Effect of ABA on the water relations of winter-wheat cultivars varying in drought resistance

Water and osmotic potentials were measured in leaves of a drought-sensitive ('Ponca') and a drought-resistant ('KanKing') cultivar of winter wheat ( Triticum aestivum L . em. Thell.) to determine if the potentials of the drought-sensitive cultivar could be made similar to those of the drought-resistant cultivar through application of abscisic acid (ABA). Stomatal resistance was also measured. Plants were sprayed with ABA and grown in soil, which was watered or allowed to dry. In well-watered plants, ABA closed the stomata of both cultivars. Stomatal resistance of plants grown without added water and with ABA was less than that of plants grown without added water and without ABA. Under ample water supply, ABA decreased water and osmotic potentials of the drought-sensitive cultivar (Ponca), but had no effect on these potentials in the drought-resistant cultivar (KanKing). Under water-deprived conditions, ABA increased water and osmotic potentials of Ponca, but did not change these potentials in KanKing. The overall effect of ABA was to decrease the differences in the water and osmotic potentials between the two cultivars.     

Effect of drought on ear and flag leaf photosynthesis of two wheat cultivars differing in drought resistance

We investigated net photosynthetic rate (P_N) of ear and two uppermost (flag and penultimate) leaves of wheat cultivars Hongmangmai (drought resistant) and Haruhikari (drought sensitive) during post-anthesis under irrigated and non-irrigated field conditions. The P_Nof ear and flag leaf were significantly higher and less affected by drought in Hongmangmai than in Haruhikari. The rate of reduction in stomatal conductance (g_s) was similar for the two cultivars, but intercellular CO₂concentration (C_i) in the flag leaf of Hongmangmai was lower than that of Haruhikari in non-irrigated treatment. No differences were observed in leaf water potential (ψ₁) and osmotic adjustment of the flag leaf of the cultivars. These results imply that differences in photosynthetic inhibition on the flag leaf at low leaf ψ₁between the cultivars were primarily due to non-stomatal effects. Hence the main physiological factor associated with yield stability of Hongmangmai under drought stress may be attributed to the capacity for chloroplast activity in the flag leaf, which apparently allows sustained P_Nof flag leaf during grain filling under drought stress. The higher P_Nof ear in Hongmangmai under drought could also be related to its drought resistance.     

Hydraulic resistance of two sorghums varying in drought resistance

Genotypes of sorghum [Sorghum bicolor (L.) Moench] vary in drought resistance. Yet it is not known if their hydraulic resistances vary. The objective of this study was to determine if the hydraulic resistance of a drought-resistant sorghum was the same as that of a drought-sensitive sorghum. Leaf water and osmotic potentials were measured daily, during a 14-d period, in leaves of a drought-resistant (‘KS9’) and a drought-sensitive (‘IA25’) sorghum, which had the roots in pots with a commercial potting soil that was either well watered or allowed to dry. Soil water potential, adaxial stomatal resistance, and transpiration rate were determined daily. Hydraulic resistance of the plants was calculated from the slope of the line relating soil water potential minus leaf water potential versus transpiration rate. When the soil was not watered, the drought-sensitive sorghum had a water potential that averaged −0.50 MPa lower and an osmotic potential that averaged −0.57 MPa lower, but a similar adaxial stomatal resistance (1.19 s mm⁻¹), compared with the drought-resistant sorghum. Seven days after the beginning of the experiment, the water potential of the soil with the drought-sensitive sorghum was −0.25 MPa lower than that of the soil with the drought-resistant sorghum. With the water-limited conditions, the drought-sensitive sorghum depleted the soil-water reserve more quickly and died 2 d before the drought-resistant sorghum. Under well watered conditions, the two sorghums had similar water potentials (−1.64 MPa), osmotic potentials (−2.83 MPa), and adaxial stomatal resistances (0.78 s mm⁻¹). The calculated hydraulic resistance of the two sorghums did not differ and averaged 3.4 × 10⁷ MPa s m⁻¹. The results suggested that the variation in susceptibility to drought between the two genotypes was due to differences in rate of soil-water extraction. Contribution No. 86-249-J from the Kansas Agricultural Experiment Station. The paper is dedicated to the memory of Dr Dan M Rodgers.     

Effect of drought on ear and flag leaf photosynthesis of two wheat cultivars differing in drought resistance

We investigated net photosynthetic rate (P_N) of ear and two uppermost (flag and penultimate) leaves of wheat cultivars Hongmangmai (drought resistant) and Haruhikari (drought sensitive) during post-anthesis under irrigated and non-irrigated field conditions. The P_Nof ear and flag leaf were significantly higher and less affected by drought in Hongmangmai than in Haruhikari. The rate of reduction in stomatal conductance (g_s) was similar for the two cultivars, but intercellular CO₂concentration (C_i) in the flag leaf of Hongmangmai was lower than that of Haruhikari in non-irrigated treatment. No differences were observed in leaf water potential (₁) and osmotic adjustment of the flag leaf of the cultivars. These results imply that differences in photosynthetic inhibition on the flag leaf at low leaf ₁between the cultivars were primarily due to non-stomatal effects. Hence the main physiological factor associated with yield stability of Hongmangmai under drought stress may be attributed to the capacity for chloroplast activity in the flag leaf, which apparently allows sustained P_Nof flag leaf during grain filling under drought stress. The higher P_Nof ear in Hongmangmai under drought could also be related to its drought resistance.This revised version was published online in March 2005 with corrections to the page numbers.     

Role of silicon in enhancing resistance to freezing stress in two contrasting winter wheat cultivars

The main objective of this study was to elucidate the roles of silicon (Si) in enhancing tolerance to freezing stress (?5 °C) in two contrasting wheat (Triticum aestivum L.) cultivars: i.e. cv. Yangmai No. 5, a freezing-susceptible cultivar and cv. Linmai No. 2, a freezing-tolerant cultivar. Shoot dry weight of the freezing-susceptible wheat was significantly lower under freezing stress than in controls, but increased significantly with Si amendment. The freezing treatment did not affect shoot dry weight of the freezing-tolerant cultivar. The leaf water content was considerably decreased by freezing stress in the freezing-susceptible cultivar, but was significantly increased by Si amendment. In contrast, freezing treatment did not significantly reduce leaf water content in the freezing-tolerant cultivar and Si played no role in water retention in this cultivar. The concentrations of H₂O₂ and free proline along with malondialdehyde (MDA) were progressively enhanced by freezing stress in the two wheat cultivars used, but were significantly suppressed by amendment with Si. The major antioxidant enzyme activities and non-enzymatic antioxidants (i.e. glutathione and ascorbic acid) in the leaves of freezing-stressed plants were decreased, but were stimulated significantly by the exogenous Si. The possible mechanisms for Si-enhanced freezing stress may be attributed to the higher antioxidant defense activity and lower lipid peroxidation through water retention in leaf tissues.     

Physiological responses of two wheat cultivars to soil drought

Young plants of the two wheat cultivars Katya and Prelom, differing in their reaction to drought in the field, were grown in soil in pots, and their water status was assessed as well as the intensity of gas exchange, chlorophyll fluorescence, and accumulation of compatible solutes and hydrogen peroxide after 7 days of dehydration. It was established that cv. Katya displayed markedly better tolerance to soil drying in comparison with cv. Prelom. This was partly due to the more effective control of water balance, activity of the photosynthetic apparatus, and metabolic activity of leaves under stress. Consequently, lower amounts of hydrogen peroxide were accumulated and a lower membrane injury index was determined.     

Effect of water stress on protein content in two maize cultivars differing in drought resistance

The changing pattern, of WSD and concomitant protein changes were scanned in twoZea mays cultivars, with differential sensitivity to water stress. Increasing protein values were recorded in resistant cv. Ageti-76 with decreasing osmotic potentials of substrate (on 3rd day of stress), although the values in susceptible cv. Vijay remained almost on a par with the controls. In another experiment both the cultivars revealed initial increase of protein on 2nd day at osmotic potentials of - 2 and -4 × 10⁶ Pa, however, values declined up to day 7 of stress. WSD showed an increasing trend in both the experiments, although slightly higher values of WSD were registered in cv. Ageti-76 in comparison with cv. Vijay. The significance of protein changes in reference to drought is discussed.     

Pressure-volume curves and drought resistance in two wheat genotypes

The water relations of two durum wheat cultivars ( Triticum durum Desf.) were studied throughout the growing season. Irrigated and unirrigated plants were compared from booting to milk stage; a period where water stress occurred naturally in the field. Modulus of elasticity (ε), turgid weight/dry weight ratio (TW/DW), relative water content at zero turgor (RWC_o) and osmotic potential at full turgor (ε) declined throughout the season while average turgor (ψ_p) increased. Water stress induced a further decrease in ψ_π¹⁰⁰ and the TW/DW ratio. The elastic modulus varied greatly. During the first stages of growth, cv. Appulo (the more resistant cultivar) showed lower ε values than cv. Valforte. At the milk stage, ε was lower for the unirrigated than the irrigated plants. Correlation coefficients between the TW/DW ratio and the osmotic potential were significant for both cultivars. In cv. Valforte, TW/DW was also correlated with the average turgor and the bulk modulus of elasticity. Structural changes that affect the TW/DW ratio seem to be important factors influencing water relations and drought tolerance in durum wheat.     

Factors affecting plant regeneration from immature inflorescence of two winter wheat cultivars

Inflorescence explants of two winter wheat cultivars, Triticum durum cv. Kızıltan-91 and T. aestivum cv. Bezostaja-01, were used to evaluate the effects of vernalization period of donor plants, callus age and medium composition on regeneration capacity. Donor plants were grown for 7 d and they were exposed to 4 °C for 1, 2, 3, 4, and 5 weeks. The maximum inflorescence formation was observed as 79 % at 4 weeks and 73 % at 5 weeks of vernalization period for Kızıltan-91 and Bezostaja-01, respectively. Among 6 different callus induction and regeneration mediums, I₁-R₁ and I₃-R₃ have to be the best responding mediums for Kızıltan-91 and Bezostaja-01, respectively. In Kızıltan-91, calli induced from donor plants, vernalized for 3 weeks, showed a significantly lower regeneration capacity than counterparts vernalized for 4 and 5 weeks. The highest regeneration capacity of 69 % was obtained from 6-week-old calli produced from 4 weeks vernalized Kızıltan-91 donor plants. In contrast to Kızıltan-91 cultures, the effects of vernalization period and callus age on regeneration capacity were not significant in Bezostaja-01 cultures. The maximum numbers of tillers were obtained from 6-and 15-week-old calli for Bezostaja-01 and Kızıltan-91, respectively. In contrast to vernalization period of donor plants, callus age had no effect on seed number.     

The expression of resistance to powdery mildew infection in winter wheat cultivars. I. Seedling resistance

Detached seedling leaf tests were used to assign 18 winter wheat cultivars to three wheat mildew resistance (WMR) groups on the basis of identified race-specific resistance characters. The same cultivars were exposed in mobile seedling nurseries from 1976 to 1979. Infection by mildew populations containing matching pathogenicity characters was used to recognise background seedling resistance in three cultivars: Maris Freeman, Maris Huntsman and Sentry. The value of background seedling resistance is discussed.     

Water stress and cell wall polysaccharides in the apical root zone of wheat cultivars varying in drought tolerance

Glycosyl composition and linkage analysis of cell wall polysaccharides were examined in apical root zones excised from water-stressed and unstressed wheat seedlings (Triticum durum Desf.) cv. Capeiti ("drought-tolerant") and cv. Creso ("drought sensitive"). Wall polysaccharides were sequentially solubilized to obtain three fractions: CDTA+Na(2)CO(3) extract, KOH extract and the insoluble residue (alpha-cellulose). A comparison between the two genotypes showed only small variations in the percentages of matrix polysaccharides (CDTA+Na(2)CO(3) plus KOH extract) and of the insoluble residues (alpha-cellulose) in water-stressed and unstressed conditions. Xylosyl, glucosyl and arabinosyl residues represented more than 90mol% of the matrix polysaccharides. The linkage analysis of matrix polysaccharides showed high levels of xyloglucans (23-39mol%), and arabinoxylans (38-48mol%) and a low amount of pectins and (1-->3), (1-->4)-beta-d-glucans. The high level of xyloglucans was supported by the release of the diagnostic disaccharide isoprimeverose after Driselase digestion of KOH-extracted polysaccharides. In the "drought-tolerant" cv. Capeiti the mol% of side chains of rhamnogalacturonan I and II significantly increased in response to water stress, whereas in cv. Creso, this increase did not occur. The results support a role of the pectic side chains during water stress response in a drought-tolerant wheat cultivar.     

Responses of two differentially sensitiveCicer arietinum L. cultivars to water stress: Protein content and drought resistance

Two cultivars ofCicer arietinum L. with contrasting sensitivities to water stress, cv. C 214 (relatively resistant) and cv. 130 (susceptible) were compared for their protein content and soluble nitrogen (sol-N) under water stress. During water stress shoots of the cv. C 214 showed an increase (over the control) in both protein and sol-N content. The total nitrogen (total-N) also increased in the cv. C 214 while remaining almost unaffected in the cv. G 130. Cultivar C 214 shoots recorded lower protein/sol-N ratios. The results have been discussed in relation to resistance to water stress.     

Inheritance of resistance to stripe rust in winter wheat cultivars Aquileja and Xian Nong 4

Winter wheat cultivars Aquileja (AQ) and Xian Nong 4(XN) were previously reported to possess durable, quantitative resistance to stripe rust disease. In the present study, AQ, XN and a susceptible wheat cultivar were reciprocally crossed in all 6 combinations. Parents, F1, F2, F3, BCP1 and BCP2 were used to determine quantitative genetic parameters for infection type and disease severity. The results showed that fixable genetic components preponderated in the inheritance of the resistance in AQ and XN for both infection type and disease severity, while the dominant component could be detected in some cases. The resistance was conditioned by oligogenes. Heritability of the resistance ranged from 50 to 79% in most cases.     

Effect of drought on chlorophyll content and antioxidant enzyme activities in leaves of three wheat cultivars varying in productivity

Seedlings of three wheat varieties (Triticum aestivum L.)—highly productive cv. Ballada, moderately productive cv. Belchanka, and low productive cv. Beltskaya—were exposed to progressive soil drought (cessation of watering for 3, 5, and 7 days) and then analyzed for chlorophyll content and activities of ferredoxin-NADP⁺ oxidoreductase (FNR) and antioxidant enzymes, namely, glutathione reductase (GR) and ascorbate peroxidase (AscP). In addition, the proline content, and the extent of lipid peroxidation were examined. In the first period of water limitation, the water loss from leaves was slight for all wheat cultivars, which is characteristic of drought-resistant varieties. After 7-day drought the leaf water content decreased by 5.2–6.8%. The total chlorophyll content expressed per unit dry weight increased insignificantly during the first two periods of drought but decreased by 13–15% later on. This decrease was not accompanied by changes in chlorophyll a/b ratio. The plant dehydration did not induce significant changes in FNR activity. Activities of GR and AscP in leaves of wheat cultivars Ballada and Belchanka increased on the 3rd and 5th days of drought. Owing to the coordinated increase in GR and AscP activities, the lipid peroxidation rate remained at nearly the control level observed in water-sufficient plants. When the dehydration period was prolonged to 7 days, activities of GR and AscP in wheat cultivars reduced in parallel with the increase in malonic dialdehyde (MDA) content, indicating that the antioxidant enzyme defense system was weakened and lipid peroxidation enhanced. Unlike Ballada and Belchanka, the wheat cv. Beltskaya did not exhibit the increase in GR and AscP activities during progressive soil drought. The increase in MDA content by 16% in this cultivar was only observed after a 7-day drought period. The proline content in leaves of all wheat cultivars increased substantially during drought treatment. Thus, in wheat cultivars examined, different responses of the defense systems were mobilized to implement plant protection against water stress. The activities of antioxidant enzyme defense system depended on wheat cultivar, duration of drought, and the stage of leaf development.     

The expression of resistance to powdery mildew infection in winter wheat cultivars. II. Adult plant resistance

Resistance to powdery mildew infection was compared in 18 winter wheat cultivars under field conditions in three seasons, 1977 to 1979. In each year, natural infection occurred from populations of Erysiphe graminis f. sp. tritici containing host-specific pathogenicity characters matching identified cultivar resistance. Leaves, ears and stems were assessed separately. The expression of resistance in leaves appeared to be independent of that in ears. There were few consistent differences between cultivars in stem infection. Resistance in adult leaves did not necessarily correspond with that in seedling leaves; resistance in ears did not appear to be correlated with plant height. Quantitative differences between cultivars having the same identified resistance characters occurred in both leaves and ears; Flinor, Avalon and Bounty were most resistant on leaves whereas Iona, Wizard and Bounty were most resistant on ears. Differences in background resistance were considered to be responsible for these observations.     

Yield response and categories of resistance to Russian wheat aphid in four Dn4 hard red winter wheat cultivars

A field experiment was conducted to determine whether resistance to Russian wheat aphid, Diuraphis noxia (Mordvilko), conferred by the Dn4 gene is affected by genetic background. This was done by comparing the yield responses to Russian wheat aphid-resistant wheat containing Dn4, derived through the backcross method, to those of the corresponding recurrent parents. Infested resistant cultivars had fewer Russian wheat aphids per tiller than infested susceptible cultivars at the Lamar and Fort Collins, CO sites but not at the Akron, CO site. At the Lamar site, resistant cultivars yielded more than the susceptible cultivars. 'Prairie Red' and 'Yumar' were more resistant than 'Prowers', especially at the higher infestation level. Resistance in these cultivars was categorized in a laboratory experiment to confirm this differential expression of resistance. Resistance in Prairie Red, 'Halt', 'Prowers 99', and Yumar was categorized at three plant growth stages. Antibiosis was expressed as reductions in maximum number of nymphs produced per 24 h and intrinsic rate of increase. The maximum number of nymphs produced per 24 h was reduced in Halt and 'Lamar'. Averaged over cultivars, the intrinsic rate of increase was less at jointing than at the seedling or tillering growth stages. Tolerance was expressed in the resistant cultivars as reduced chlorosis and leaf rolling. Growth reductions in infested Prowers 99 plants was less than the other cultivars. This study confirms that some cultivars containing Dn4 may express antibiosis and tolerance, whereas others may not show the same categories. Thus, expression is affected by genetic background.     

Antioxidant response of wheat roots to drought acclimation

Wheat (Triticum aestivum L.) seedlings of a drought-resistant cv. C306 were subjected to severe water deficit directly or through stress cycles of increasing intensity with intermittent recovery periods. The antioxidant defense in terms of redox metabolites and enzymes in root cells and mitochondria was examined in relation to membrane damage. Acclimated seedlings exhibited higher relative water content and were able to limit the accumulation of H₂O₂ and membrane damage during subsequent severe water stress conditions. This was due to systematic up-regulation of superoxide dismutase, ascorbate peroxidase (APX), catalase, peroxidases, and ascorbate–glutathione cycle components at both the whole cell level as well as in mitochondria. In contrast, direct exposure of severe water stress to non-acclimated seedlings caused greater water loss, excessive accumulation of H₂O₂ followed by elevated lipid peroxidation due to the poor antioxidant enzyme response particularly of APX, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and ascorbate–glutathione redox balance. Mitochondrial antioxidant defense was found to be better than the cellular defense in non-acclimated roots. Termination of stress followed by rewatering leads to a rapid enhancement in all the antioxidant defense components in non-acclimated roots, which suggested that the excess levels of H₂O₂ during severe water stress conditions might have inhibited or down-regulated the antioxidant enzymes. Hence, drought acclimation conferred enhanced tolerance toward oxidative stress in the root tissue of wheat seedlings due to both reactive oxygen species restriction and well-coordinated induction of antioxidant defense.     

Influence of water stress on leaf photosynthetic characteristics in wheat cultivars differing in their susceptibility to drought

A gradual reduction in leaf water potential (Ψ_leaf), net photosynthetic rate (P _N), stomatal conductance, and transpiration rate was observed in two drought tolerant (C 306 and K 8027) and two susceptible (RW 893 and 899) genotypes subjected to water stress. The extent of reduction was lower in K 8027 and C 306 and higher in RW 893 and RW 899. Rewatering the plants after 5 d of stress restored P _N and other gas exchange traits in all four cultivars. Water stress had no significant effect on variable to maximum fluorescence ratio (F_v/F_m) indicating that water stress had no effect on primary photochemistry of photosystem 2 (PS2). However, water stress reduced the efficiency of excitation energy transfer (F′_v/F′_m) and the quantum yield of electron transport (Φ_PS2). The reduction was more pronounced in susceptible cultivars. Water stress had no significant effect on photochemical quenching, however, the non-photochemical quenching increased by water stress.     

Identification of quantitative trait loci conferring resistance to tan spot in a biparental population derived from two Nebraska hard red winter wheat cultivars

Tan spot, caused by Pyrenophora tritici-repentis (Ptr), is a destructive foliar disease in all types of cultivated wheat worldwide. Genetics of tan spot resistance in wheat is complex, involving insensitivity to fungal-produced necrotrophic effectors (NEs), major resistance genes, and quantitative trait loci (QTL) conferring race-nonspecific and race-specific resistance. The Nebraska hard red winter wheat (HRWW) cultivar ‘Wesley’ is insensitive to Ptr ToxA and highly resistant to multiple Ptr races, but the genetics of resistance in this cultivar is unknown. In this study, we used a recombinant inbred line (RIL) population derived from a cross between Wesley and another Nebraska cultivar ‘Harry’ (Ptr ToxA sensitive and highly susceptible) to identify QTL associated with reaction to tan spot caused by multiple races/isolates. Sensitivity to Ptr ToxA conferred by the Tsn1 gene was mapped to chromosome 5B as expected. The Tsn1 locus was a major susceptibility QTL for the race 1 and race 2 isolates, but not for the race 2 isolate with the ToxA gene deleted. A second major susceptibility QTL was identified for all the Ptr ToxC-producing isolates and located to the distal end of the chromosome 1A, which likely corresponds to the Tsc1 locus. Three additional QTL with minor effects were identified on chromosomes 7A, 7B, and 7D. This work indicates that both Ptr ToxA-Tsn1 and Ptr ToxC-Tsc1 interactions are important for tan spot development in winter wheat, and Wesley is highly resistant largely due to the absence of the two tan spot sensitivity genes.