ABSTRACTIn an attempt to further elucidate the molecular mechanisms that determine the loss of dough strength associated with heat stress of growing wheat, the roles of heat‐shock proteins (HSP) and heat‐shock elements upstream of glutenin genes were investigated. A range of genotypes differed in the extent of synthesis of high molecular weight glutenin subunits (HMW‐GS) and HSP during heat stress. The concentration of HSP 70 remaining in mature grain increased as a result of a few days' heat stress of wheat plants. The amount of HSP 70 in mature grain samples from heat‐stressed plants of 45 genotypes was not strongly correlated with loss of dough strength. There was much less evidence for this mechanism than for other molecular hypotheses from the literature, particularly, changes in glutenin‐to‐gliadin ratio, size distribution of the glutenin polymer, and the involvement of HSP and chaperones during grain‐protein synthesis. HSP 70 was purified from heat‐stressed grain, and was added to (or incorporated into) dough in the direct‐drive mixograph. The HSP behaved similarly to several other hydrophilic proteins when added at a level of 2 mg/2 g of flour. It showed no dramatic effects on dough properties that could constitute a major explanation for the dough‐weakening effects of heat stress, even though the level of addition was well above the maximum levels that might be encountered in field‐grown, mature grain. Furthermore, sequencing of the genes (upstream of the coding region) for HMW‐GS failed to show the presence of heat‐shock promoters, even for genotypes that differed considerably in their reactions to heat stress. The findings simplify the range of possibilities that cause heat‐related loss of dough strength, focusing attention on the degree of polymerization of the glutenin chains, and on the roles of HSP and chaperones in the developing grain.

The effect of CO2 enrichment and irradiance on the growth and gas exchange of two tropical grasses, Panicum laxum (C3) and Panicum antidotale (C4) were investigated. The two species were grown at either 350 (low) or 700 (high) µL L-1 CO2 concentration, under 40% (low) or 100% (high) of direct sunlight and supplied with ample water and nutrition. Elevated CO2 enhanced plant dry weight at both irradiances in the C3 species (1.41-fold and 1.71-fold increase at low and high light, respectively) but only at high light in the C4 species (1.28 fold increase). CO2 enrichment had no effect on the dry weight of P. antidotale, when stem development was suppressed by growth under artificial lighting. When measured at the CO2 concentration at which they were grown, assimilation rates were similar in the low and high CO2 grown plants, for both species. However, when measurements made at low CO2 were compared, CO2 assimilation rates of the high light, high CO2 grown C3 and C4 species were lower than those of their low CO2 grown counterparts. High CO2 strongly reduced the stomatal conductance of both species, while it affected the Rubisco content (30% decrease) of the high light C3 species only. This work shows clearly that C4 species can respond to CO2 enrichment under favourable growth conditions, and that acclimation to elevated CO2 in pasture grasses does not necessarily involve accumulation of non-structural carbohydrates or reduction of total N or soluble proteins in source leaves.

SummaryMany studies have reported the effect of crop load on leaf water relations of fruit trees but none have dealt with the effect of crop load on water relations of the fruit and how such an effect might influence fruit growth. This is of interest because it could improve our understanding of the relationship between crop load and fruit size. In this study, we measured fruit water potential, fruit osmotic potential, fruit turgor potential and fruit growth, and leaf water potential and leaf conductance on nine-year-old ‘Yanco Queen’ peach ‘Prunus persica (L) Batsch’ trees carrying different levels of crop load. The trees were trained on Tatura trellis and had been either left unthinned (heavy crop load: 450 fruit per tree in 1986 and 560 fruit per tree in 1987) or were thinned (light crop load: 310 fruit per tree). Trees carrying the heavy crop load had lower fruit water potential and higher fruit osmotic potential, and consequently lower fruit turgor potential compared with trees carrying a light crop ...

Elevated levels of atmospheric CO2 have been shown to increase grain yield and reduce grain nitrogen concentration. The object of this study was to determine whether elevated CO2 levels would modify other aspects of grain quality relevant to processing, particularly protein and starch quality. Wheat of two genotypes (Hartog and Late Hartog) was grown in the field in controlled-atmosphere tunnels at either the ambient level of CO2 (350 mu l/L) or an elevated level (700 mu l/L). This elevated level of CO2 produced significant increases in grain yield, but decreases in 1,000-kernel weight. Grain grown in the elevated CO2 atmosphere produced poorer dough and decreased loaf volume, farinograph development time, and dough extensibility. These changes were largely attributable to the lower protein content of the grain grown at elevated CO2. There did not appear to be major changes in protein composition or in the functional properties of the protein. Grain produced at elevated CO2 yielded starch with a significantly higher proportion of large (A-type) starch granules but no overall change in amylose-to-amylopectin ratio. These studies indicate that elevated levels of CO2 may result in decreased quality of bread wheats largely due to lowered protein content.

When wheat coleoptiles or plants are subjected to a period of heat stress (e.g. at > 35 degrees C for 1 h or more), there is a reduction in normal protein synthesis, accompanied by de nova synthesis of the classical range of heat-shock proteins (based on radioactive tracer experiments) in virtually all parts of the plant. Study of coleoptile elongation rates indicates that this synthesis is related to a protective effect, whereby a preliminary heat shock provides a degree of protection against a later lethal shock. This thermotolerance is also associated with the appearance in coleoptiles and roots of a small peptide (detected without radioactive labelling) whose amino acid sequence (12 residues) is the same as the N-terminal sequence of the alpha- and beta-gliadin proteins of the endosperm.Heat stress during grain filling lends to important changes in the synthesis of gluten proteins with reduced synthesis of the high molecular weight (HMW) subunits of glutenin, and continuing synthesis of other gluten proteins, particularly various gliadin proteins. This latter group of polypeptides is thus presumed to be acting as heat-shock proteins, and indeed, multiple heat-shock elements are present in the published sequences of representative genes, up-stream of the coding regions.HPLC analysis (with or without radioactive labelling) shows that there is a resulting change from the normal balance of gluten polypeptides immediately after the shock as well as in the mature grain. As a result, there is a lower proportion of large-sized aggregates of glutenin and weaker dough properties. This scenario indicates that it should be possible to identify genotypes that would be tolerant to stress-related variations in quality by analysis of gluten composition and, at the gene level, by screening for heat-stress elements in the genes encoding HMW-glutenin subunits. In addition, heat stress modifies the particle size distribution of the starch fraction of mature grain, producing an increase in the proportion of large (A-type) starch granules. No change in chemical structure was detectable as a result of heat stress.

There have been a few notable occasions when the Australian wheat segregation system (mainly based on specification of variety and protein content) has failed to produce grain which gives dough properties expected for the wheat grade. The reasons for this are likely to relate to growing and storage conditions; of these, variations in temperature during grain filling appear to be a major factor. Observations of crop statistics, field and glasshouse experiments indicate that as growth temperatures increase up to 30°C, there is a general increase in dough strength (as indicated by Extensograph maximum resistance, Rmax, and as Farinograph development time and stability). However, a decline in dough strength is observed following periods of heat stress (e.g. a few days with maxima of over 35°C). Increasing temperatures during grain filling have also been observed to produce grain with a higher protein content, but this observation is not as consistent nor as marked as the effects on dough strength. We have sought to identify genotypes that do not follow this general trend in response to heat stress, and thus could be used as parents to breed for heat tolerance and greater stability of dough quality. A glasshouse experiment involving 45 genotypes has indicated that there is some variation in the response to heat stress, with a few genotypes being promising sources of tolerance. A second important approach to minimising the effects of heat stress is to develop a model to predict grain-quality changes, thus enabling a marketing authority to be forewarned of significant variation from the quality attributes normally expected for a wheat grade, and assisting breeders to better interpret the results of quality testing of lines grown at various sites.

High temperatures (e.g. daily maxima over 35°C occurring during grain filling) have been identified as major source of variation in dough properties in wheat. Such conditions may occur in the wheat-cropping areas of many countries with sufficient frequency for this phenomenon to be a significant marketing factor. The association of heat stress with dough weakening was indicated by analysis of historical data from crop statistics, cultivar trials and field experiments. Glasshouse and growth-cabinet experiments have further validated the conclusions, and have led to the formulation of a molecular mechanism to explain these results. This involves the identification of heat-shock elements upstream of the coding region of certain gliadin proteins, but not for glutenin polypeptides. As a result, gliadin synthesis continues at a greater rate than glutenin synthesis during a period of heat stress. Consequently, the mature grain has a higher ratio of gliadin: glutenin and produces weaker dough. These results now provide a basis for formulating strategies to minimize variations in dough properties due to growing conditions.

The effect of drought stress on the non-structural carbohydrate (NSC) composition and water relations of the wheat (Triticum aestivum L.) stem has been investigated. Five separate parts of the stem were sampled: the unsheathed portion of the peduncle (Stem 1a), the sheathed portion of the peduncle (Stem 1b), the penultimate internode (Stem 2), the lower internodes (Stem 3 + 4) and the flag-leaf-pulvinus (pulvinus). The NSC was analysed as two fractions, an ethanol-soluble carbohydrate (ESC) fraction containing mono- and di-saccharides and some low molecular weight oligosaccharides, and a water-soluble carbohydrate (WSC) fraction containing mostly fructans and some high molecular weight oligosaccharides.In Stems 1b, 2 and 3 + 4, the imposition of drought midway through grain filling resulted in a shift in soluble carbohydrate from the WSC to the ESC fraction indicating hydrolysis of fructans. In Stem 2 on day 29 of grain filling, the WSC/ESC ratio was 7.6 +/- 1.5 in well watered plants in contrast to 0.5 +/- 0.1 in droughted plants on day 30, even though NSC concentration did not differ. The NSC content of Stem 1a and the pulvinus increased threefold under drought, although levels were significantly lower than in the rest of the stem. The WSC/ESC ratio in these tissues was low compared to the rest of the stem but still declined noticeably under drought stress. Turgor (P) was fully maintained in Stem 2 and the pulvinus of droughted plants. Under drought, P in the pulvinus was maintained at higher levels (1.9-2.4 MPa) than in Stem 2 (0.8-1.2 MPa).

Wheat plants exposed to higher than usual temperatures during ripening produced grain with weaker dough properties in glasshouse, field experiments and crop samples. In a review of Prime Hard wheat samples from 1960/61 to 1988/89, those seasons when the dough properties were particularly weak coincided with the years when the number of hours over 35-degrees-C during the grain filling period (October to December) was greatest.A five-day period of heat stress in 1988 provided an opportunity to directly investigate the effects of heat stress in the field. A weakening of dough properties was shown, for four varieties, by longer dough development times and faster breakdown in the Farinograph and also by shorter resistance to extension (at 5 cm) in the Extensograph.These (and similar changes for glasshouse grown grain) were accompanied by an increase in the proportion of gliadin (monomeric) proteins. That this increase was associated with the heat stress was shown by demonstrating increased accumulation of C-14 amino acids into the gliadin fraction for heat-stressed heads in culture.These results support the hypothesis that episodes of high temperature during grain filling activate the heat shock elements of gliadin genes in wheat causing the mature grain to contain more gliadin and thus to produce weaker doughs.

Grain quality results for variety trials extending over 27 years (3 sites and 5 varieties) were compared with the temperature profiles during the grain filling period (56 days prior to harvest) to determine the effects on quality of high temperatures (> 35-degrees-C) during this period of growth. Heat stress episodes have been frequent at two (Narrabri, N.S.W., and Turretfield, S.A.) of the three sites studied; spring temperatures were more moderate at the third site, Wongan Hills, W.A. There were highly significant (P < 0.01) correlations of heat stress (as hours above 35-degree-C, during grain filling) with protein content (positive) and with grain yields (negative) at Narrabri for all varieties. In many combinations of site and variety, heat stress correlated negatively with loaf volume, and with dough strength (as R(max, resistance to stretching with the Extensograph). Heat stress episodes in the Narrabri (N.S.W.) region in 1981 and 1982 gave further opportunity to examine these relationships. Results showed very clearly that high temperatures late in grain filling were associated with weaker dough properties (lower R(max) in the resulting grain. These trends may form the basis of a predictive system by which to estimate crop quality and to interpret the results of variety trials.

Four half-sib families of Pinus radiata (D. Don) (20080, 20010, 20022 and 20062) were grown in pots under conditions where P was either deficient or adequate and water was supplied daily. Seedlings of all four families were exposed to 340 (ambient levels) or 660 μl CO2l−1 in controlled-environment chambers for 16 weeks. Families 20010 and 20062 were grown at the same CO2 concentrations for 22 weeks. A subset of plants grown in the chambers were subject to drought and their water potentials and relative water contents measured. In the glasshouse experiment, photosynthesis, conductance and elongation of needles were measured. The fresh and dry-weights of the tops of the plants were measured in both experiments.

Unpredictable temperatures and rainfall associated with climate change are expected to affect wheat (Triticum aestivum L.) production in various countries. The development of climate-resilient spring wheat cultivars able to maintain grain yield and quality is essential to food security and economic returns. We tested 54 CIMMYT spring bread wheat genotypes, developed and/or released over a span of 50 years, in the field for two years under optimum sowing dates, as well as using two delayed sowing dates to expose crops to medium and severe heat-stress conditions. The grain yield and yield components were severely affected as the heat-stress increased. Two contrasting groups of 10 lines each were identified to determine the effect of heat-stress on bread-making quality. The first set included entries that produced high yields in optimal conditions and maintained higher yields under heat-stress (superior-yielding lines), while the second set included genotypes that did not perform well in the environment with high temperature (inferior-yielding lines). We identified genotypes exhibiting bread-making quality stability, as well as the quality traits that had higher correlation with the loaf volume in the environment without stress and under heat-stress. Of all the quality traits tested, dough extensibility (AlvL) and grain protein content had a significant influence in heat-stress adaptation. Most of the lines from the superior-yielding group were also able to maintain and even improve quality characteristics under heat-stress and therefore, could be used as parents in breeding to develop high-yielding and stable quality wheat varieties.

Detached wheat heads (7 days post-anthesis) were grown in liquid culture containing nitrogen concentrations of 0.025% or 0.1% in the form of glutamine, ammonium nitrate or asparagine. With each form of the nitrogen, increasing the concentrations of nitrogen in the culture medium led to increases in the total nitrogen and the non-protein nitrogen in the grain. Protein contents (N × 5.7) were approximately 16% and 21% on a dry weight basis in the low and high treatments respectively for all nitrogen sources. Amino acids from the endosperm cavity, the ethanol-soluble extract of the grain, and the grain protein were analysed by HPLC techniques to define the site of transfer between amino acid forms. The results indicated that amino nitrogen from glutamine, ammonium nitrate, or asparagine enters the grain and is found in the endosperm cavity fluid mainly in the form of glutamine, alanine and, to a lesser extent, aspartate (including asparagine). These amino acids are then converted into the various other amino acids required for protein synthesis, as is demonstrated by the increases found in the others in the ethanol-soluble free amino acid fraction with different nitrogen regimes. These variations in the composition of the free amino acids occurred without altering the amino acid composition of the protein component of the grain.

A low molecular weight peptide, induced by exposure of coleoptiles to heat stress at 41°C, has been detected by reversed-phase high performance liquid chromatography of extracts from coleoptiles of five wheat (Triticum aestivum) cultivars. This component is detected within 1 h of a 41°C heat shock, is not detected 48 h after cessation of the heat shock, and remains present during a continuous 24 h heat treatment. The appearance of this component is also induced by exposure of the coleoptile to 0.1 M sodium arsenite or 10% ethanol. When other species such as barley (Hordeum vulgare), soybean (Glycine max), sorghum (Sorghum bicolor), maize (Zea mays), mungbean (Vigna radiata), or rice (Oryza sativa) were examined for the presence of a component eluting in the same position, it was only detected in maize. The amino acid sequence for the heat-induced peptide from wheat was determined to be: V-L-V-P-V-P-Q-L-Q-P-Q-N-Q-P/Q. The sequence of 12 of these amino acids is the same as the N-terminal sequence of α- and β gliadins (wheat endosperm storage proteins). The production of this heat-induced peptide in aneuploids of Chinese spring wheat indicated that the peptide gene was located on the same chromosome arm as one of the gliadin genes. The presence of this gliadin-like peptide in heat-stressed coleoptiles may be due to the presence of five heat shock elements in the gene sequence of gliadins. The potential heat inducibility of the gliadin gene has important implications for end-use quality of wheat. The results also imply that seed proteins may have a function other than storage of nitrogen.

Abstract Advanced selections (families 20010 and 20062) of P. radiata D. Don were exposed to either 340 or 660 μmol CO2 mol 1 for 2 years to establish if growth responses to high CO2 would persist during the development of woody tissues. The experiment was carried out in glasshouses and some of the trees at each CO2 concentration were subjected to phosphorus deficiency and to periodic drought. CO2 enrichment increased whole‐plant dry matter production irrespective of water availability, but only when phosphorus supply was adequate. The greatest increase occurred during the exponential period of growth and appeared to be tied to increased rates of photosynthesis, which caused accelerated production of leaf area. The increase in whole‐plant dry matter production was similar for both families; however, family 20010 partitioned larger amounts of dry weight to the trunks than family 20062. which favoured the roots and branches. Wood density was generally increased by elevated CO2 and for family 20010 this increase was due to thickening of the tracheid walls. Tracheid length was similar at both CO2 levels but differed between families. These results suggest that, as the atmospheric CO2 concentration rises, field‐grown P. radiata should produce more dry weight at sites where phosphorus is not acutely deficient, even where drought limits growth; however, increases in wood production are likely only for genotypes which continue to partition at least the same proportion of dry weight to wood in the trunk.

The exposure of wheat (Triticum aestivum) coleoptiles to a transient high temperature stress results in the synthesis of a group of proteins known as the heat shock proteins (hsps). The appearance of these proteins is associated with a concomitant reduction in normal protein synthesis and has been correlated with the acquisition of thermotolerance (assessed as growth of coleoptiles). Pretreatment with a sublethal heat shock confers protection to a subsequent heat shock that would otherwise have been lethal. In addition, we find that increasing the time interval between the sublethal heat treatment and the subsequent heat shock from 0 to 72 h reduces the protective effect of the sublethal heat treatment considerably. The five cultivars examined (Sunelg, Sunco, Hartog, Vulcan, Halberd) showed differences in the degree of protection acquired, and in the length of time for which protection was maintained. Hartog was found to be the most thermotolerant, and acquired the greatest degree of protection from exposure to a sublethal heat treatment, but the duration of this acquired protection was shorter than in the remaining cultivars. Sunelg was most susceptible to a heat shock but the duration of acquired protection was the greatest.

Abstract. Seedlings of Pinus radiata D. Don were grown in growth chambers for 22 weeks with two levels of phosphorus, under either well‐watered or water‐stressed conditions at CO2 concentrations of either 330 or 660mm3 dm−3. Plant growth, water use efficiency and conductance were measured and the relationship between these and needle photosynthetic capacity, water use efficiency and conductance was determined by gas exchange at week 22. Phosphorus deficiency decreased growth and foliar surface area at both CO2concentrations; however, it only reduced the maximum photosynthetic rates of the needles at 660 mm3 CO2 dm−3 (plants grown and measured at the same CO2 concentration). Water stress reduced growth and foliar surface area at both CO2 concentrations. Increases in needle photosynthetic rates appeared to be partly responsible for the increased growth at high CO2 where phosphorus was adequate. This effect was amplified by accompanying increases in needle production. Phosphorus deficiency inhibited these responses because it severely impaired needle photosynthetic function. The relative increase in growth in response to high CO2 was higher in the periodically water‐stressed plants. This was not due to the maintenance of cell volume during drought. Plant water use efficiency was increased by CO2 enrichment due to an increase in dry weight rather than a decrease in shoot conductance and, therefore, transpirational water loss. Changes in needle conductance and water use efficiency in response to high CO2 were generally in the same direction as those at the whole plant level. If the atmospheric CO2 level reaches the predicted concentration of 660 mm3 dm−3 by the end of next Century, then the growth of P. radiata will only be increased in areas where phosphorus nutrition is adequate. Growth will be increased in drought‐affected regions but total water use is unlikely to be reduced.

The reactivity of leaf growth to changes in plant water status has been analysed in terms of leaf development, water transport and turgor. The different growth patterns of monocotyledonous and dicotyledonous leaves result in fundamental differences in the water relations of expanding leaves. Most monocotyledonous leaf cells complete their expansion phase within the protective older leaf bases, while the majority of dicotyledonous leaf cells expand in an exposed evaporative environment. The consequence of this morphological difference is that expanding monocotyledonous leaves behave similarly to other enclosed tissue during water stress by exhibiting turgor maintenance through osmotic adjustment. Expanding dicotyledonous leaves do not exhibit this response. The maintenance of turgor in monocotyledons in the absence of leaf expansion suggests that growth is controlled by the yield threshold of the cell wall during episodes of water stress.