( Fig. 2 ) suggest that bell pepper plants were under heat stress, particularly those that were unshaded. Root zone temperature under plastic mulch affects plant growth and yield in several vegetable crops ( DÃaz-Pérez et al., 2008 ). Root zone
reduced root zone temperature to a value closer to optimal root zone temperature (≈27 °C) for bell pepper compared with unshaded conditions ( DÃaz-Pérez, 2013 ). Decreased disease associated with shading may be related to amelioration of heat stress of
Temperature and chlorophyll fluorescence characteristics were determined on leaves of various horticultural species following a dark adaptation period where dark adaptation cuvettes were shielded from or exposed to solar radiation. In one study, temperature of Swietenia mahagoni (L.) Jacq. leaflets within cuvettes increased from ≈36C to ≈50C during a 30-minute exposure to solar radiation. Alternatively, when the leaflets and cuvettes were shielded from solar radiation, leaflet temperature declined to 33C in 10 to 15 minutes. In a second study, 16 horticultural species exhibited a lower variable: maximum fluorescence (Fv: Fm) when cuvettes were exposed to solar radiation during the 30-minute dark adaptation than when cuvettes were shielded. In a third study with S. mahagoni, the influence of self-shielding the cuvettes by wrapping them with white tape, white paper, or aluminum foil on temperature and fluorescence was compared to exposing or shielding the entire leaflet and cuvette. All of the shielding methods reduced leaflet temperature and increased the Fv: Fm ratio compared to leaving cuvettes exposed. These results indicate that heat stress from direct exposure to solar radiation is a potential source of error when interpreting chlorophyll fluorescence measurements on intact leaves. Methods for moderating or minimizing radiation interception during dark adaptation are recommended.
Forty hybrid broccoli [Brassica oleracea L. (Italica Group)] accessions were screened for heat tolerance and holding ability over three planting dates in 1988 at the Long Island Horticultural Research Laboratory in Riverhead, N.Y. Holding periods were quantified using the number of consecutive days between the time individual heads reached 10 cm diameter and cutting, which occurred when the sepals had fully expanded and had just begun to separate. In 1989 and 1991, heat stress was applied at various weeks during maturation to determine the most sensitive stage or stages of plant development in terms of reduction in holding period and head weight. Field studies and heat stress experiments indicate that heat stress may be most critical during the time the immature inflorescence measures 5 to 10 mm in diameter. This stage corresponds to ≈ 3 weeks before harvest for summer plantings in the northeastern United States.
Exposure of stratified apple (Malus domestics Borkh. cv. Golden Delicious) seeds to 30C induces secondary dormancy. To determine if an increase in abscisic acid (ABA) content was associated with the loss in germination capacity, stratified seeds (3,- 6, or 9 weeks at 5C) were held at 30C for 0, 3, or 6 days. Stratification at 5C either had no effect or increased ABA content in embryonic axes, cotyledons, and seed coats. Exposure to 30C after stratification either did not affect or decreased ABA content of embryonic axes and seed coats; in contrast, cotyledonary ABA was increased. Seed coats, cotyledons, and embryonic axes stratified for 3, 6, or 9 weeks at 20C contained the same or higher levels of ABA in comparison with nonstratified seeds or seeds stratified at SC. Changes in ABA levels were not consistently correlated with changes in germination capacity during stratification or after exposure to 30C. These data suggest that changes in ABA are not related to changes in dormancy. Chemical names used: abscisic acid (ABA); butylated hydroxy-toluene (BHT); n-(trichloromethyl) thio-4-cyclohexene-1,2-dicarboximide(Captan).
The demand for new and/or improved herbaceous annuals and perennials continues to increase, making information on production and viability of these plants a necessity. In Louisiana and the Southern U.S., one of the greatest impediments to production of marketable herbaceous plants and their longevity is high temperature. Herbaceous plants have various stages of vegetative growth and flowering; high temperatures during these developmental stages can have a tremendous impact on plant metabolism, and thus plant growth and development. The goal of this research was to better understand the differences between heat tolerant (HT) and heat sensitive (HS) species and cultivars at various high temperatures in terms of whole plant growth, flowering, photosynthesis, carbohydrate content, electrolyte leakage, chlorophyll content and plant small heat shock proteins (HSP) expression levels. Salvia splendens Vista Series (HT), Sizzler series (HS); Viola witrokiana `Crystal Bowl Purple' (HT), `Majestic Giant Red Blotch' (HS), F1 Nature Series (HT) and F1 Iona Series (HS); Gaillardia × grandiflora `Goblin' (HT) and Coreopsis grandiflora `Early Sunrise' (HS) were grown from seed in growth chambers under 25/18 °C (day/night) cycles. Plants at 4, 6, and 8 weeks after germination were subjected to different high temperature treatments of 25 (control), 30, 35, 40, and 45 °C for 3 h. Results show that there was a significant difference in net photosynthesis, electrolyte leakage, soluble carbohydrate content and HSP levels between HT and HS cultivars. Effects of high temperature on plant growth, chlorophyll content, and number of days to flower, flower size, and marketable quality were also significantly different.
Herbaceous perennials are one of the fastest growing ornamental sectors in the United States. Current production recommendations do not address the effect of environmental factors, such as high temperature, on growth of herbaceous perennials. The focus of this research was to determine how supra-optimal temperatures effect growth and photosynthesis. Plants were exposed to a high temperature of 35 °C and photosynthesis measurements were recorded over a 6-week period at 1100, 1300, and 1500 hr. Results indicate that the time of day the measurements were taken made little difference on rate of photosynthesis and that there was a similar trend in photosynthetic rate over the 6-week period. Photosynthesis decreased as the plants began to flower and then increased until the onset of flower senescence. Plants grown at supraoptimal and optimal conditions had a similar trend and rate of photosynthesis throughout the 6-week period. Plant growth significantly decreased as the duration of high temperature increased for both species; however, Gaillardia was more heat tolerant then Coreopsis.
Gamete selection was examined as a breeding tool in developing Phalaenopsis hybrids that are more extreme temperature tolerant. Four pairs of hybrid cultivars were cross-pollinated, and then exposed to two temperature extremes, 30 °C/25 °C and 14 °C/9 °C, during initial pollen tube growth. One of each pollinated orchid cultivar was placed in either of two growth chambers and exposed to an 11-hour photoperiod with an irradiance of 180 mmol·m-2·s-1 and a relative humidity of 70% for 3–7 days. The capsules were collected after 150 days. Protocorm development was evaluated after 73 days on a thermogradient table ranging from 10 to 30 °C. For cold-pollinated seeds, protocorm development was optimum at 22 and 28 °C (means of 290 and 250 protocorms per plate, respectively), whereas the greatest protocorm development for warm-pollinated seeds occurred at 20 °C (103 protocorms/plate). Protocorms were evaluated for leaf and root formation 125 days after initial plating. Transfer to warm or cold incubators occurred as seedlings matured. One year after the initial plating, seedlings were evaluated on the following criteria: weight, number of leaves, leaf width, leaf length, leaf area, number of roots, and root length. The pollination treatment significantly affected the number of roots per seedling, whereas table position during germination significantly affected weight. The weight, number of leaves, and average root length were significantly affected by the pollination treatment and incubator interaction. The interaction between pollination treatment and table position significantly affected weight, number of roots, and average root length. Additional replication is required to confirm the greater germinability of cold-pollinated seed at higher temperatures.
Roots of hydroponically grown Hibiscus Rosa-sinesis L. cuttings were exposed to 22, 30, 40, or 50C for 20 minutes, after which they were inoculated with zoospores of Phytophthora parasitica Dastur. Visual assessment of root discoloration and culturing of randomly selected root pieces 10 to 13 days after treatment showed that roots exposed to 40 or 50C had a significantly higher incidence of infection than those exposed to 20 or 30C. Plants were also grown in pots containing University of California (UC) mix or washed, graded sand and exposed to solar radiation for 1 day or 3 weeks, respectively. Root systems of plants in direct sunlight heated to 52C, while roots of shaded plants heated to 40C. Assessment of infection severity was done visually or by means of a Phytophthora-specific antibody probe. In all experiments, infection severity was highest in sun-exposed plants and was insignificant to moderate in shaded plants.
`Early Calwonder' pepper (Capsicum annuum L.) and `Jubilee' corn (Zea mays L.) leaf disks exposed to high temperature stress produced ethylene, ethane, methanol, acetaldehyde, and ethanol based on comparison of retention times during gas chromatography to authentic standards. Methanol, ethanol, and acetaldehyde were also identified by mass spectroscopy. Corn leaf disks produced lower levels of ethylene, ethane, and methanol, but more acetaldehyde and ethanol than pepper. Production of ethane, a by-product of lipid peroxidation, coincided with an increase in electrolyte leakage (EL) in pepper but not in corn. Compared with controls, pepper leaf disks infiltrated with linolenic acid evolved significantly greater amounts of ethane, acetaldehyde, and methanol and similar levels of ethanol. EL and volatile hydrocarbon production were not affected by fatty acid infiltration in corn. Infiltration of pepper leaves with buffers increasing in pH from 5.5 to 9.5 increased methanol production.