Genotypic Differences In Tolerance Research Articles

Photosynthetic Limitations and Growth Traits of Four Arabica Coffee (Coffea arabica L.) Genotypes under Water Deficit

Climate change increases the risk of coffee yield due to the genotype-dependent effects of water deficit on coffee physiology. The goal of this research was to evaluate how water deficit altered the physiological and growth characteristics of arabica coffee (Coffea arabica L.). Water status, photosynthetic response to CO2 intercellular concentration (A/Ci curves) and growth parameters were evaluated in seedlings of four genotypes (Catimor ECU 02, Cavimor ECU, red Caturra and Sarchimor 4260). Most of the physiological traits evaluated differed significantly among genotypes. Between control and water deficit plants, significant variations occurred in the A/Ci parameters, showing a wide range of values for net photosynthetic rate, stomatal conductance, and water use efficiency, with decreases ranging from 4 to 74%. Maximum electron transport rate through photosystem II, highest rate of RuBisCO carboxylation, and triose phosphate utilization rate were all strongly decreased by water deficit 61% (red Caturra and Sarchimor 4260), followed by Cavimor ECU (35%) and Catimor ECU 02 (24%). Differences in response to water deficit among genotypes suggest possible genotypic differences in tolerance. The results indicated that Catimor ECU 02 and Cavimor ECU were less sensitive to water deficit, while red Caturra and Sarchimor 4260 were the most susceptible.

Mechanisms of genotypic differences in tolerance of iron toxicity in field-grown rice

Mechanisms of genotypic differences in tolerance of iron toxicity in field-grown rice

Seasonal changes in gas exchange and yield of 21 genotypes of Coffea arabica

Background: Coffee breeding programs in Ecuador have information on production and disease tolerance in many genotypes; however, they lack physiological information, especially on photosynthetic characteristics and their response to drought.
 Questions: Whether high genetic variability among coffee genotypes will explains the photosynthetic and production differences expected? Will the physiological response to the dry season (DS) be different between genotypes?
 Studied species: Coffea arabica L.
 Study site and dates: Pichincha canton, Manabí province, Ecuador during March-April 2017 (rainy season, RS) and June-July 2017 (DS).
 Methods: Leaf relative water content (RWC) and gas exchange of 21 coffee genotypes were measured during DS and RS. Coffee production during a period of three years was evaluated.
 Results: Significant differences were found in RWC, photosynthetic rate (A), stomatal conductance (gs) and water use efficiency (WUE) among genotypes, between seasons, an interaction effect of genotype × season. Drought caused a significant reduction in A and gs of 30 and 44 % respectively, while WUE was not affected. A positive linear relationship was found between A and gs, and a negative relationship between A and the leaf-air vapor pressure gradient (D W ) and between gs and D W . Differences in coffee production were found among genotypes.
 Conclusions: The high genetic variability of C. arabica genotypes may explain the significant differences in RWC and gas exchange and interaction genotypes x season, suggesting a differential response of each genotype to drought. Eleven of the 21 coffee genotypes were sensitive to drought, but showed different responses, suggesting possible genotypic differences in tolerance.
 
 
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Cultivar Differences in the Biochemical and Physiological Responses of Common Beans to Aluminum Stress.

Soil conditions leading to high levels of available aluminum are detrimental to plant growth, but data are limited on genotypic differences in tolerance to aluminum stress in some crops. The aim of this study was to examine the morphological, biochemical, and physiological changes in roots and shoots of 25 common bean (Phaseolus vulgaris L.) cultivars (Pinto market class) under aluminum (Al) treatment. Additionally, this study aimed to assess the range of responses amongst the common bean cultivars relative to their Al toxicity tolerance and sensitivity. Plants were grown hydroponically using a simplified nutrient solution with or without 20 µM AlCl3. Reactive oxygen species (ROS), activities of the antioxidant enzymes superoxide dismutase (SOD) and guaiacol peroxidase (POD), and malondialdehyde (MDA) concentration, an indicator of lipid peroxidation, were measured to establish the effects of Al treatment on the plants. In addition, growth parameters such as shoot and root dry weight, root-to-shoot ratio, root elongation, and root volume changes were also investigated. The cultivar effect was significant for all the measured parameters, except for shoot dry weight. Inhibition of the root and shoot dry weight for selected common bean cultivars shows that the response of common bean to Al stress is genotype-specific. Additionally, Al-induced root elongation inhibition and root volume changes varied among the cultivars. Most cultivars had significantly higher SOD activity (20 of 25 cultivars) and POD activity (12 cultivars) under AlCl3 treatment compared to the controls. A positive significant correlation was observed between MDA and ROS, showing that Al stress induced the accumulation of ROS along with an increase in lipid peroxidation. According to the results of this study, Arapaho and AC Island cultivars could potentially be used in the future production of common beans under Al stress. Therefore, these two cultivars could also be included in Al tolerance breeding programs.

Photosynthetic activity of oil palm (Elaeis guineensis) and interspecific hybrid genotypes (Elaeis oleifera × Elaeis guineensis), and response of hybrids to water deficit

Climate change raises significant challenges for oil palm performance, as its physiology can be differentially affected by water deficit depending on genotype. Therefore, the aim of the present study was to assess the leaf gas exchange and photochemical activity in oil palm seedlings of two genotypes of Elaeis guineensis (G) and four interspecific hybrids of E. oleifera × E. guineensis (OG) and evaluate the effect of water deficit for 15 days on water status and photosynthetic traits in a subset of two OG genotypes. Significant differences in gas exchange and photochemical activity variables occurred among well watered seedlings of G and OG genotypes, showing a wide range of variation in photosynthetic rate, stomatal conductance, water use efficiency and electron transport rate. In the two selected OG genotypes, water deficit caused a reduction in leaf relative water content, photosynthetic rate, stomatal conductance, transpiration rate and water use efficiency of 20, 99, 60, 52 and 98 %, respectively; maximum rate of RuBisCO carboxylation, maximum electron transport rate, and triose phosphate utilization rate were markedly reduced (70 – 80 %) by water deficit. The response of gas exchange and biochemical related variables to water deficit seemed to be bimodal, with stomatal closure being the first response and reduction of biochemical photosynthetic capacity the second one. There were differences in physiological performance among genotypes in well watered plants, and both OG genotypes were sensitive to WD but showed different responses, suggesting possible genotypic differences in tolerance.

Assessing the variation in traits for manganese deficiency tolerance among maize genotypes

Deficiency of manganese (Mn) is a serious problem reducing crop yields on calcareous and sandy soils throughout the world. In maize, limited knowledge is available on genotypic differences in tolerance to low-Mn supply and the physiological mechanisms underlying this tolerance. In the present study we have evaluated twelve maize genotypes (inbred lines) for their tolerance to Mn deficiency. The evaluation was based on measurements of how low-Mn supply affected shoot biomass, leaf Mn concentrations, maximum quantum efficiency of photosystem II (Fv/Fm), photosynthetic net CO2 assimilation, root length, Mn uptake and root-shoot Mn translocation. Tolerant genotypes were able to maintain optimum Fv/Fm values under a longer period of Mn deficiency with less reduction of foliar Mn concentration and photosynthetic rate, resulting in less reduction of shoot biomass, compared to sensitive genotypes. Efficient root uptake of Mn and root-to-shoot translocation of Mn also contributed to improved tolerance to Mn-deficiency. The metal transport genes YSL, NRAMP, ZIP, CAX and MTP, involved in root Mn uptake, root-to-shoot Mn translocation and vacuolar Mn homeostasis, were more highly expressed in the efficient genotype K22compared to sensitive genotype BY815. With respect to breeding of maize cultivars with improved Mn-efficiency, the time-course of changes in Fv/Fm values in response to Mn-deficiency provides a useful screening index for low-Mn tolerance.

Grain yield, root growth habit and lodging of eight oilseed rape genotypes in response to a short period of heat stress during flowering

Increasing temperature due to global climate change is detrimental to both the growth and development of canola (Brassica napus L.) that ultimately leads to a disastrous reduction in productivity. Understanding how these increasing temperatures will impact canola growth in terms of root traits, seed yield and lodging resistance is urgently required to maximize production in the coming decades. A set of eight B. napus genotypes was evaluated under controlled environments with two temperature treatments: 23/17 °C as the optimum (control) and 27.0/24.3 °C mean day/night temperatures as the heat stress. Root traits were assessed using both destructive and non–destructive methods, such as electrical capacitance (EC) and electrical impedance (EI). Simulated root and stem lodging tests were conducted to determine the risk of root lodging and stem lodging, respectively. High temperature stress significantly suppressed root morphological traits, and reduced pod fertility by an average of 28% and seed yield by 34%, along with a reduction in EC (10%) and an increased EI (17%). A significant relationship of EC with root traits (R2=0.53–0.59⁎⁎) and seed yield (R2=0.29*) was also observed. The high temperature treatment reduced the stem bending strength by 7% and stem safety factor by 11%, while did not show a consistent impact on root anchorage strength and root safety factor. The present study suggests that the risk of both yield reductions due to lower fertilization success ratio and crop lodging especially stem lodging increases under short periods of high temperature stress. Furthermore, EC could be applied as a non–destructive technique for evaluating root morphology and genotypic differences in tolerance of canola plants to heat stress.

Effect of tolerance to Septoria tritici blotch on grain yield, yield components and grain quality in Argentinean wheat cultivars

Septoria tritici blotch (STB) caused by Zymoseptoria tritici (Mycosphaerella graminicola) is a major disease of wheat worldwide due to significant losses in grain yield and quality. Disease tolerance is the ability to maintain yield performance in the presence of disease symptoms. Therefore, it could be a useful tool in the management of the disease. Although it is known, that there is disease tolerance to STB in some wheat cultivars, this aspect has not been studied among Argentinean cultivars. The aims of this study were to evaluate genotypic differences in tolerance to STB among Argentinean cultivars, considering the relationship between the area under disease progress curve or the green leaf area or the non-green leaf area duration with the grain yield. In addition the effect of the disease on yield, yield components, test weight, grain protein concentration, wet and dry gluten concentration and the influence of tolerance on these traits was investigated. Field experiments were carried out with ten cultivars in a split-split-plot design during 2010 and 2011. Inoculation treatments were the main plots and cultivars, the subplots. STB significantly reduced grain yield, their components, test weight and increase grain protein and gluten concentration. Cultivar Baguette 10 showed major tolerance to STB, indicated by a consistent low regression slope between the green area duration and yield, while Klein Chaja was non-tolerant due to a high regression slope. However, many cultivars such as Buck Brasil, Buck 75 Aniversario, Klein Escorpion and Klein Flecha had considerably similar regression slopes to Baguette 10, provided good levels of tolerance. Other cultivars presented no significant differences. The correlation coefficient between tolerance and grain yield potential was not significant, suggesting that tolerant high-yielding cultivars can be obtained. No relationship was found between quality group or tolerance with the increase in protein and gluten concentration due to STB either.

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

Abstract 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.

Sorghum genotypes differ in high temperature responses for seed set

Significant genotypic differences in tolerance of pollen germination and seed set to high temperatures have been shown in sorghum. However, it is unclear whether differences were associated with variation in either the threshold temperature above which reproductive processes are affected, or in the tolerance to increased temperature above that threshold. The objectives of this study were to (a) dissect known differences in heat tolerance for a range of sorghum genotypes into differences in the threshold temperature and tolerance to increased temperatures, (b) determine whether poor seed set under high temperatures can be compensated by increased seed mass, and (c) identify whether genotypic differences in heat tolerance in a controlled environment facility (CEF) can be reproduced in field conditions. Twenty genotypes were grown in a CEF under four day/night temperatures (31.9/21.0°C, 32.8/21.0°C, 36.1/21.0°C, and 38.0/21.0°C), and a subset of six genotypes was grown in the field under four different temperature regimes around anthesis. The novelty of the findings in this study related to differences in responsiveness to high temperature—genotypic differences in seed set percentage were found for both the threshold temperature and the tolerance to increased maximum temperature above that threshold. Further, the response of seed set to high temperature in the field study was well correlated to that in the CEF (R2=0.69), although the slope was significantly less than unity, indicating that heat stress effects may have been diluted under the variable field conditions. Poor seed set was not compensated by increased seed mass in either CEF or field environments. Grain yield was thus closely related to seed set percentage. This result demonstrates the potential for development of a low-cost field screening method to identify high-temperature tolerant varieties that could deliver sustainable yields under future warmer climates.

Genome-wide transcription profiles reveal genotype-dependent responses of biological pathways and gene-families in Daphnia exposed to single and mixed stressors.

The present study investigated the possibilities and limitations of implementing a genome-wide transcription-based approach that takes into account genetic and environmental variation to better understand the response of natural populations to stressors. When exposing two different Daphnia pulex genotypes (a cadmium-sensitive and a cadmium-tolerant one) to cadmium, the toxic cyanobacteria Microcystis aeruginosa, and their mixture, we found that observations at the transcriptomic level do not always explain observations at a higher level (growth, reproduction). For example, although cadmium elicited an adverse effect at the organismal level, almost no genes were differentially expressed after cadmium exposure. In addition, we identified oxidative stress and polyunsaturated fatty acid metabolism-related pathways, as well as trypsin and neurexin IV gene-families as candidates for the underlying causes of genotypic differences in tolerance to Microcystis. Furthermore, the whole-genome transcriptomic data of a stressor mixture allowed a better understanding of mixture responses by evaluating interactions between two stressors at the gene-expression level against the independent action baseline model. This approach has indicated that ubiquinone pathway and the MAPK serine-threonine protein kinase and collagens gene-families were enriched with genes showing an interactive effect in expression response to exposure to the mixture of the stressors, while transcription and translation-related pathways and gene-families were mostly related with genotypic differences in interactive responses to this mixture. Collectively, our results indicate that the methods we employed may improve further characterization of the possibilities and limitations of transcriptomics approaches in the adverse outcome pathway framework and in predictions of multistressor effects on natural populations.

Do genotype responses always converge from lethal to nonlethal toxicant exposure levels? Hypothesis tested using clones ofDaphnia magnastraus

This study examines the hypothesis that genotypic differences in tolerance will converge with decreasing levels of toxicity: from lethal to sublethal responses and from high to low sublethal exposure levels of toxic chemicals. Four laboratory Daphnia magna clones and three chemicals with different modes of action and known sublethal effects—cadmium, copper, and fluoranthene—were selected to test the previously mentioned hypothesis. Specifically, we compare how genetic variability in feeding responses among the studied Daphnia clones changed across increasing nonlethal exposure levels (expressed as increased exposure levels of feeding inhibition, EC) and how these changes converge relative to acute lethal responses (expressed in terms of the 48-h LC50). Our data show that cadmium fully supports the convergence hypothesis and that data with fluoranthene may also support it. Nevertheless, the most important findings of the present study are to have provided experimental evidence that (1) lethal responses in acute exposures and sublethal responses such as feeding inhibition are not related, (2) genetic variability for sublethal responses tend to be lower than acute responses, and (3) trade-offs between low and high sublethal exposure levels rather than genetic heterogeneity governed sublethal responses. The first two findings have important implications for risk assessment if further supported with more data on other toxicants and clones since they suggest that concerns voiced about the need to include more genetic variability in chronic toxicity tests may be groundless. The third finding may indicate that selection under low and variable environmental levels of toxicants with sublethal effects of feeding may select genotypes with high phenotypic plasticity in response to toxic stress rather than genotypes that perform well within particular toxicant exposure levels.

Analysis of leaf water relations in leaves of two olive (Olea europaea) cultivars differing in tolerance to salinity.

One-year-old rooted cuttings of olive (Olea europaea L. cvs. Frantoio and Leccino) were grown either hydroponically or in soil in a greenhouse. Plants were exposed to NaCl treatments (0, 100, and 200 mM) for 35 days, followed by 30 to 34 days of relief from salt stress to determine whether previously demonstrated genotypic differences in tolerance to salinity were related to water relations parameters. Exposure to high salt concentrations resulted in reductions in predawn water potential (Psi(w)), osmotic potential at full turgor (Psi(piFT)), osmotic potential at turgor loss point (Psi(piTLP)), and relative water content (RWC) in both cultivars, regardless of the growth substrate. Leaf Psi(w) and RWC returned to values similar to those of controls by the end of the relief period. The effect of salinity on Psi(pi) appeared earlier in Leccino than in Frantoio. Values for Psi(piFT) were -2.50, -2.87, and -3.16 MPa for the 0, 100, and 200 mM salt-treated Frantoio plants, respectively, and -2.23, -2.87, and -3.37 MPa for the corresponding Leccino plants. Recovery of Psi(pi) was complete for plants in the 100 mM salt treatment, but not for plants in the 200 mM salt treatment, which maintained an increased pressure potential (Psi(pi)) compared to control plants. Net solute accumulation was higher in Leccino, the salt-sensitive cultivar, than in Frantoio. In controls of both cultivars, cations contributed 39.9 to 42.0% of the total Psi(piFT), mannitol and glucose contributed 27.1 to 30.8%, and other soluble carbohydrates contributed 3.1 to 3.6%. The osmotic contribution of Na(+) increased from 0.1-2.1% for non-treated plants to 8.6-15.5% and 15.6-20.0% for the 100 mM and 200 mM salt-treated plants, respectively. The mannitol contribution to Psi(piFT) reached a maximum of 9.1% at the end of the salinization period. We conclude that differences between the two cultivars in leaf water relations reflect differences in the exclusion capacities for Na(+) and Cl(-) ions.

Biomass accumulation and allocation in soybean associated with genotypic differences in tolerance of nitrogen fixation to water deficits

Nitrogen fixation in soybean (Glycine max [L.] Merr.) is more sensitive to water deficits than many physiological processes and may therefore limit yield under nonirrigated conditions. Tolerance of nitrogen fixation to water deficits has been observed in the cultivar Jackson, however, the physiological basis for this is unclear. It was hypothesized that genotypes that could continue biomass production on limited soil water would prolong nitrogen fixation by continued photosynthate allocation to nodules. An initial greenhouse experiment compared biomass and N accumulation in six genotypes over an 8 d water deficit. Low stress intensity minimized genotypic expression of water-deficit tolerance; nevertheless, Jackson was clearly one of the most tolerant genotypes. In a second experiment, Jackson was compared to SCE82-303 at more severe stress levels. Biomass and N accumulation continued during water deficits for Jackson but ceased in SCE82-303. Individual nodule mass tended to increase during water deficits in Jackson and tended to decrease in SCE82-303, indicating greater allocation of photosynthate to Jackson's nodules in response to water deficits. Biomass accumulation of Jackson was contrasted with the USDA plant introduction (PI) 416937, which also has demonstrated tolerance to water deficits. For water-deficit treatments, total biomass accumulation was negligible for PI416937, but biomass accumulation continued at approximately 64% of the control treatment for Jackson. Transpirational losses for Jackson and PI416937 were approximately the same for the water-deficit treatment, indicating that Jackson had higher water use efficiency (WUE). Isotopic discrimination of 13C relative to12 C also indicated that Jackson had superior WUE during water deficits. Carbon-14 allocation in Jackson was compared to KS4895, a cultivar that was identified as sensitive to water deficits in an initial experiment. The comparison of water-deficit treatments of Jackson with KS4895 indicated that Jackson exported significantly greater amounts of14 C from labeled leaves and allocated approximately four times greater amounts of 14C per g of nodule. Results indicated that Jackson's sustained biomass production during water deficits resulted in the continued allocation of photosynthate to nodules and prolonged nitrogenase activity.

Sorghum genotypic differences in tolerance to excess manganese

Manganese (Mn) toxicity can be a growth limiting constraint for many plants grown on acid soil. Plant species/genotypes tolerant to Mn could help overcome detrimental Mn toxicity effects on plants grown on high Mn soils. Thirty-seven sorghum [Sorghum bicolor (L.) Moench] genotypes from a broad germplasm base were grown in solution culture (pH 4.5) with 0, 3.0, and 6.0 mM of added Mn above the basic solution concentration (18 μM) to determine genotypic differences in tolerance to excess Mn. Dry matter (DM) was used to evaluate 24-day-old plants (10 days in Mn treatments) for Mn toxicity responses. Wide variability among genotypes for differential DM was noted at 3.0 and 6.0 mM Mn. Sorghum generally tolerated high levels of Mn. Genotypes showing relatively high tolerance to excess Mn in solution were NB 9040, Wheatland, IS 7180, IS 7755, and IS 7809. Those genotypes showing relatively low tolerance to high Mn were ICA-Nataima, Martin, IS 7173c (SC 283), IS 7321, IS 9187, IS 9785, and IS 9828. IS 7173c, an aluminum (Al)-tolerant standard genotype, was sensitive to high Mn. Wide variability was noted among tissue culture generated lines derived from a common parent. Laboratory screening for tolerance to Mn toxicity was effective with sorghum, but results need to be verified in the field

Root demography of mature citrus trees: the influence of Phytophthora nicotianae

The amount of root mortality caused by root pathogens such as Phytophthora nicotianae (syn. Phytophthora parasitica) has typically been inferred from the net change in root length density in sequential soil cores. Because such measurements give information only on net changes in root populations, the actual rate of root turnover is often underestimated. We used minirhizotrons to track the fate of a large number of individual fine roots of mature field-grown citrus trees over a 6-month period. This method enabled us to examine the effect of P. nicotianae population levels on fine-root mortality. Seasonal and genotypic variation in patterns of citrus fine root mortality were associated with variation in population levels of P. nicotianae. Fine root lifespans were shorter when populations of P. nicotianae were high. Fine roots of the Phytophthora-susceptible rootstock, rough lemon (Citrus jamibhiri), had shorter median lifespans and supported larger populations of P. nicotianae than the fine roots of the more tolerant rootstock, Volkamer lemon (Citrus volkameriana). Rates of root mortality were either relatively constant for roots of all ages, or increased with age; the latter pattern was most pronounced for Volkamer lemon roots. Differences in the age-dependence of root mortality may, therefore, play a role in genotypic differences in tolerance of Phytophthora root rot by these two rootstocks. H Lambers Section editor

CROP TOLERANCE TO WEEDS

Plant breeders have successfully increased crop tolerance to many pests and physiological stresses. However, very little work has examined the potential for increasing levels of tolerance to weeds, despite weed control costs of several billion dollars annually. Evidence will be summarized from the literature supporting the contention that genotypic differences in tolerance to neighboring plants exist. Approaches to screening for tolerance, choice of selection environment, and genotype identification will be discussed.

Genotypic Differences in Tolerance of Ice Encasement, Low Temperature Flooding, and Freezing in Winter Wheat1

A key to improved winter survival of cereals in eastern Canada and northeastern USA appears to be resistance to a combination of winter stresses. Three traits associated with winterhardiness in winter wheat (Triticum aestivum L. emend Thell.) were measured on 34 genotypes in late fall and early spring in 2 successive yr, to determine whether the environmental controls of icing and flooding tolerances are more or less complex than those for freezing tolerance. Significant genotypic differences were observed for all three traits, and significant genotype ✕ environment (year and sampling) interactions were noted for icing and freezing tolerances. The environmental controls for the expression of icing and freezing tolerances thus appear relatively complex, whereas the control of low temperature flooding tolerances appears somewhat simpler. Genotypes with high freezing tolerances tended to exhibit high tolerance of both ice encasement and low temperature flooding, but there were some exceptions. ‘Norstar’ and ‘Roughrider’, which exhibit good winterhardiness in western North America, did not express the same relative stress tolerance in this study.

Reproductive Responses of Cowpea to Heat Stress: Genotypic Differences in Tolerance to Heat at Flowering1

The effect of high temperature on ability of cowpea [Vigna unguiculata (L.) Walp.] to set pods was investigated under field conditions and in controlled environments. During hot weather in the Imperial Valley, Calif., the cowpea strains TVu 4552, PI 204647, and ‘Prima’ produced substantial numbers of pods, while 55 other strains produced no or few pods, even though they produced numerous open flowers. Studies in growth chambers demonstrated that TVu 4552 has greater tolerance to high night and high day temperatures at flowering than the California cultivar, ‘California Blackeye No. 5’ (CBS), whereas Prima is intermediate. Susceptibility to high temperatures was associated with low pollen viability and indehischent anthers for CB5 and with anther indehiscence for Prima. Artificial pollination demonstrated that pistil viability of all three genotypes was not significantly affected by high temperature. Thus, the flower abscission observed with CB5 and Prima at high temperatures was mainly caused by male sterility. In addition, substantial levels of embryo abortion were observed with CB5 and Prima at high day temperatures. Apparently, Imperial Valley provides a suitable field environment during the summer for screening cowpeas for heat tolerance. This environment is presently being used to screen the segregating populations from crosses designed to incorporate the heat tolerance of TVu 4552 and Prima into genetic backgrounds suitable for the tropics and subtropics.

Effects of short-term temperature fluctuations on leaf photosynthesis in corn (Zea mays)

The effects of short-term temperature fluctuations on net photosynthetic rates (Pn) are described for a single-cross corn (Zea mays L.) hybrid (Harrow 691). The hybrid was grown at 25 °C and subjected to 4 days at either 15 or35 °C, and then maintained at 25 °C. Genotypic differences in tolerance of Pn to low temperature were studied using 12 hybrids of different maturities grown at 25 °C and subjected to 4 days at 15 °C.Following low temperature treatment. Pn was initially reduced by about 15% for leaves 5. 9. and 13 of hybrid Harrow 691. but completely recovered after 1 day at 25 °C. Following high temperature treatment, leaves 5 and 9 showed similar reductions in Pn whereas leaf 13 was not affected. The photosynthetic activity of the heat-stressed leaf 9 recovered after 1 day, but that of leaf 5 showed no recovery for 14 days at 25 °C.Among 12 hybrids. Pn of the newly matured leaf 7 was reduced after the low temperature treatment, but the reduction was more drastic for two late hybrids than for three early hybrids. Most hybrids showed complete recovery of Pn after 1 day at 25 °C. but genotypic differences were evident. The results reported indicate that further evaluation of the amount of genetic variation in temperature stability in corn could be carried out usefully.