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A greenhouse study was designed to determine the relative heat tolerance of 10 lima bean cultivars and to evaluate the effects of high temperature on lima bean yield. Cultivars were arranged in a randomized complete block with three blocks per treatment. The temperature treatments were 25C day/15C night and 35C day/25C night. Cultivars varied in their response to the higher temperature, allowing for classification into three heat response groups: intolerant, average, and tolerant. Heat-intolerant plants did not experience a significant reduction in number of pods, but number of beans and total bean weight were reduced at the higher temperature. Number of seeds per pod and average weight per bean also tended to decrease in intolerant plants at 35C. In future experiments, these data will be correlated with random amplified DNA (RAPD) markers. These markers will be evaluated for their potential for heat tolerance screening.

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Temperature sensitivity of net photosynthesis (Pn) was evaluated among 4 taxa of rhododendron including Rhododendron hyperythrum, R. russatum, and plants from two populations (northern and southern provenances) of R. catawbiense. Measurements were conducted on individual leaves at temperatures ranging from 15 to 40C. Temperature optima for Pn ranged from a low of ∼21 C for R. russatum to a high of ∼27C for R. hyperythrum. At 40C, Pn rates for R. hyperythrum, R. catawbiense (northern provenance), R. catawbiense (southern provenance), and R. russatum were 7.8, 5.7, 3.5, and 0.2 μmol·m-2·s-1, respectively. R. catawbiense from the southern provenance did not appear to have greater heat tolerance than plants from the northern provenance. There was no difference in temperature sensitivity of dark respiration among the taxa. Variations in heat tolerance among species appeared to result from a combination of stomatal and nonstomatal limitations on Pn at high temperatures.

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Heat at any growth stage can damage green beans (Phaseolus vulgaris L.), but plants are most susceptible at or near bloom. The effect of heat during the bloom period resulted in reduction of a yield in proportion to the duration of the heat period. The most critical growth stage was found to be 2 to 3 days before anthesis, rather than at anthesis itself. By subjecting F1 plants to heat during the bloom period, genetic selection for heat tolerance was moderately effective. The heritability for heat tolerance was quite low. Broad-sense heritability was 19% to 79% and narrow-sense heritability 0% to 14%. These values are probably conservative, since during the genetic study the heat period was initiated on the first day of bloom, which resulted in some escapes and excess variability.

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both amino acids and proteins between different cultivars of plants contrasting in heat tolerance will enable the identification of the key metabolic processes controlling genetic variations in heat tolerance. Free amino acids are constituents of

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Temperature sensitivity of net photosynthesis (PN) was evaluated among four taxa of rhododendron including Rhododendron hyperythrum Hayata, R. russatum Balf. & Forr., and plants from two populations (northern and southern provenances) of R. catawbiense Michx. Measurements were conducted on leaves at temperatures rauging from 15 to 40C. Temperature optima for PN ranged from a low of 20C for R. russatum to a high of 25C for R. hyperythrum. At 40C, PN rates for R. hyperythrum, R. catawbiense (northern provenance), R. catawbiense (southern provenance), and R. russatum were 7.8,5.7,3.5, and 0.2 μmol·m-2·s-1, respectively (LSD0.05 = 1.7). Rhododendron catawbiense from the southern provenance did not appear to have greater heat tolerance than plants from the northern provenance. Differences in dark respiration among taxa were related primarily to differences in tissue weight per unit leaf surface area. Temperature coefficients (Q5) for respiration did not vary in temperature response among taxa. Differences in heat tolerance appeared to result from a combination of stomatal and nonstomatal limitations on PN at high temperatures.

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Our objective was to determine heat tolerance and performance of 245 summer-flowering annual plant cultivars installed 16 Mar. 1995 in beds receiving full sun located at the E.V. Smith Research Center in Shorter, Ala. (lat. 32°30′N, long. 85°40′W). No maintenance, with the exception of one midseason pruning of petunias, was performed. Catharanthus roseus L. `Blush Cooler' had the highest mean rating (4.1 of 5.0). Salvia farinacea Benth. `Victoria Blue' and Petunia ×hybrida `Fantasy Pink' both performed well with 3.5 mean ratings. `Purple Wave', a compact spreading cultivar of P. ×hybrida, had a 3.1 mean rating, but had a 5.0 rating before pruning. We do not recommend pruning `Purple Wave'. Of the 34 marigold cultivars evaluated, Tagetes erecta L. `Antigua Mixed' had the highest mean rating. Tagetes erecta `Inca Yellow' and `Perfection Gold' tied with the second highest mean rating.

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In the summer of 1995 and 1996, 245 and 400 annual plant cultivars were evaluated for heat tolerance and landscape performance. Nine transplants of each cultivar were installed in raised beds amended with controlled-release fertilizer as per soil analysis recommendations, under full-sun and overhead irrigation, at the E.V. Smith Research Center in Shorter, Ala. (lat. 32° 30′ N, long. 85° 40′ W). No mainte-nance, with the exception of one midseason pruning of petunia, was performed on any of the cultivars. Catharanthus roseus 'Blush Cooler' was the best performer in 1995 with a mean rating of 4.1 (of 5.0). Salvia farinacea `Victoria Blue' and Petunia ×milliflora `Fantasy Pink' performed well, with a mean rating of 3.5. In 1996, the cultivar with the highest mean rating was Gomphrena globosa `Lavender Lady' (4.1). Second highest was G. globosa `Strawberry Fields' (4.0). Other cultivars that performed well in 1996 and had high mean ratings were Verbena × speciosa `Imagination' (3.6) and Melampodium paludosium `Derby' and `Medallion' (3.5 and 3.5).

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Leaf gas exchange and chlorophyll fluorescence measurements were used as indices for evaluating heat tolerance among five species of birch: paper (Betula papyrifera), European (B. pendula), Japanese (B. platyphylla var. japonica `Whitespire'), Himalayan (B. jacquemontii), and river (B. nigra). Measurements were conducted on individual leaves at temperatures ranging from 25C to 40C. Carbon exchange rates (CER) were depressed for all species at 40C. However, there was considerable variation in both absolute and relative (percent of maximum) CER among species at 40C; river birch maintained the highest absolute and relative CER while CER of paper birch was reduced the most. Although stomatal conductance of paper birch decreased at higher temperatures, internal leaf CO2 increased indicating that reduced stomatal conductance was not responsible for decreased CER. Stomatal conductance of river birch increased at higher temperatures which provided for enhanced uptake of CO2 and greater evaporative cooling. Variable chlorophyll fluorescence decreased similarly for both species with increasing temperatures. Measurements of dark respiration rates over the range of 25C to 40C suggested that the primary factor influencing variation in CER at higher temperatures was due to variation in respiration rates at higher temperatures.

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Abstract

Seedlings of Kentucky bluegrass (Poa pratensis L.), perennial ryegrass (Lolium perenne L.), red fescue (Festuca rubra L.), and weeping alkaligrass [Puccinellia distans (L.) Parl.] were exposed to water stress prior to measuring heat tolerance of leaf blade segments. Heat tolerance was determined using an electrolyte leakage assay. Water stress pretreatments did not increase in vitro heat tolerance of turfgrass leaves.

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Bermudagrass (Cynodon spp.) turf is often overseeded with a cool-season species such as perennial ryegrass (Lolium perenne L.) to provide an improved winter surface for activities such as golf or athletic events. Perennial ryegrass can become a persistent weed in overseeded turf due to the heat and disease tolerance of improved cultivars. Intermediate ryegrass is a relatively new turfgrass that is a hybrid between perennial and annual ryegrass (L. multiflorum Lam.). Very little information is available on intermediate ryegrass as an overseeding turf. Greenhouse, field, and growth chamber studies were designed to compare two cultivars of intermediate ryegrass (`Transist' and `Froghair') with three cultivars of perennial ryegrass (`Jiffie', `Racer', and `Calypso II') and two cultivars of annual ryegrass (`Gulf' and `TAM-90'). In a greenhouse study, the perennial ryegrass cultivars had finer leaf texture (2.9-3.2 mm), shorter collar height (24.7-57.0 mm), and lower weight/tiller (29-39 mg) than the intermediate and annual cultivars. In the field studies, the intermediate cultivar Transist exhibited improved turfgrass quality (6.1-7.1) over the annual cultivars (4.5-5.8) and the other intermediate cultivar Froghair (5.4-5.7). However, neither of the intermediate cultivars had quality equal to the perennial ryegrass cultivars (7.0-7.9). The perennial ryegrass cultivars exhibited slow transition back to the bermudagrass compared to the annual and intermediate ryegrass cultivars. In the growth chamber study, the annual and intermediate cultivars all showed increased high-temperature stress under increasing temperatures compared to the perennial cultivars, which did not show stress until air temperature exceeded 40 °C. Collectively, these studies indicate that the intermediate ryegrass cultivar Transist may have promise as an overseeding turfgrass due to its improved quality compared to annual types and a lack of heat tolerance relative to perennial cultivars, but with transition qualities similar to perennial ryegrass.

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