Putrescine Increases Floral Retention, Pod Set and Seed Yield in Cold Stressed Chickpea

Chickpea (Cicer arietinum L.) is sensitive to cold stress (<8 °C) at its reproductive phase that results in flower abortion, poor pod set and thus reduced yield. Early maturing genotypes are especially more sensitive. In this crop, the metabolic causes underlying cold injury that are imperative to induce cold tolerance are not known. In the present study, the endogenous levels of putrescine (diamine), spermidine (triamine) and spermine (tetramine) were examined in early maturing chickpea genotype ICCV 96029, subjected to chilling temperatures of field (12–15/4–6 °C; average maximum and minimum temperature respectively), at flowering or early podding stage. These were compared with controls growing in warmer conditions (28/12 °C) of the glasshouse. The polyamine levels increased six to nine times because of stress. Relatively, putrescine (PUT) elevation was the highest but short-lived and its decrease appeared to match with the onset of flower and pod abscission in stressed plants. Compared with controls, chilling injury, observed as electrolyte leakage (EL), increased by 60 % while cellular respiration declined by 68 % in stressed plants. Exogenous application of 10 mm PUT to stressed plants reduced the EL by 29 % and elevated the cellular respiration by 40 %. PUT application at flowering stage resulted in increase of 30, 31, 23 and 25 % in floral retention, pod set, pod retention and fertile pods respectively. At the early podding stage, PUT treatment increased the seed yield per plant, seed number per 100 pods and individual seed weight by 50, 17 and 19 % respectively. The number of single-seeded pods per plant increased from 4.4 in stressed plants to 12.2 in PUT-treated plants while the number of double-seeded pods reduced from 6.2 to 4.3. The number of infertile pods declined from 8.2 in stressed plants to 3.1 in PUT-treated plants.     

Effects of Low Temperature Treatment at Seedling Stage on Soybean Growth, Development and Final Yield

The after effect of continuous and variable chilling temperatures acting periodically for 7 days at seedling stage (day/night: 2/2°C, 5/5°C, 15/2°C, 15/5°C, 15/15°C and control–22/18°C) on growth, development and final yield of soybean cultivar Polan and Progres were investigated. The temperature decrease lengthened the duration of the vegetation period before flowering for both cultivars, which allowed compensation for the reduced–as an effect of chilling–rate of daily increments in the leaf area and the dry weight of the above-ground parts of the plants. The most important symptom of chilling treatment was a considerable increase in the number of axillary branches after flowering, which caused an increase in the dry weight of plants. The subsequent reaction of the growth of leaves to chilling at seedling stage was, after flowering, variety dependent. The area of leaves bigger at Polan or similar to control at Progres ensured good seed filling in an increased number of pods developed on axillary branches. Chill-induced delay in plants development allowed the avoidance of low temperature occurring during flowering in this experiment. This chilling during flowering was one of the reasons of worse-pod setting and reduction of seeds per pod number at control plants.     

Temperature adaptation of tropical highland maize (Zea mays L.) during early growth and in controlled conditions

For economic and ecological reasons, the chilling tolerance of maize must be further improved. This seems to be possible by the introgression of tropical highland germplasm if chilling and heat tolerance can be combined to obtain a high yield consistency. Two adapted and two semi-exotic highland varieties were grown at 16, 25 and 30°C until the third and sixth leaf stage. Thereafter, some plants were transplanted and grown at 24°C in the glasshouse until anthesis. Exotic germplasm improved the leaf appearance and shoot growth during the early heterotrophic phase at all temperatures, with a marked advantage at low temperature. This superiority was almost completely lost during the succeeding autotrophic growth phase with some residual effects at low temperature and a marked relative retardation in leaf appearance and growth at high temperature. Relative growth rates (RGR) of exotic germplasm were not superior at low temperature or inferior at high temperature. At low temperature, their comparatively reduced leaf area ratio (LAR) was still compensated by a high net assimilation rate (NAR). Their expression of the latter trait was comparatively decreased at a high temperature, which explained the low RGR. Early growth at low and medium temperatures had similar effects on final leaf number for all varieties; a high temperature increased the final leaf number up to 20%, especially in exotic germplasm. In conclusion, the good early vigor of Mexican highland germplasm seems to be mainly restricted to the heterotrophic phase; a relation to adaptative mechanisms, such as a low LAR, must be overcome for an efficient utilization in conventional breeding programs.     

Exposure to heat stress during flowering period reduces flower yield and pyrethrins in Pyrethrum (Tanacetum cinerariifolium)

Heat stress is one of the major limitations to crop productivity worldwide. Global warming effects are expected to increase the number of hot days and increase the probability and intensity of heat stress events. Short periods (3–5 days) of heat stress with maximum temperatures exceeding 35°C often occur during late spring and early summer in some pyrethrum growing regions of Australia. These heat stress events usually coincide with pyrethrum flowering period. Pyrethrum is a perennial herbaceous plant which is commercially grown for extraction of pyrethrins which accumulate in the achenes of the flowers and are used as a natural insecticide. This experiment was conducted to understand the effects of timing of short periods of heat stress on flower development and pyrethrum yield. Plants were subjected to short periods of high temperature treatments (12 hr at 35–40°C) for three consecutive days at three flower maturity stages (early, mid, late). Control plants were grown at ambient temperature (10–25°C) throughout the flowering period. Exposure of pyrethrum plants to short periods of high temperature during the flowering period caused a significant reduction in the flower and pyrethrin yield. This was associated with the reduction in flower size and accelerated flower senescence. Exposure of pyrethrum plants to heat stress significantly increased the rate of flower development resulting in a shorter flowering period. Overall, plants grown under control treatment showed slower rate of flower development and longer duration flowering period. This resulted in longer duration of pyrethrin accumulation and higher yield of pyrethrins per flower. Timing and duration of heat stress significantly influenced pyrethrin yield per flower. Heat stress caused more severe yield reductions at early flowering than later in the flowering period. Research focusing on agronomic strategies, phenology and breeding for tolerance to heat stress is therefore important to cope with future climate changes and to obtain maximum pyrethrin yield.     

Environmental effects on genetic variation of chilling resistance in cucumber

Environmental effects on genetic variation for chilling resistance were studied in nine cultivars and breeding lines (referred to as cultigens hereafter) of cucumber (Cucumis sativus L.). Five experiments were carried out in controlled-environment chambers to measure the effects of growth temperature, photoperiod, duration of chilling, light level during chilling, and watering frequency on chilling resistance of seedlings at the cotyledon and first true leaf growth stages. Significant interactions were found between cultigen and all environmental factors studied except for the photoperiod and watering frequency. Cultigen rank was affected by growth temperature before chilling, chilling duration, and light level during chilling, but shifts in rank were not consistent. Genetic variation was largest when the plants were grown at 22/18 °C, most pronounced after a chilling duration of 5 to 9 hours and a light level during chilling of 270 μmol·m^-2·s^-1. Variation was larger at the first true leaf stage than at the cotyledon stage. Differences among cultigens in chilling damage were largest 5 days after chilling. Therefore, it seems that testing for genetic variation in chilling damage can be restricted to one set of environmental conditions. We recommend the following conditions for screening cucumber for genetic variation in chilling resistance: grow the plants at 22/18 °C, under a 9-hour photoperiod with a 3-hour night interruption, water them once daily, subject them at the first true leaf stage to a chilling treatment of 7 hours at 4°C at a light level of 270 μmol·^-2·s^-1, and evaluate damage 5 days after treatment. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genotypic difference in spikelet sterility response to air temperature during the reproductive stage of rice

This experiment was conducted to evaluate the varietal differences of spikelet sterility response to air temperature during the reproductive stage. Six rice varieties differing in maturity group (early-maturing; Unkwangbyeo, Odaebyeo, medium-maturing; Andabyeo, Hwasungbyeo, and mid-late maturing; Donganbyeo, Chuchungbyeo) were grown under ambient temperature (AT) conditions before being transferred to the temperature-controlled plastic houses. For the synchronization of the growth stage, 15 rice seedlings (2011) and 10 rice seedlings (2012) per pot were transplanted in a circle and only main stems were grown by removing tillers at early stage of their emergence. At the initial heading stage, pots for each variety were transferred to the four plastic houses that were controlled to AT, AT + 1.5°C, AT + 3.0°C, and AT + 5.0°C, respectively. Spikelet fertility was significantly decreased due to high temperature-induced spikelet sterility at AT + 3.0 and/or AT + 5.0°C treatment during flowering time in 2011. Spikelet fertility in 2012 was much lower than in 2011 even at the AT treatment because of high temperature-induced spikelet sterility at the micosporogenesis stage. Critical temperature (Tc) that induces 50% spikelet sterility at flowering time was estimated by fitting the temperature response of spikelet fertility to a logistic function. Tc ranged from 34.6°C (Odaebyeo) to 39.7°C (Hwasungbyeo), Odaebyeo being significantly more sensitive to high temperature-induced spikelet sterility than the other varieties. This result has shown that response of spikelet sterility to higher temperature is different according to rice varieties. However, further study should be done to arrive at a concrete conclusion.     

Grain development and endoreduplication in maize and the impact of heat stress

The post-anthesis development of growing maize kernels is strongly affected by heat stress. The maize cultivar “Spezi” was used to quantify this effect in kernels from 14 days after flowering until maturity. Day/night temperature of control plants was 25/20°C. Stress of 40/25°C was given for seven days or continuously up to maturity. Kernels were analysed weekly for dry matter and extractable DNA. In addition the ploidy levels and the DNA content in intact cell nuclei were determined by flow cytometry. The dry matter reduction started immediately after heat treatment and reached, at maturity, 40% for temporary heat stress and 60% for permanent heat stress. The reduction of extractable DNA started later and was not as extensive. Endopolyploidy was found in all kernel tissues, namely embryo, endosperm and pericarp. In endosperm, 3C nuclei reached their maximum number at approximately 14–17 days, and cells with higher ploidy levels between 21 and 26 days after flowering. Later on 6C nuclei were dominant. The DNA content in intact cells of the endosperm reached a maximum 21 days after flowering. This maximum was lower for heat stress variants and decreased more rapidly. Heat stress can vary from year to year under field conditions. Since heat stress changes the ratio between embryo and endosperm DNA in the direction of a higher portion of embryo DNA at maturity, this has an influence on the measured content of GM DNA from GM pollen transfer into conventional maize fields.     

QTL associated with heat susceptibility index in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) under short-term reproductive stage heat stress

Heat stress adversely affects wheat production in many regions of the world and is particularly detrimental during reproductive development and grain-filling. The objective of this study was to identify quantitative trait loci (QTL) associated with heat susceptibility index (HSI) of yield components in response to a short-term heat shock during early grain-filling in wheat. The HSI was used as an indicator of yield stability and a proxy for heat tolerance. A recombinant inbred line (RIL) population derived from the heat tolerant cultivar ‘Halberd’ and heat sensitive cultivar ‘Cutter’ was evaluated for heat tolerance over 2 years in a controlled environment. The RILs and parental lines were grown in the greenhouse and at 10 days after pollination (DAP) half the plants for each RIL received a three-day heat stress treatment at 38°C/18°C day/night, while half were kept at control conditions of 20°C/18°C day/night. At maturity, the main spike was harvested and used to determine yield components. A significant treatment effect was observed for most yield components and a HSI was calculated for individual components and used for QTL mapping. QTL analysis identified 15 and 12 QTL associated with HSI in 2005 and 2006, respectively. Five QTL regions were detected in both years, including QTL on chromosomes 1A, 2A, 2B, and 3B. These same regions were commonly associated with QTL for flag leaf length, width, and visual wax content, but not with days to flowering. Pleiotropic trade-offs between the maintenance of kernel number versus increasing single kernel weight under heat stress were present at some QTL regions. The results of this study validate the use of the main spike for detection of QTL for heat tolerance and identify genomic regions associated with improved heat tolerance that can be targeted for future studies.     

Chilling Tolerance in Hybrid Maize Induced by Seed Priming with Salicylic Acid

The optimum temperature for maize germination is between 25 and 28 °C. Poor and erratic germination at suboptimal temperature is the most important hindrance in its early sowing. This study was conducted to induce chilling tolerance in hybrid maize (Zea mays L.) by seed priming with salicylic acid (SA) and to unravel the background biochemical basis. For seed priming, maize hybrid (Hycorn 8288) seeds were soaked in 50, 100 and 150 ppm (mg l^?1) aerated solutions of SA for 24 h and were dried back. Treated and untreated seeds were sown at 27 °C (optimal temperature) and at 15 °C (chilling stress) under controlled conditions. Performance of maize seedlings was hampered under chilling stress. But seed priming with SA improved the seedling emergence, root and shoot length, seedling fresh and dry weights, and leaf and root score considerably compared with control both at optimal and chilling temperatures. However, priming in 50 mg l^?1 SA solution was more effective, followed by priming in 100 mg l^?1 SA solution. Seed priming with SA improved the chilling tolerance in hybrid maize mainly by the activation of antioxidants (including catalase, superoxide dismutase and ascorbate peroxidase). Moreover, maintenance of high tissue water contents and reduced membrane permeability also contributed towards chilling tolerance.     

Acquired Thermo‐Tolerance and Trans‐Generational Heat Stress Response at Flowering in Rice

In rice, pre‐exposure to sublethal treatment followed by harsh lethal treatment is known to improve tolerance of different abiotic stresses at the vegetative stage within and across generations. Our major aim was to test the phenomenon of thermo‐tolerance at flowering across (trans)‐generations and within generation using rice cultivars contrasting for heat stress tolerance at flowering. To test trans‐generational response, plants were exposed to higher temperature at flowering stage and seeds obtained from previous generations were exposed to heat stress during flowering, which recorded significantly lower fertility when exposed to the same degree of stress in their subsequent generations. A pre‐acclimation to moderately high acclimating temperatures imposed over three different durations during the vegetative and initial reproductive stage showed positive response in the tolerant N22, particularly under severe heat stress (40 °C). This finding indicates the possibility of acquiring ameliorative thermo‐tolerant mechanisms at anthesis, restricted to tolerant genetic backgrounds to combat subsequent harsh conditions within the same generation. However, trans‐generational memory was ineffective in mitigating spikelet sterility losses in both tolerant and susceptible backgrounds. Rice is extremely sensitive to heat stress during flowering; hence, similar exercise across other crops of interest needs to be carried out before generalizing conclusions.     

Activation of Antioxidant System by KCl Improves the Chilling Tolerance in Hybrid Maize

As maize is a chilling-sensitive crop, low temperatures during the early stages of development can be injurious to crop growth and development. Prime mechanism behind chilling-induced damage is oxidative stress. This study was undertaken to improve the chilling tolerance in hybrid maize by seed priming with KCl. For priming, seeds of the maize hybrid Hycorn 8288 were soaked in 50, 100 and 150 mg l⁻¹ aerated solution of KCl for 24 h and then re-dried close to original weight. Primed and untreated seeds were sown at 27 °C (optimal temperature) and at 15 °C (chilling stress) under controlled conditions. Seed priming improved the performance of maize under both normal and stress conditions. It was found that the chilling tolerance in maize is well associated with the enhanced capacity of the anti-oxidative system. Priming with KCl significantly improved the chilling tolerance mainly by the activation of antioxidants including catalase, superoxide dismutase and ascorbate peroxidase enzymes. KCl treatments also improved the germination rate and time, root and shoot length, and fresh and dry weights of seedlings compared with control. Soluble sugars and α-amylase activity determined as general metabolic indicators of stress were also improved by seed priming with KCl. Other possible bases of chilling tolerance in maize included maintenance of high tissue water contents, reduced electrolyte leakage and carbohydrate metabolism. Seed treatment with 100 mg l⁻¹ KCl was the best treatment to improve the performance of hybrid maize both under normal and chilling stress conditions.     

Differential heat sensitivity of two cool-season legumes,chickpea and lentil,at the reproductive stage,is associated with responses in pollen function,photosynthetic ability and oxidative damage

Increasing temperatures are adversely affecting various food crops, including legumes, and this issue requires attention. The growth of two cool-season food legumes, chickpea and lentil, is inhibited by high temperatures but their relative sensitivity to heat stress and the underlying reasons have not been investigated. Moreover, the high-temperature thresholds for these two legumes have not been well-characterised. In the present study, three chickpea (ICCVO7110, ICC5912 and ICCV92944) and two lentil (LL699 and LL931) genotypes, having nearly similar phenology with respect to flowering, were grown at 30/20°C (day/night; control) until the onset of flowering and subsequently exposed to varying high temperatures (35/25, 38/28, 40/30 and 42/32°C; day/night) in a controlled environment (growth chamber; 12 hr/12 hr; light intensity 750 µmol m⁻² s⁻¹; RH-70%) at 108 days after sowing for both the species. Phenology (podding, maturity) was accelerated in both the species; the days to podding declined more in lentil at 35/25 (2.8 days) and 38/28°C (11.3 days) than in chickpea (1.7 and 7.1 days, respectively). Heat stress decreased flowering–podding and podding–maturity intervals considerably in both the species. At higher temperatures, no podding was observed in lentil, while chickpea showed reduction of 14.9 and 16.1 days at 40/30 and 42/32°C, respectively. Maturity was accelerated on 15.3 and 12.5 days at 38/28°C, 33.6 and 34 days at 40/30°C and 45.6 and 47 days at 42/32°C, in chickpea and lentil, respectively. Consequently, biomass decreased considerably at 38/28°C in both the species to limit the yield-related traits. Lentil was significantly more sensitive to heat stress, with the damage—assessed as reduction in biomass, reproductive function-related traits (pollen viability, germination, pollen tube growth and stigma receptivity), leaf traits such as membrane injury, leaf water status, photochemical efficiency, chlorophyll concentration, carbon fixation and assimilation, and oxidative stress, appearing even at 35/25°C, compared with 38/28°C, in chickpea. The expression of enzymatic antioxidants such as superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase and non-enzymatic antioxidants declined remarkably with heat stress, more so in lentil than in chickpea. Carbon fixation (assessed as Rubisco activity) and assimilation (assessed as sucrose concentration, sucrose synthase activity) were also reduced more in lentil than in chickpea, at all the stressful temperatures, resulting in more inhibition of plant biomass (shoot + roots), damage to reproductive function and severe reduction in pods and seeds. At 38/28°C, lentil showed 43% reduction in biomass, while it declined by 17.2% in chickpea at the same time, over the control temperature (30/20°C). At this temperature, lentil showed 53% and 46% reduction in pods and seed yield, compared to 13.4% and 22% decrease in chickpea at the same temperature. At 40/30°C, lentil did not produce any pods, while chickpea was able to produce few pods at this temperature. This study identified that lentil is considerably more sensitive to heat stress than chickpea, as a result of more damage to leaves (photosynthetic ability; oxidative injury) and reproductive components (pollen function, etc.) at 35/25°C and above, at controlled conditions.     

A strategy of ideotype development for heat-tolerant wheat

Heat stress significantly limits yield in many wheat-growing areas globally including north-western NSW. While various traits linked to high-temperature tolerance have been identified, the combination of traits that optimize the heat tolerance of wheat has not been established in most environments. A total of 554 genotypes were evaluated in the field at different times of sowing in north-western NSW for three consecutive years to develop a heat-tolerant wheat ideotype for this environment. The later sown experiments were exposed to higher temperatures at the critical reproductive and grain-filling stages of development. The impact of high temperature was greatest at anthesis, and eventual grain yield was reduced by between 4% and 7% with every 1°C rise in average maximum temperature above the optimum of 25°C. High temperature reduced yield, plant height, grain weight and days to anthesis and maturity, and increased the percentage of screenings and grain protein content. Genotypes that produced higher yield under heat stress had shorter days to flowering and maturity, higher NDVI during grain filling, greater chlorophyll content at the milk stage of grain fill, taller plants, greater grain weight and number, and lower screenings compared with the benchmark cultivar Suntop. The genotype closest to the predicted heat-tolerant wheat ideotype identified from trait ranges had 79.6% similarity.     

QTL analysis of flooding tolerance in soybean at an early vegetative growth stage

Soybean cultivars are sensitive to flooding stress and their seed yields are substantially reduced in response to the stress. This study was conducted to investigate the genetic basis of flooding tolerance at an early vegetative growth stage. Sixty recombinant inbred lines derived from a cross between a relatively tolerant cv. ‘Misuzudaizu’ and a sensitive cv. ‘Moshidou Gong 503’ were grown in pots in a vinyl plastic greenhouse in 2002 and 2003. At the two‐leaf stage, half of the pots were waterlogged by water placed in plastic containers and adjusted to 5 cm above the soil surface. After 3 weeks of treatment, the pots were returned to the greenhouse and grown until maturity. Flooding tolerance was evaluated by dividing the seed weight of the treated plants by that of the control plants. Quantitative trait loci (QTL) analysis using 360 genetic markers revealed three QTLs for flooding tolerance, ft1 to ft3 in 2002. The ft1 (molecular linkage group C2) was reproducible and an additional four QTLs, ft4 to ft7, were found in 2003. The ft1 had a high LOD score in both years (15.41 and 7.57) and accounted for 49.2% and 30.5% of the total variance, respectively. A large QTL for days to flowering was consistently observed across treatments and years at a similar position to ft1. Comparing the relative location with markers, the maturity gene probably corresponds to E1. Late maturity may have conferred a longer growth period for recovery from flooding stress.     

Soybean Pollen Anatomy,Viability and Pod Set under High Temperature Stress

High temperature (HT) stress is one of the major environmental factors influencing yield of soybean (Glycine max L. Merr.) in the semi‐arid regions. Experiments were conducted in controlled environments to study the effects of HT stress on anatomical changes of pollen and their relationship to pollen function in soybean genotype K 03‐2897. Objectives of this study were to (a) quantify the effect of HT stress during flowering on pollen function and pod set and (b) observe the anatomical changes in pollen grains of soybean plants grown under HT stress. Plants were exposed to HT (38/28 °C) or optimum temperature (OT, 28/18 °C) for 14 days at flowering stage. HT stress significantly decreased in vitro pollen germination by 22.7 % compared to OT. Pollen from HT stress was deformed; it had a thicker exine wall and a disintegrated tapetum layer. HT stress decreased pod set percentage (35.2 %) compared to OT. This study showed that decreases in pollen in vitro germination by HT stress were caused by anatomical changes in pollen, leading to decreased pod set percentage under HT stress.     

Nitrogen Partitioning in Entire Plants of Different Spring Wheat Cultivars

The aim of this study was to investigate nitrogen partitioning in entire plants, including roots, of spring wheat in two temperature regimes during grain filling. Six cultivars, genetically different and with varying grain protein concentration, were grown in solution culture to full maturity. After anthesis, half the plants were grown in high temperature (23/17 °C, day/night) and half in low temperature (18/12 °C). Root nitrogen concentration was genetically influenced. The roots had ability to redistribute nitrogen to aboveground plant parts. At maturity the roots contained 10–20 % of the total nitrogen amount in the plants. Harvest index (HI) and harvest index for the entire plant (HI_tot) for cv. Heta were significantly higher at low temperature than at high. Cv. Heta had a rapid development rate from planting to maturity. Due to slow senescence at low temperature, cv. Kärn II showed lower HI and nitrogen harvest index (NHI) at low, compared with high, temperature. Cvs Kärn II and Sport showed higher nitrogen amount in the roots and shoots at low, compared with high, temperature. A negative correlation was found between NHI and NHI_tot vs. root weight, total shoot weight and root N amount. Because of the latter correlation, breeding for low root N concentration is suggested.     

Glycinebetaine Improves Chilling Tolerance in Hybrid Maize

Plant growth and development is hampered by various environmental stresses including chilling. We investigated the possibility of improving chilling tolerance in hybrid maize by glycinebetaine (GB) seed treatments. Maize hybrid (Hycorn 8288) seeds were soaked in 50, 100 and 150 mg l^?1 (p.p.m.) aerated solution of GB for 24 h and were dried back. Treated and untreated seeds were sown at 27 °C (optimal temperature) and at 15 °C (chilling stress) under controlled conditions. Germination and seedling growth was significantly hindered under chilling stress. Moreover, chilling stress significantly reduced the starch metabolism and relative water contents (RWC), and increased the membrane electrolyte leakage. However, activities of antioxidants (catalase, superoxide dismutase and ascorbate peroxidase) were increased under stress conditions. Seed treatments with GB improved the germination rate, root and shoot length, seedling fresh and dry weights, leaf and root scores, RWC, soluble sugars, α‐amylase activity and antioxidants significantly compared with untreated seeds under optimal and stress conditions. Induction of chilling tolerance was attributed to maintenance of high tissue water contents, reduced membrane electrolyte leakage, and higher antioxidant activities and carbohydrate metabolism. Seed treatment with 100 mg l^?1 GB was the best treatment for improving the performance of hybrid maize under normal and stress conditions compared with control and other levels used.     

Differences between wheat genotypes in damage from freezing temperatures during reproductive growth

Cereal crops in the reproductive stage of growth are considerably more susceptible to injury from freezing temperatures than during their vegetative growth stage in the fall. While damage resulting from spring-freeze events has been documented, information on genotypic differences in tolerance to spring-freezes is scarce. Ninety wheat genotypes were subjected to a simulated spring-freeze at the mid-boot growth stage under controlled conditions. Spring-freeze tolerance was evaluated as the number of seeds per head at maturity after plants were frozen at −6 °C. Plants that froze, as confirmed by infrared (IR) thermography, died shortly after thawing and consequently the heads did not mature. Only in plants that had no visible freezing (super-cooled) were heads able to reach maturity and produce seeds. In plants that super-cooled four genotypes had significantly higher seed counts after being exposed to freezing than three with the lowest. In addition, significant differences between genotypes were found in whole plant survival among those that had frozen. Genotypes with high whole-plant freezing survival were not necessarily the same as the super-cooled plants with the highest seed counts. Spring-freeze tolerance was not correlated with maturity suggesting that improvement in freezing tolerance could be selected for without affecting heading date. Spring-freeze tolerance was not correlated with freezing tolerance of genotypes of plants in a vegetative state, either under non-acclimated or cold-acclimated conditions indicating that vegetative freezing tolerance is not a good predictor of spring-freeze tolerance.     

Gas Exchange of Five Warm‐Season Grain Legumes and their Susceptibility to Heat Stress

Objective of this study was to compare the heat stress performance of four pulses from dry and hot areas (mungbeans, limabeans, and teparybeans and cowpeas) with that of soybeans. Two experiments were conducted in growth chambers, and data were pooled because results of both experiments were similar. Plants were raised up to flowering at 24/17 °C (day/night) and were then either exposed to these temperatures until maturity or stressed with 33/24 °C for 2 weeks starting at day 1 or 15 after onset of flowering (early vs. late stress). Before, during and after these stress intervals, gas exchange of representative upper leaves was examined; additionally, immediate effects of increasing leaf temperatures from 24 to 32 or 40 °C on chlorophyll fluorescence were assessed. Without heat stress rates of photosynthesis (P_n), and of transpiration (TR), stomatal and mesophyll conductance (g_s, g_m) and intrinsic transpiration efficiency (iTE) differed significantly among the five crops at each date. However, because of crop‐specific time‐courses ranking among unstressed crops was instable with time, so values were integrated or averaged over time. This procedure revealed high P_n potentials in mung‐ and teparybeans and high iTE values in limabeans compared to the other crops. Heat stress lowered P_n and g_s considerably, but increased TR in all five crops. Relative lowering of P_n during heat stress displayed a crop‐specific pattern with limabeans being least susceptible to both early and late heat stress, while cowpeas were highly susceptible to early stress. Effects on P_n were mainly attributable to lowering of g_s and only in part to g_m. The latter was supported by very small changes (<10 %) of various chlorophyll fluorescence signals shortly after raising leaf temperature to 32 °C in all species. However, in limabeans, a decreased electron transport rate (e‐rate, ?18 %) and an increased non‐photochemical quenching (Q_N, +16 %) pointed to an adaptive mechanism to avoid oxidative strains under heat. Leaf temperatures of 40 °C immediately provoked stronger changes in all fluorescence signals than 32 °C; substantial damages at 40 °C were indicated by effective quantum yield, photochemical quenching and ratio of fluorescence decrease in mungbeans and low ones in cowpeas and soybeans. Nevertheless, some adaptive responses of e‐rates and Q_N were observed in all crops and were most expressed in limabeans.     

Exploring the Role of Calcium to Improve Chilling Tolerance in Hybrid Maize

Abiotic stresses, including chilling, impede the plant growth and development mainly by oxidative damage. In this study, seed priming with CaCl₂ was employed to reduce the damage caused by chilling stress in hybrid maize. Maize hybrid (Hycorn 8288) seeds were soaked in 50, 100 and 150 mg l⁻¹ (ppm) aerated solution of CaCl₂ for 24 h and dried. Treated and untreated seeds were sown at 27 °C (optimal temperature) and 15 °C (chilling stress) under controlled conditions. Seed priming with CaCl₂ significantly reduced the chilling damage and improved the germination rate, root and shoot length, and seedling fresh and dry weights. Activities of antioxidants, including catalase, superoxide dismutase and ascorbate peroxidase, were also improved. Soluble sugars and α-amylase concentrations determined as general metabolic indicators of stress were also increased by seed priming with CaCl₂. Priming also improved the performance of maize at optimal temperature. Maintenance of tissue water contents, reduction in membrane leakage and increase in antioxidant activities, and carbohydrate metabolism seemed to induce chilling tolerance by CaCl₂. Seed priming with 100 mg l⁻¹ CaCl₂ was the optimal concentration in improving the performance of hybrid maize both under optimal and stress conditions.