after through validation using bi-parental populations.     

Sucrose Utilization via Invertase and Sucrose Synthase with Respect to Accumulation of Cellulose and Callose Synthesis in Wheat Roots under Oxygen Deficiency

The sucrose cleavage by sucrose synthase (SuSy) and neutral invertase was studied in wheat roots (Triticum aestivum L.) subjected to hypoxia or anoxia for 4 days. By in situ activity staining, increased SuSy activity was observed in the tip region and stele of root axes while the activity of invertase decreased. Cellulose content significantly increased in hypoxically treated roots. The cellulose deposition was correlated with regions of high SuSy activity, being mainly located in the pericycle and endodermis. Invertase activity was distributed along the root without clear difference between cortex and stele. Under root hypoxia, a significant increase in the structural carbohydrates, callose and especially cellulose, was shown. Increasing levels of soluble carbohydrates were partially used to synthesize cellulose for secondary wall thickening and callose to counteract the tissue injury following low-oxygen stress. Under strict anoxia, the roots were much more injured but sustained a high level of cellulose and callose while the soluble carbohydrates almost disappeared.     

Metabolic changes associated with vernalization of wheat. I. Carbohydrate and nitrogen patterns

A spring wheat (Triticum aestivum) and an obligate winter wheat (Triticum compactum) variety were each grown for 5 weeks in controlled environments at 2° and 25°. The threshold for flower induction in the winter wheat was 4 to 5 weeks at 2°, whereas the spring wheat had no low temperature requirement for flowering. Changes in the levels of carbohydrate and nitrogen fractions in the wheat leaves were determined during their growth in the cold and warm environments. There was an enhanced accumulation of the 5 carbohydrate fractions in both wheat varieties grown at 2° compared to 25°. Highly significant differences in the levels of sucrose, oligosaccharides, and starch were found between the spring and winter varieties grown at 2°. The winter wheat seedlings grown at 2° accumulated much more of these carbohydrates than the corresponding spring wheat. The carbohydrate patterns in both varieties grown at 25° were nearly identical except for the final 2 weeks of growth.

The level of nitrogenous substances in the tissues grown at 2° was much higher than in the corresponding tissues grown at 25°. The only significant difference between the spring and winter varieties was in the soluble protein fraction. This fraction rose nearly 3-fold in the winter variety grown at 2°, whereas it remained nearly constant in the similarly grown spring wheat. Most of the changing chemical patterns observed in relation to the vernalization treatment appear to be metabolic alterations associated with low temperature rather than alterations directly related with the vernalization response.

     

Salinity and drought interaction in wheat (Triticum aestivum L.) is affected by the genotype and plant growth stage

Salinity and drought are important agro-environmental problems occurring separately as well as together with the combined occurrence increasing with time due to climate change. Screening of bread wheat genotypes against salinity or drought alone is common; however, little information is available on the response of wheat genotypes to a combination of these stresses. This study investigates the response of a salt-resistant (SARC-1) and a salt-sensitive (7-Cerros) wheat genotype to drought at different growth stages under non-saline (ECe 2.1 dS m^?1) and saline soil (ECe 15 dS m^?1) conditions. Drought was applied by withholding water for 21 days at a particular growth stage viz. tillering, booting, and grain filling stages. At booting stage measurements regarding water relations, leaf ionic composition and photosynthetic attributes were made. At maturity grain yield and different yield, components were recorded. Salinity and drought significantly decreased grain yield and different yield components with a higher decrease in the case of combined stress of salinity × drought. The complete drought treatment (drought at tillering + booting + grain filling stages) was most harmful for wheat followed by drought at booting stage and grain filling–tillering stages, respectively. The salt-resistant wheat genotype SARC-1 performed better than the salt-sensitive genotype 7-Cerros in different stress treatments. A decrease in the water and turgor potentials, photosynthetic and transpiration rates, stomatal conductance, leaf K⁺, and increased leaf Na⁺ were the apparent causes of growth and yield reduction of bread wheat due to salinity, drought, and salinity × drought.     

Plant Morphological and Biochemical Responses to Field Water Deficits: III. Effect of Foliage Temperature on the Potential Activity of Glutathione Reductase

Activity of glutathione reductase has been related to stress tolerance; however, these enzyme assays are generally conducted at 25°C. Foliage temperature varies greatly in the field in response to soil water availability and ambient conditions and this may affect enzyme response. This study was conducted to determine the effect of changing foliage temperature on glutathione reductase activity of wheat under field conditions. Wheat leaf glutathione reductase was purified and the temperature response of the enzyme was determined at 2.5°C intervals between 12.5 and 45°C. These data, in conjunction with continuous measurements of field-grown wheat foliage temperatures, were used to compare the temperature-related changes in potential glutathione reductase activities in water stressed and control plants. Assuming saturating substrate levels, the results indicate that early in the season the daily potential enzyme activity of the irrigated and stressed plants could never have reached the daily activity predicted from the 25°C (room temperature) measurements. Later in the season, the daily potential activity of the irrigated plants was lower, and the daily potential activity of the stressed plants was higher, than the activities predicted from the 25°C determinations. These results suggest that a better understanding of the regulation of plant metabolism will be obtained by linking continuous temperature measurements of plant foliage with enzyme responses to temperature.     

Evaluation of Wheat Chromosome Translocation Lines for High Temperature Stress Tolerance at Grain Filling Stage

High temperature (HT, heat) stress is detrimental to wheat (Triticum aestivum L.) production. Wild relatives of bread wheat may offer sources of HT stress tolerance genes because they grow in stressed habitats. Wheat chromosome translocation lines, produced by introgressing small segments of chromosome from wild relatives to bread wheat, were evaluated for tolerance to HT stress during the grain filling stage. Sixteen translocation lines and four wheat cultivars were grown at optimum temperature (OT) of 22/14°C (day/night). Ten days after anthesis, half of the plants were exposed to HT stress of 34/26°C for 16 d, and other half remained at OT. Results showed that HT stress decreased grain yield by 43% compared with OT. Decrease in individual grain weight (by 44%) was the main reason for yield decline at HT. High temperature stress had adverse effects on leaf chlorophyll content and Fv/Fm; and a significant decrease in Fv/Fm was associated with a decline in individual grain weight. Based on the heat response (heat susceptibility indices, HSIs) of physiological and yield traits to each other and to yield HSI, TA5594, TA5617, and TA5088 were highly tolerant and TA5637 and TA5640 were highly susceptible to HT stress. Our results suggest that change in Fv/Fm is a highly useful trait in screening genotypes for HT stress tolerance. This study showed that there is genetic variability among wheat chromosome translocation lines for HT stress tolerance at the grain filling stage and we suggest further screening of a larger set of translocation lines.     

黄瓜幼苗非结构性碳水化合物代谢对干旱胁迫与CO2倍增的响应

以‘津优1号’黄瓜水培幼苗为试材,采用裂区设计,主区设大气CO₂浓度(约380 μmol·mol^-1)和倍增CO₂浓度(760±20 μmol·mol^-1)2个CO₂浓度处理,裂区设无干旱胁迫、中度干旱胁迫和重度干旱胁迫3个水分处理(以PEG 6000模拟根际干旱胁迫),研究了黄瓜幼苗非结构性碳水化合物代谢对干旱胁迫和CO₂倍增的响应.结果表明: CO₂倍增促进了黄瓜叶片中非结构性碳水化合物(葡萄糖、果糖、蔗糖、水苏糖)的积累,降低了渗透势,提高了黄瓜的耐旱性.在干旱胁迫处理过程中,叶片中蔗糖合成酶、可溶性酸性转化酶和碱性转化酶活性先上升后下降;根中可溶性酸性转化酶和碱性转化酶活性则逐渐上升,蔗糖磷酸合成酶活性先上升后下降.CO₂倍增提高了蔗糖合成酶的活性而降低了蔗糖磷酸合成酶的活性,这两种酶和转化酶相互配合,促进了蔗糖的分解和抑制蔗糖合成,导致己糖积累,从而降低了细胞的渗透势,增强吸水能力.因此,CO₂倍增能缓解干旱胁迫造成的不利影响,提高黄瓜的耐旱性,并且这种缓解效应在干旱胁迫严重时表现更为明显.
     

Genome-wide association mapping of yield and yield components of spring wheat under contrasting moisture regimes

Key message

A stable QTL that may be used in marker-assisted selection in wheat breeding programs was detected for yield, yield components and drought tolerance-related traits in spring wheat association mapping panel.

Abstract

Genome-wide association mapping has become a widespread method of quantitative trait locus (QTL) identification for many crop plants including wheat (Triticum aestivum L.). Its benefit over traditional bi-parental mapping approaches depends on the extent of linkage disequilibrium in the mapping population. The objectives of this study were to determine linkage disequilibrium decay rate and population structure in a spring wheat association mapping panel (n = 285–294) and to identify markers associated with yield and yield components, morphological, phenological, and drought tolerance-related traits. The study was conducted under fully irrigated and rain-fed conditions at Greeley, CO, USA and Melkassa, Ethiopia in 2010 and 2011 (five total environments). Genotypic data were generated using diversity array technology markers. Linkage disequilibrium decay rate extended over a longer genetic distance for the D genome (6.8 cM) than for the A and B genomes (1.7 and 2.0 cM, respectively). Seven subpopulations were identified with population structure analysis. A stable QTL was detected for grain yield on chromosome 2DS both under irrigated and rain-fed conditions. A multi-trait region significant for yield and yield components was found on chromosome 5B. Grain yield QTL on chromosome 1BS co-localized with harvest index QTL. Vegetation indices shared QTL with harvest index on chromosome 1AL and 5A. After validation in relevant genetic backgrounds and environments, QTL detected in this study for yield, yield components and drought tolerance-related traits may be used in marker-assisted selection in wheat breeding programs.     

Purification and Characterization of Wheat beta(2-->1) Fructan:Fructan Fructosyl Transferase Activity

Fructans are the major storage carbohydrate in vegetative tissues of wheat (Triticum aestivum L.). Fructan:fructan fructosyl transferase (FFT) catalyzes fructosyl transfer between fructan molecules to elongate the fructan chain. The objective of this research was to isolate this activity in wheat. Wheat (cv Caldwell) plants grown at 25°C for 3 weeks were transferred to 10°C to induce fructan synthesis. From the leaf blades kept at 10°C for 4 days, fructosyl transferase activity was purified using salt precipitation and a series of chromatographic procedures including size exclusion, anion-exchange, and affinity chromatography. The transferase activity was free from invertase and other fructan-metabolizing activities. Fructosyl transferase had a broad pH spectrum with a peak activity at 6.5. The temperature optimum was 30°C. The activity was specific for fructosyl transfer from β(2→1)-linked 1-kestose or fructan to sucrose and β(2→1) fructosyl transfer to other fructans (1-FFT). Fructosyl transfer from oligofructans to sucrose was most efficient when 1-kestose was used as donor molecule and declined as the degree of polymerization of the donor increased from 3 to 5. 1-FFT catalyzed the in vitro synthesis of inulin tetra- and penta-saccharides from 1-kestose; however, formation of the tetrasaccharide was greatly reduced at high sucrose concentration. 6-Kestose could not act as donor molecule, but could accept a fructosyl moiety from 1-kestose to produce bifurcose and a tetrasaccharide having a β(2→1) fructose attached to the terminal fructose of 6-kestose. The role of this FFT activity in the synthesis of fructan in wheat is discussed.     

Sucrose Metabolism in Tubers of Potato (Solanum tuberosum L.): Effects of Sink Removal and Sucrose Flux on Sucrose-Degrading Enzymes

Excision of developing potato (Solanum tuberosum L.) tubers from the mother plant, followed by storage at 10°C, resulted in a rapid, substantial decrease in sucrose synthase activity and considerable increases in hexose content and acid invertase activity. A comparison of the response of three genotypes, known to accumulate different quantities of hexoses in storage, showed that both sucrose synthase activity and the extent to which activity declined following excision were similar in all cases. However, there was significant genotypic variation in the extent to which acid invertase activity developed, with tubers accumulating the highest hexose content also developing the highest extractable activity of invertase. Similar effects were found in nondetached tubers when growing plants were maintained in total darkness for a prolonged period. Furthermore, supplying sucrose to detached tubers through the cut stolon surface prevented the decline in sucrose synthase activity. Maltose proved to be ineffective. Western blots using antibodies raised against maize sucrose synthase showed that the decline in sucrose synthase activity was associated with the loss of protein rather than the effect of endogenous inhibitors. Although there were indications that maintaining a flux of sucrose into isolated tubers could prevent the increase in acid invertase activity, the results were not conclusive.     

Effect of dietary carbohydrate source on soluble protein glucose concentration and enzyme activity (aldolase) of European eels (Anguilla anguilla L.)

This study was undertaken to determine the influence of dietary carbohydrate sources: wheat meal, bread meal, soluble corn starch, native potato starch and sorghum meal, on soluble protein, enzyme activity (aldolase) and glucose concentration in muscle and liver of European eels (Anguilla anguilla). There was less soluble protein in both muscle and liver of eels fed 30% wheat meal or bread meal than the other experimental groups. However, eels fed 30% bread meal or soluble corn starch had a higher glucose concentration in muscle and liver than the other experimental groups. High enzyme activity (aldolase) was found in the liver of eels fed 30% wheat meal, bread meal or soluble starch.     

SUCROSE INVERSION BY BAKERS' YEAST AS A FUNCTION OF TEMPERATURE

Inversion of sucrose by bakers'' yeast follows the same course as inversion catalyzed by yeast invertase. Rate of inversion increases exponentially with temperature; the temperature characteristic in the Arrhenius equation is 10,700 below 13–17°C., and 8,300 above that temperature. Temperature inactivation occurs above 40°C. The effects of temperature upon rate of inversion were the same using Fleischmann''s yeast cake, the same yeast killed with toluene, and a pure strain (G. M. No. 21062) of bakers'' yeast. The last differed from the other two only in the fact that its critical temperature was 13°C. as compared with 17°C. for the others. The catalytic inversion is associated with enzyme activity inside the cell, not in the medium, and is independent of any vital processes inside the cell such as respiration and fermentation. Since invertase activity is the same inside the cell as it is after extraction, it appears possible to relate the temperature characteristics for physiological processes to the catalytic chemical systems which determine their rate. At least two enzymes are capable of inverting sucrose in the yeast cell. The familiar yeast invertase (µ = 10,700) is active below 13–17°C. while a second enzyme (M = 8,300) plays the dominant role above that temperature.     

Soluble acid invertase activity in leaves is independent of species differences in leaf carbohydrates, diurnal sugar profiles and paths of phloem loading

Leaf sucrose, starch, hexose and maximum extractable soluble acid invertase activity were compared throughout the day in source leaves of 13 plant species chosen for their putative phloem-loading type (apoplastic or symplastic). Four species which represent the different phloem-loading types (tomato, barley, maize and Fuchsia ) were studied in detail. Using this information we wished to determine whether a positive correlation between foliar carbohydrates and acid invertase activity exists in leaves from different species and, furthermore, whether this relationship is determined by phloem-loading type. Acid invertase activity was relatively constant throughout the day in all species. The extent of sucrose, hexose and starch accumulation and the sucrose: starch ratio measured at a given time were species-dependent. No correlations were found between foliar soluble acid invertase activity and the hexose, sucrose or starch content of the leaves in any of the species, regardless of phloem-loading type. The species examined could be divided into three distinct groups: (1) high sucrose, low invertase; (2) low sucrose, low invertase; and (3) low sucrose, high invertase. The absence of an inverse relationship between leaf sucrose, hexose or starch contents and endogenous soluble acid invertase suggests that this enzyme is not directly involved in carbon partitioning in leaves but serves an auxiliary function.