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  • Author or Editor: Abbas Lafta x
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Thermoinhibition of lettuce (Lactuca sativa L.) seed germination is a common problem associated with lettuce production. Depending on lettuce cultivars, seed germination may be inhibited when temperatures exceed 28 °C. The delay or inhibition of seed germination at high temperatures may reduce seedling emergence and stand establishment of lettuce in the field, leading to a reduction in economic yield. To identify heat-tolerant lettuce genotypes, lettuce varieties and germplasm accessions were screened for the ability to germinate under high-temperature stress. Twenty-four to 26 genotypes were selected from each lettuce types (crisphead, romaine, butterhead, loose leaf, and wild species) and their seeds were placed in petri dishes to test their ability to germinate at high temperatures (29 and 34 °C) as compared with controls at 24 °C. Some lettuce genotypes showed thermotolerance to 34 °C (less than 20% reduction in germination) such as Elizabeth, PI 342533, PI 358025, Florida Buttercrisp, Kordaat, Corsair, FL 50105, PRO 425, PI 278070, Noemie, Picarde, Gaillarde, L. serriola (PI 491112, UC96US23, PI 491147), L. virosa (PI 274378 D), L. saligna (PI 491159), and primitive (PI 187238 A, PI 289063 C). The germination rates were consistent with the germination percentage at the high temperatures. Seed germination in the field was very low and positively correlated with seed germination at 29 and 34 °C. The highest field germination percentages (greater than 40%) were observed in Belluro, Mantilia, Mid Queen, Headmaster, PRO 874, PRO 425, FL 50105, Corsair, Romaine SSC 1148, Romaine Romea, Green Forest, Grenadier, FL 43007, Squadron, Xena, Noemie, Green Wave, Picarde, and Red Giant. The results of this study indicated that lettuce genotypes differ greatly in their ability to germinate at high temperatures as determined by the percentages and the rates of germination. Our research indicates that thermoinsensitive varieties could be used to expand lettuce production seasons in warm and low land cost areas and reduce the need for seed priming, lowering the production costs. The information may also be useful for growers to better choose cultivars for warm environments and for lettuce breeders to improve the crop for adaptation to global warming and climate change.

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Potato plants are sensitive to heat stress, which reduces tuber yield and alters whole plant partitioning. This study was conducted to determine the effect of high temperature on activity of enzymes related to sucrose metabolism in source and sink tissues of potato plants. Potato plants were exposed to two temperature regimes (20 and 28 C) for 3 days. High temperatures resulted in an increase in sucrose synthase and acid and neutral invertase activities in young growing leaves (< 1.5 cm). However, there were no significant changes in these enzyme activities in stems or mature leaves. The results indicate that both sucrose synthase and invertases are involved in sucrose breakdown in young leaves. We will also discuss the role of sucrose phosphate synthase in mature source leaves and how it is affected by temperature and altered partitioning pattern. Activity of sucrose synthase was affected more than ADP-glucose pyrophosphorylase in small growing tubers exposed to high temperatures.

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Lettuce (Lactuca sativa L.) is a cool season crop that is vulnerable to high temperature stress, which promotes bolting and decreases yield and quality. It is anticipated that climate change may lead to higher temperatures in current lettuce growing areas in the United States, thereby negatively affecting lettuce production and possibly resulting in adverse impacts on global food production. Therefore, it is important to identify lettuce germplasm with tolerance to temperatures higher than those that have occurred over the past century. We evaluated 25 crisphead lettuce cultivars for tolerance to high temperature stress in the San Joaquin, Imperial, and Salinas Valleys, CA. Genetic variation was identified for yield and horticultural traits, such as core length, head diameter, tipburn, bolting, and market maturity, of crisphead lettuce grown in warmer conditions. Significant genotype × environment interaction did not account for most of the variation; the main differences were found for environments and only a small proportion of the variation was due to genotypes. Cultivar Primetime is a good source of heat tolerance for crisphead lettuce, as it presented the best yield and exhibited other desirable characteristics across warmer conditions. These results provide insight into the cultivars that respond well to hot environments. Moreover, the data can be used by breeders to develop new heat-tolerant lettuce cultivars.

Open Access

Warmer temperatures during crop production are not desirable for a cool-season crop such as lettuce (Lactuca sativa L.). Lettuce is among the top 10 most consumed vegetables in the United States. Production of this vegetable is concentrated mostly in temperate areas of California, and during the wintertime in Arizona and Florida as a result of their mild climatic conditions. Heat-tolerant cultivars are needed for the leafy vegetable industry to continue thriving. However, there is very little information on heat-tolerant germplasms of lettuce that can be used as a source to improve heat tolerance in lettuce. This is particularly important in romaine and butterhead lettuce, which are two morphological types with increasing demand in the market. Therefore, research was conducted to identify germplasm that performs acceptably in warmer regions in the western United States. This investigation also aimed to understand the reaction of varieties to different environments, which could help plant breeders select and evaluate lettuce plants during the breeding process. Twenty-three and 25 accessions of romaine and butterhead lettuce, respectively, were planted in five trials near Holtville, CA, USA: Five Points, CA, USA, under warmer temperatures and Salinas, CA, USA, under cooler temperatures. Romaine genotypes Bambi, Blonde Lente a Monter, Medallion MT, and Red Eye Cos; and butterhead genotypes Butter King and Margarita had no bolting, an acceptable head weight, short cores, and acceptable head height. Head weight and related traits (including core length, height, width, etc.) and heat-related disorders were significantly different across multiple experiments, indicating genetic variation. The major component of the phenotypic variation in these experiments was a result of environmental factors. Therefore, plant breeders may still need to evaluate progeny in multiple trials and multiple locations to select heat-tolerant romaine and butterhead lettuce effectively.

Open Access

Potato (Solanum tuberosum L.) responds to heat stress with a shift in partitioning from tubers to shoots. Enzymes responsible for sucrolysis previously have been used as indicators of sink strength and are likely involved in controlling flow of carbon into developing organs. Changes in activity of enzymes involved in sucrose metabolism were investigated in shoots of two potato cultivars that previously were characterized as susceptible and tolerant to heat stress. Enzyme activity of sucrose synthase (SS) and invertases was determined for mature leaves, young leaves, and stems of plants adapted to 21/19 °C, or after transferring plants to 29/27 °C for 3 days. High temperatures resulted in a nonsignificant increase in activities of SS, acid, and neutral invertase in young growing leaves but not in stems or mature leaves. The combined activity of the two invertases was ≈40 times higher than SS activity in young leaves. There was no temperature genotype interaction with regard to these enzymes in the tissues investigated. A previously reported increase in activity of sucrose-phosphate synthase in mature leaves of plants subjected to high temperature was reversed after these plants were returned to a normal growing temperature.

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Growth chamber and greenhouse experiments were conducted to investigate the effect of temperature and irradiance on foliar glycoalkaloids of three potato genotypes (Solanum tuberosum L.) that differ in glycoalkaloid content. Two genotypes (ND4382-17 and ND4382-19) produced the acetylated glycoalkaloids, leptine I and II, that contribute resistance to the Colorado potato beetle (CPB, Leptinotarsa decemlineata Say). The glycoalkaloids were separated and quantified by high performance liquid chromatography. Exposure of plants to high temperature (32/27 °C, 14-hour day/10-hour night) for 3 weeks under a 14-hour photoperiod with an irradiance of 475 μmol·m-2·s-1 significantly increased the levels of leptines I and II, solanine, and chaconine compared to that at low temperature (22/17 °C). Increases in foliar leptines and total glycoalkaloids at high temperature were 90% and 169%, respectively. Growing potato plants at low irradiance (75% reduction) for 2 or 4 weeks resulted in a significant reduction in the levels of leptine I and II (46%), solanine (43%), and chaconine (38%) compared to nonshaded plants. Transferring plants from high to low irradiance or from low to high irradiance for 2 weeks caused a decrease and an increase in glycoalkaloid concentration, respectively. Therefore, both temperature and irradiance influenced foliar levels of glycoalkaloids in potato plants without changing the leptines and solanine to chaconine ratios. Thus, irradiance and temperature influenced glycoalkaloid compounds that can effect resistance to CPB, especially leptine I and II.

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Global warming poses serious threats and challenges to the production of leafy vegetables. Being a cool-season crop, lettuce is particularly vulnerable to heat stress. To adapt to climate change, this study was conducted to evaluate the performance of leaf lettuce genotypes for heat tolerance by growing them in different locations within California that differ in temperatures during the growing season. Fifteen green leaf and 21 red leaf lettuce genotypes were selected to evaluate their performance under these environments. These genotypes were planted in March and May in Five Points (San Joaquin Valley) and El Centro (Imperial Valley) and in June 2012 in Salinas (Salinas Valley). The results suggest that lettuce planting can be extended from January to March beyond the normal growing seasons in San Joaquin and Imperial Valleys, where yield may be higher than in the Salinas Valley. The further delay in planting date from March to May in Five Points and El Centro resulted in reduction of yield and an increase in susceptibility to bolting and heat-related disorders such as tipburn and leaf desiccation in most genotypes. The susceptibility to these disorders depends on the genotype and the temperature during lettuce growth and maturation. However, heat-tolerant leaf lettuce genotypes adapted to these regions were identified. Results of this research should be useful for the development of heat-tolerant lettuce cultivars and for extending the growing season in warmer but lower land cost areas to reduce production costs.

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The influence of potassium (K) nutrition on the growth and sugar contents of carrot (Daucus carota L.) cv. Navajo was investigated in a greenhouse study. Seeds were germinated in 15-cm plastic pots (volume1.5 L) containing a peatlite mix (2 parts peat:1 part vermiculite:1 part perlite, v/v). Starting at 6 true-leaf stage (5 weeks from germination), plants were watered with nutrient solutions containing 0, 1, 2, 4, or 8 meq K/L for 10 weeks. While plants receiving no potassium had the lowest biomass yield, there was little or no difference in shoot or root biomass yields between different K concentrations. Root glucose and sucrose contents were the highest when plants grown with 8 meq K/L and 4 meq K/L, respectively, from the nutrient solution. The influence of nutrient solution K concentration on tissue content of K and other macronutrient elements was also determined.

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The relative concentrations of sucrose, glucose, and starch in the xylem and cortex tissues of carrot (Daucus carota) roots were evaluated after harvest and during storage. For the three cultivars (Apache, Bolero, Danvers 126) tested, the cortex tissue contained 76.6, 49.1, and 33.6 mg·g–1 dry weight of sucrose, glucose, and starch, respectively. In comparison, the average contents of sucrose, glucose, and starch in xylem tissues were 57.4, 52.4, and 11.6 mg·g–1 dry weight, respectively. In general, cortex tissue contained higher concentrations of sucrose and starch than the xylem tissues. The glucose concentrations in cortex and xylem were similar. In `Apache', for example, the cortex tissue contained 40% and 57% higher concentrations of sucrose and starch, respectively, than the xylem tissues, whereas glucose content of the cortex was only 7.5% higher than that of the xylem. Since sweetness is largely influenced by sucrose, the relative volume of cortex to xylem must be considered in evaluating carrot cultivars for sweet taste.

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