Heat stress has been defined as the rise of temperature for a period of time higher than a threshold level, thereby permanently affecting the plant growth and development. Day or night temperature is considered as the major limiting factor for plant growth. Earlier studies reported that night temperature is an important factor in the heat reaction of the plants. Tomato cultivars capable of setting viable fruits under night temperatures above 21 °C are considered as heat-tolerant cultivars. The development of breeding objectives is generally summarized in four points: (a) cultivars with higher yield, (b) disease resistant varieties in the 1970s, (c) long shelf-life in 1980s, and (d) nutritive and taste quality during 1990s. Some unique varieties like the dwarf “Micro-Tom”, and the first transgenic tomato (FlavrSavr) were developed through breeding; they were distributed late in the 1980s. High temperature significantly affects seed, pollen viability and root expansion. Researchers have employed different parameters to evaluate the tolerance to heat stress, including membrane thermo stability, floral characteristics (Stigma exertion and antheridia cone splitting), flower number, and fruit yield per plant. Reports on pollen viability and fruit set/plant under heat stress by comparing the pollen growth and tube development in heat-treated and non-heat-stressed conditions are available in literature. The electrical conductivity (EC) have been used to evaluate the tolerance of some tomato cultivars in vitro under heat stress conditions as an indication of cell damage due to electrolyte leakage; they classified the cultivars into three groups: (a) heat tolerant, (b) moderately heat tolerant, and (c) heat sensitive. It is important to determine the range in genetic diversity for heat tolerance in tomatoes. Heat stress experiments under field conditions offer breeders information to identify the potentially heat tolerant germplasm.

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Tomato is a mild season crop and high temperature stress impacts productivity negatively. However, the development of cultivars with improved heat tolerance is possible as genetic variability has been consistently reported. This study aimed to identify candidate genes that impact various traits under heat stress. Genome-wide association studies (GWAS) were conducted on a diverse set of 144 tomato genotypes collected from various germplasm centers and breeding programs. The genotypes were grown under control and heat stress in poly tunnels having mean temperatures of 30°C and 45°C for two seasons and phenotypic data were collected on seven agro-physiological traits. All individuals were genotyped withthe80K DArTseq platform using 31237 SNP markers. Data were analysed using a mixed model based on restricted maximum likelihood (REML). Pattern analysis of the phenotypic data showed five primary clusters each with genotypes from multiple origins. Based on the genotypic data, three wild tomato genotypes showed a degree of un-relatedness with the other materials as they were distantly located from the rest of the genotypes in the scatter plot. Control treatment data were used to ascertain markers that are exclusively important under high temperature stress. A large number of markers were significantly associated with various traits under heat stress. These included strong marker associations for number of inflorescence/plant (IPP), number of flowers/inflorescence (FPI), fresh fruit weight (FFrW), and electrolyte leakage (EL). High association with EL was found due to two SNPs 7858523|F|0-25:G>A-25:G>A and 4705224|F|0-60:C>G-60:C>G located on Chr 6. Other less pronounced marker-trait associations were observed for plant dry weight (PDW), and number of fruit/plant (FrPP).

Tomato is an important vegetable crop worldwide and Fusarium wilt is a significant disease of tomato in many countries. One hundred and fifty three diverse tomato genotypes collected from local and exotic sources were screened for resistance to F. solani in the greenhouse. Significant variation in genotype response to inoculation was observed. The organic metabolite profiles of resistant and susceptible genotypes were assessed to determine the basis of resistance. Significant genetic variation was observed for ABA, malic acid, citric acid, fructose, glucose, sucrose, L-proline and myo-inositol. The disease treatment produced significant changes in fructose, glucose, proline and sucrose and significant genotype-by-treatment interaction was observed for ABA, citric acid, fructose, glucose, malic acid and sucrose, indicating that genetic improvement of these traits is feasible. Disease incidence was strongly associated with citric acid (R 2 = 0.84), sucrose (R 2 = 0.72) and L-proline (R 2 = 0.76). Principal component analysis confirmed that citric acid and L-proline were important in determining plant disease response. Genetic variation for Fusarium wilt resistance can be used to develop new tomato cultivars with improved disease resistance.

The identification of heat tolerance traits that express across environments is key to the successful development of high temperature tolerant tomatoes. A replicated experiment of 145 tomato genotypes was established at two temperature regimes in two planting seasons using hydroponics in a poly greenhouse to assess high temperature tolerance. Electrolyte leakage, number of inflorescences, number of flowers and fruits, fresh fruit weight and fresh and dry plant weight were measured and genotype and temperature treatment differences were observed for all traits. Planting season impacted all traits except electrolyte leakage and number of flowers. High temperature reduced number of fruits by 88.8%, flower fruit set ratio by 77.2% and fresh fruit weight by 79.3%. In contrast, traits little impacted by temperature included number of flowers per inflorescence (1.3%) and plant dry weight (11.1%). The correlation between plant dry weight under both high and optimal temperature was significant (R 2 = 0.82). To assess the effectiveness of plant dry weight and flower-fruit set ratio for selection under heat stress two subsets of genotypes (A and B) comprising ten and six genotypes respectively, were subsequently selected on the basis of their dry weight loss and flower-fruit set ratio under high temperature. Organic metabolite analyses of set A and B respectively, showed a significant change (%) in citric acid (77.4 and 15.4), L-proline (117.8 and 40.2), aminobutyric acid (68.6 and 11.8), fructose (24.9 and 21.3), malic acid (50.3 and 42.7), myo-inositol (55.1 and 6.1), pentaerythitol (54.1 and 39.0) and sucrose (34.7 and 25.8). The change (%) in all metabolites was greater in heat tolerant genotypes with the exception of fructose and sucrose where sensitive genotypes produced a higher variation. The change in sucrose in tolerant genotypes was variable in subset A and more uniform in subset B. Flower-fruit set ratio was found as a reliable trait for discriminating between How to cite this paper:

The incidence of Fusarium wilt on tomato is increasing with rising global temperatures. To assess the impact of this disease, a factorial experiment of ten tomato genotypes grown at two temperatures and two levels of disease severity was established using hydroponics in a poly tunnel house. Fusarium wilt inoculum was used to promote disease and the heat treatment produced temperatures >40 o C for most of the growing season. Genotypes varied significantly for disease incidence and response to heat stress. Significant temperature x disease treatment interactions were observed for number of inflorescences per plant (IPP), fruit set ratio (FSR), number of fruits per plant (FPP), fresh fruit weight (FFW), plant dry weight (PDW) and disease severity index (DSI). A highly significant correlation (R 2 = 0.98) was observed between disease incidence under both control and high-temperature treatments, even though the treatments were significantly different. Biomass was reduced under both heat and disease stress and was correlated across treatments (R 2 = 0.86). The genotype LA3847, characterized by relatively low fruit set inhibition and high fruit yield, was classified as tolerant to both heat and disease stress. Concurrent selection for improved disease and heat tolerance appears possible as the disease severity index and the heat stress response were not correlated (R 2 = 0.11).Keywords: tomato, genetic variability, heat stress, Fusarium wilt. Abbreviations: IPP_ number of inflorescence per plant; DSI_ disease severity index; FSR_ fruit set ratio; FPP_ fruit per plant; FFW_ fresh fruit weight; PDW_ plant dry weight.

Fusarium pseudograminearum and Fusarium graminearum commonly cause crown rot (FCR) and head blight (FHB) in wheat, respectively. Disease infection and spread can be reduced by the deployment of resistant cultivars or through management practices that limit inoculum load. Plants deficient in micronutrients, including zinc, tend to be more susceptible to many diseases. On the other hands, and zinc deficiency in cereals is widespread in Australian soils. Zinc deficiency may have particular relevance to crown rot, the most important and damaging Fusarium disease of wheat and barley in Australia. Four wheat genotypes; Batavia, Sunco and two lines from the International Maize and Wheat Improvement Center (CIMMYT) were tested for response to FHB and FCR under differing levels of Zn,1 and 2 g/kg and its correlation with disease severity. Sunco and CIMMYT line 146 were previously rated resistant to crown rot and Zn efficient. Zn application 2 g/kg soil enhanced resistance to FCR of the disease susceptible and Zn in-efficient in Batavia and 48 as its recorded 0.75 and 0.5 respectively compared to Sunco and CIMMYT line 146 as it recorded 0.2 and 0.3 respectively, but did not increase resistance to FHB. However, Zn application did enhance the resistance of Zn efficient genotypes to FHB. Results suggest that higher levels of Zn fertilization could reduce the expression of Fusarium diseases in wheat.