The optimum temperature for the growth of each species varies widely depending on genotype and the original natural habitat of the species (Franz et al. 1984, Herath et al. 1979, Lincoln and Langenheim 1978, Putievsky 1983a). Accordingly, basil, native to warm climates, accumulates the greatest yields under warm conditions (30°C) while biennial caraway and oregano, naturally found in colder climates, produce the largest yields under moderate (18/24°C) temperatures (Putievsky 1983a). For the biennial caraway, low temperatures (16/20°C) at flowering and seed binding stages are most favorable. On the other hand, the annual caraway which is mainly grown in the Mediterranean region requires more heat for seed production. The plant does not commonly produce fruits when grown at the northern locations (Halva et al. 1986, Svab 1992). However, a fruit yield was harvested in Canada but the content of the main compound, carvone was lower than desired by the industry. The low carvone content is attributed to the fruit that was not wholly ripe at the harvest due to the short growing season (Wahab 1997). In this case the irrigation clearly increased the fruit yield as also reported earlier in plants grown in Israel (Putievsky 1978). Bernath and Hornok (1992) concluded in their review that the influence of temperature is relative. Accordingly, the content of active substances will either increase or decrease depending on the optimum temperature for the given species.
The optimum temperature will decrease at low light intensity. The optimum temperature for the leaf development and essential oil concentration in peppermint is approximately 21°C (Biggs and Leopold 1955, Clark and Menary 1980b). The optimum temperature for flower head and oil production in chamomile is 15°C. Also, a constant high temperature (25°C) favored the essential oil production in chamomile (Saleh 1970), In Japanese mint (Mentha arvensis L. var. piperascens Holmes), a combination of high day and low night temperatures (35/13°C) produced the greatest number of leaves and the highest amount of essential oil per plant. A day temperature of 30°C, however, produced the maximum dry weight regardless of the night temperature. At lower day temperatures, the oil production increases with increasing night temperature (Duriyaprapan et al. 1986).
In cold conditions, the growth rate is slow and the plants become more branched than under warm conditions. In pennyroyal, also more flowers will develop at each node at low temperatures (Firmage 1981). According to Saleh (1970), chamomile will produce more but smaller flower heads at high temperatures.
The difference in oil content in herbs is sometimes explained by the so called latitude effect. Temperature and day length cause the main climatic differences between the southern and northern locations. The carvone/limonene ratio in wild caraway in Finland is found to be higher at northern latitudes compared to southern latitudes. The same applies to the ratio on higher versus lower elevations in the Alps (Galambosi and Peura 1996). The term latitude-effect, however, seems to be too general to describe any variation in the growth and oil concentration of herbs as shown in the case of caraway and dill (Halva et al. 1986, Halva 1993). The plants were grown on similar subsrates in northern and southern locations. The growing site did not explain the variation in the oil content.
The factors that affected the fresh herb production the most, were the number of degree days and the amount of rainfall. Also, warm conditions increased the essential oil concentration and the contents of major oil components in dill. The latitude of the growing alone site did not explain the variation in the herb and oil production (Halva et al. 1988). In the growth chambers, contrary to the field conditions, dill yield tended to be highest under cool conditions. Under cool conditions, the growth period was longer, and the longer growth period could possibly have helped to accumulate more plant tissue than the shorter growth periods under warmer conditions. Under cool conditions, low respiration can preserve the assimilates and lead to greater biomass production. Also, the relatively low light intensity in the chambers may have affected the result, because the optimum temperature for plants is known to decrease at low light intensities.
The oil concentration in dill decreased with increasing fresh biomass production and tended to correlate with dry matter production and the mean daily temperature. The correlation between the dry matter production and oil concentration, again, implies a close relation between oil production and photosynthesis. The fresh herb yields decreased the further north the dill was grown. The low yields in the north were due to less favorable soil and cooler weather. Even though the fresh herb yield seemed much greater in the southern locations, the oil concentration was, in fact, unaffected by the latitude (Halva 1993).
Franz et al. (1984) have reported that temperature does not significantly affect the oil concentration in peppermint whereas Clark and Menary (1980b) have observed that a high day temperature tends to increase the oil accumulation. Oil concentrations both in peppermint and Japanese mint increase in linear proportion with increasing dry matter production and with increasing mean daily temperatures (Burbott and Loomis 1967, Duriyaprapan et al. 1986). A similar relation between oil production and dry weight has been reported in chamomile under various temperatures (Saleh 1970).
Though night temperature has little effect on total oil concentration in peppermint, it does significantly affect the oil composition. Also, an interaction between temperature and photoperiod affects the proportion of compounds. Short warm days combined with warm nights favor pulegone and menthofuran, while cool nights favor the formation of menthone. Under long days, the effect of night temperature on the oil composition in peppermint is less important (Burbott and Loomis 1967). According to Firmage (1981), american pennyroyal accumulates the largest concentration of menthol and isomenthone at cold conditions (9°C) even though the total oil concentration decreased at low temperatures.
The oil concentration in chamomile is highest at constant high temperature and decreases in linear proportion with decreasing temperature. The total oil production as well as chamazulene content per plant are, however, greatest under cool temperatures due to increased size of flower heads. The maximum chamazulene production will occur at constant moderate (20°C) temperature. In another experiment, cool nights (15°C) with day temperatures of 25°C produced both the highest total oil and chamazulene contents (Saleh 1970). The effect of temperature is controlled by the genotype of the plant as shown in the case of chamazule in chamomile oil (Svab et al. 1967).
Burbott and Loomis (1967) have observed an increase in the synthesis of reduced compound at low temperatures and they suggested that the increase may be attributed to decreased respiration. Warm nights decrease the amounts of respiratory substrates (oxidizing conditions) while cool nights preserve higher amounts of respiratory substrates (reducing conditions). The oxidation—reduction level of monoterpenes reflects the oxidation—reduction state of the respiratory co-enzymes of the monoterpene-producing cells, which, in turn, depends on the concentration of respiratory substrates in the cells.
This theory has been later supported by Clark and Menary (1980b). They have reported that high light intensity (>500,amol m-2 s-1), cool nights, and a day temperature of 20°C will maintain high levels of photosynthetic products which favor the reduction of pulegone to menthone in peppermint. Duriyaprahan et al. (1986), on the other hand, reported that the menthone content in Japanese mint will increase under high temperatures and that the menthol content tends to be stable under various temperatures.
The optimum temperature of 16-18°C is reported for the oil production of coriander, and 19-21°C for lavender (Lavandula angustifolia Mill.) (Savchuk 1976). Huopalahti (1984) has concluded that the oil concentration in dill herb will increase with decreasing temperature. The same applies to the content of anethofuran (i.e. the benzofuranoid), which is known to be responsible for the dill aroma. Lincoln and Langenheim (1978) reported that a low temperature (15°C) favors the oil production of 'yerba buena'. Low temperatures also increased the oil concentration in peppermint (Rabak 1916, Biggs and Leopold 1955), lavender (Savchuk 1976), and citronella (Cymbopogon nardus (L.) Rendle) (Herath et al. 1979).
The effect of temperature has been studied by counting the number of oil glands. Increasing temperatures clearly increased the number of glands in peppermint leaves (Biggs and Leopold 1955). The number of oil glands is, however, not necessarily proportional to the oil concentration because the oil will evaporate more rapidly at high temperatures (Hockings and Edwards 1943, Duriyaprahan et al. 1986). Accordingly, in Japanese mint, the number of oil glands was greater at high day temperatures regardless of the night temperatures, but there was no obvious correlation between the number of oil glands and oil production (Duriyaprahan et al. 1986).
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