Cuttings of Dendranthema ×grandiflorum `Paragon' were used as a model system to assess the effects of root heating on disease severity. Roots were exposed to single episodes of heat stress, after which they were inoculated with zoospores of Phytophthora cryptogea Pethyb. & Laff. Root damage resulting from heat stress, or heat stress plus Phytophthora, was quantified 5 to 7 days after treatment. Roots of hydroponically grown plants, immersed for 30 min in aerated, temperature-controlled nutrient solutions, were severely damaged at 45C or above. Relatively little phytophthora root rot developed on inoculated plants exposed to 25 or 35C, but infection was severe in roots heated to 40C. Plants grown in potting mix were exposed to heat stress by plastic-wrapping the containers in which they were growing and placing them in heated water baths until roots achieved desired temperatures for 30 min. This system heated roots more slowly than in the hydroponic experiments, and 45 and 50C were less damaging. The amount of Phytophthora-induced root damage was insignificant in containerized plants heated to 25 or 35C, but was highly significant in those heated to 40C or higher. In field experiments, plants were positioned so their containers were either fulIy exposed to the late afternoon sun or heavily shaded to prevent sun exposure. The root zones of sun-exposed pots heated to 45 to 47C, while those of shaded pots never exceeded 34 to 36C. There was a large, highly significant increase in phytophthora root rot severity in the sun-exposed pots compared to shaded plants. These experiments showed that temperatures of 40C or higher, which commonly occur in container-grown plants exposed to solar radiation, can predispose chrysanthemum roots to severe Phytophthora infection.
Chlorophyll fluorescence was measured under both laboratory and greenhouse conditions in an effort to develop a quick, reliable, and inexpensive laboratory procedure capable of predicting heat stress experienced by tomato (Lycopersicon esculentum Mill.) under greenhouse conditions. The laboratory tests consisted of measurements of the ratio of variable to maximal chlorophyll fluorescence (Fv/Fm) performed on leaf discs taken from whole tomato leaves and placed on a temperature controlled plate. Comparisons were made with greenhouse measurements of the same parameter conducted on intact leaves of whole plants exposed to different temperature treatments imposed by manipulation of the aerial environment of the greenhouse. Dark adaption periods ranging from 15 min to all day in the greenhouse and temperature exposure periods ranging from 5 min to 60 min in the laboratory were compared to find the best correlation between the two tests. Best agreement was obtained with 60 min treatment times in the laboratory and 60 min dark adaption periods in the greenhouse. Fv/Fm decreased quadratically with increasing leaf temperature in a similar fashion in both tests, suggesting that the laboratory approach can adequately predict plant response to greenhouse heat stress.
Bell pepper (Capsicum annuum L.) was grown in 1989, 1990, and 1991. Cultural modifications were used in an effort to alleviate heat stress, improve fruit set, reduce sunscald, and improve yield quantity and quality. Treatments included bare soil, plastic mulch (both black and white), straw mulch, living rye (Secale cereale) mulch, and row covers (white and black) suspended above the foliage. Soil temperature at 2.5, 10, and 20 cm, soil moisture at 20 cm, and yield parameters were recorded. In general, plots containing white rowcovers produced good yields each year, straw mulched plots produced good yields two out of three years, plots with black plastic mulch gave poor yields two out of three years, and plots with living rye gave consistently poor yields. Yield inconsistency from year to year was correlated with, and can be explained by, soil temperatures. Sunscald was reduced by rowcovers.
Ethanol concentration and chlorophyll fluorescence (CF) were measured as signs of heat stress in apple fruit [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.]. `McIntosh', `Cortland', `Jonagold', and `Northern Spy' apples were placed in trays and exposed to 46 °C for 0, 4, 8, or 12 hours. Following treatments, fruit were stored in air at 0 °C and evaluated after 0, 1, 2, or 3 months. Ethanol and ethylene production, CF, peel and flesh browning, firmness, skin color, soluble solids, and titratable acidity were measured. Increases in ethanol were apparent immediately following 12-hour heat treatments as well as after 3 months. After 3 months, ethanol concentrations were 16-, 52-, 6-, and 60-fold higher in `McIntosh', `Cortland', `Jonagold', and `Northern Spy' apples than in controls, respectively. The concentrations of ethanol accumulated reflected the degree of heat-induced fruit injury. Heat treatments reduced ethylene production relative to control values. After 3 months of storage ethylene production of fruit exposed to 46 °C for 12 h was <0.48 μmol·kg-1·h-1 compared to >4.3 μmol·kg-1·h-1 for controls. Heat treatments also reduced CF which was expressed as Fv/Fm, where Fv is the difference between the maximal and the minimal fluorescence (Fm - Fo), and Fm is the maximal fluorescence. After 3 months storage at 0 °C, Fv/Fm was ≈0.2 in fruit held at 46 °C for 12 hours compared with 0.5-0.6 for control fruit. Exposure to 46 °C for 12 hours caused severe peel and flesh browning in all cultivars. Severity of peel and flesh browning increased with increasing duration of heat treatment and subsequent storage at 0 °C. `Northern Spy' apple fruit were most susceptible to heat stress based on the degree of flesh browning. Heat treatments of 8 and 12 hours reduced firmness of `McIntosh', `Cortland', and `Northern Spy', but not `Jonagold' apples. Hue angle of the green side of fruit was also reduced in `Cortland', Jonagold' and `Northern Spy' apples receiving the 8- and 12-hour treatments. Heat treatments caused a decrease in fruit tiratable acidity, but had no effect on soluble solids content. The increase in ethanol production and decrease in CF correlated with heat-induced injury, and were apparent before browning was visually apparent. Ethanol and CF have the potential to be used to nondestructively predict the severity of injury that develops during storage.
High temperature effects potato production by reducing overall growth and partitioning of photosynthate to tubers. Recent studies from our laboratory demonstrated that these effects can be reduced by increasing rhizospheric calcium. This present study was conducted to determine if this mitigation of heat stress effect on potato is due to modulation of heat shock protein by calcium during stress. An inert medium and nutrient delivery system capable of maintaining precise rhizospheric calcium levels were used. Biomass was measured and protein samples were collected from potato leaves. Using electroblotting, heat shock proteins were detected by antibodies to Hsp21 and Hsp70 (obtained from Dr. Elizabeth Vierling). Injury by prolonged heat stress was mitigated at calcium concentration >5 ppm. The calcium concentration of leaf and stem tissues were twice as high in 25 ppm calcium-treated plant compared to 1 ppm calcium-treated plants. Total foliage fresh weight was 33% higher and dry weight 20% higher in plants supplied with 25 ppm of calcium than supplied with 1 ppm of calcium. HSP21 was expressed only at high temperature and at greater concentrations in 25 ppm calcium treatment. HSP70 was expressed in both control, 20 °C/15 °C (day/night) and heat-stressed tissue, 35 °C/25 °C (day/night) under various calcium treatments (1 to 25 ppm). Also, there were some differences in HSPs expression patterns between young and mature leaves. Young tissue responded immediately to the heat stress and started to express HSP21 within 1 day. Mature tissue started to express HSP21 after 2 days. HSP21 of young tissue disappeared sooner than mature tissue when heat stress-treated plants were returned to normal conditions. These results support our earlier studies indicating that an increase in rhizospheric calcium mitigate heat stress effects on the potato plant. Furthermore these results suggest that this mitigation may be due to modulation of HSP21by rhizospheric calcium during heat stress.
23 °C. Temperatures above 30 °C are quite common during summers in the southeastern states of the United States ( Weather Channel, 2008 ). Elevated air temperatures (heat stress) have been found to cause foliar bleaching in ivy geraniums ( Dhir et al
Rubisco, the primary enzyme governing carbon assimilation, is dependent upon Rubisco activase. The heat sensitivity of activase, including its expression and thermal stability, varies among species and is considered a key component governing photosynthetic performance in response to moderate heat stress (32-35 °C). However, the Rubsico-Rubsico activase association has yet to be examined among woody plants or varieties within a species, the understanding of which will assist cultivar improvement strategies. Using molecular and physiological techniques to study the role of activase in thermal regulation of photosynthesis, we found that net photosynthesis decreased in Acer rubrum L. `Northwood' at 31°C, whereas the southern variety, A. rubrum `Florida Flame', maintained optimal assimilation rates up to 36 °C. Additionally, the maximal carboxylation rate of Rubisco (Vcmax) at 35 °C was 31.7% lower for Northwood in comparison to Florida Flame. The cloned activase sequences from both cultivars show 97% nucleotide homology and 98% amino acid identity, indicating the potential for similar protein product formation and function. Interestingly, sequence analysis indicates that both cultivars produce at least two isoforms of activase derived from alternative transcript splicing. We will discuss activase mRNA processing and protein isoform abundance in relation to Rubisco kinetic properties as a function of heat tolerance in these two thermally contrasting woody plant genotypes.
Ethanol production and chlorophyll fluorescence were measured as signals of freezing and heat stress in apple fruit. `Cortland' and `Jonagold' apples were held at –8.5 °C for 0, 6, 12 or 24 h (freezing treatments), or at 46 °C for 0, 4, 8 or 12 h (heat treatments). Following treatments, fruit were stored at 0 °C and evaluated after 0, 1, 2, or 3 months. Following storage, fruit samples were kept for 12 h at 20 °C and then analyzed for ethanol production, chlorophyll fluorescence, and visible injury. Severity of flesh browning increased with increasing treatment time for both freezing and heat treatments. Freezing for 24 h and heating for 12 h caused severe flesh browning in both cultivars. Severity of heat-induced browning increased during storage. Increases in ethanol production were apparent 12 h following treatments and reflected the degree of stress-induced fruit injury. After 2 months of storage, ethanol concentrations peaked and were as much as 400-fold greater than that of controls. These stress treatments also reduced ethylene production and chlorophyll fluorescence. The degree of increase in stress-induced ethanol production and decrease in chlorophyll fluorescence correlated with stress-induced injury and could be used to predict the severity of injury that develops during storage. Other volatile production and their relationship to fruit stress will also be discussed.
Artichoke is a cool-season perennial crop that is grown as an annual from seed in southern California. Growing artichokes as annuals from seed allows growers to harvest during the winter from November to March. Artichoke seed is planted in May, transplants are moved to the field in July, and harvesting begins as early as November in years with relatively cool fall weather. Hot fall weather during September and October suppresses plant growth and causes premature flowering, which lowers yield and average bud size. Plant growth regulator (PGR) treatments were evaluated in annual artichokes to determine if they could reduce the adverse effects of hot weather during September and October. Treatments included multiple applications of apogee (gibberellin inhibitor), retain (ethylene inhibitor), apogee + retain, cytokinin, and control plots. Harvestable buds were counted as a measure of earlier flowering induced by hot weather. Apogee and cytokinin show promise in reducing heat stress during hot fall artichoke production. Other PGR treatments increased the number of harvestable buds compared to control plots.
Fifty-three commercial New Guinea Impatiens cultivars (Impatiens hawkeri Bull.) from six different breeding series were tested for level of heat tolerance. Five floral (flower number, flower length, flower width, floral dry weight, and flower bud number) and five vegetative characteristics (leaf dry weight, stem dry weight, total dry weight, number of nodes, and number of branches) were evaluated with emphasis placed on continued flowering under long term heat stress. Significant differences among cultivars were found in each data category (P ≤ .0001). Flower number varied from 0 to 6, flower length varied from 10 to 51 mm, flower width varied from 10 to 47 mm, floral dry weight varied from 0 to 0.5 g, and flower bud number varied from 0 to 14. Four heat tolerant (Celebration Cherry Red, Celebration Rose, Lasting Impressions Shadow, and Paradise Moorea) and three nonheattolerant (Lasting Impressions Twilight, Danziger Blues, and Pure Beauty Prepona) cultivars were identified using a Weighted Base Selection Index. These cultivars were used as parents in a full diallel crossing block with reciprocals and selfs. One hundred seedlings from each of 49 crosses were evaluated for heat tolerance. General and specific combining abilities of the parents were evaluated as was heritability. It was found that the four heat tolerant cultivars had higher general combining abilities. Heat tolerance has low heritability and is controlled by many genes. Superior genotypes were identified (selection intensity of 0.05) and retained for further evaluation and breeding efforts.