Abstract
Seasonal variations (April to October) in leaf structure, photosynthesis and essential oils of four Lamiaceae species (Origanum vulgare, Mentha spicata, Nepeta nuda, Clinopodium vulgare) were studied at the altitudinal gradient (200-1760m) of Mt. Pangeon. The raising elevation resulted in a progressive decrease of the plant height. High-altitude plants have further a shorter growing period and a bloom with a monthly time lag. Blade surface reached the highest values at early summer for O. vulgare and M. spicata leaves and mid summer for N. nuda leaves. Leaves from lowland were thinner during the whole growing period (in O. vulgare) or only at the beginning of it (in M. spicata and C. vulgare). In M. spicata and N. nuda leaves, non-glandular hair density was higher in low-altitude plants, a fact associated with adaptation at the xeromorphic conditions. The density of the essential oil- producing glandular hairs progressively increased in leaves of O. vulgare and M. spicata during the se ...
Seasonal variations (April to October) in leaf structure, photosynthesis and essential oils of four Lamiaceae species (Origanum vulgare, Mentha spicata, Nepeta nuda, Clinopodium vulgare) were studied at the altitudinal gradient (200-1760m) of Mt. Pangeon. The raising elevation resulted in a progressive decrease of the plant height. High-altitude plants have further a shorter growing period and a bloom with a monthly time lag. Blade surface reached the highest values at early summer for O. vulgare and M. spicata leaves and mid summer for N. nuda leaves. Leaves from lowland were thinner during the whole growing period (in O. vulgare) or only at the beginning of it (in M. spicata and C. vulgare). In M. spicata and N. nuda leaves, non-glandular hair density was higher in low-altitude plants, a fact associated with adaptation at the xeromorphic conditions. The density of the essential oil- producing glandular hairs progressively increased in leaves of O. vulgare and M. spicata during the seasonal gradient, particularly in low-altitude plants. At higher altitudes (N. nuda populations included), glandular hair density was the highest in summer. N. nuda leaves were covered, except for glandular peltate hairs, also with glandular capitate hairs. The majority of stomata of the species studied belonged to the diacytic type and a few belonged to the anisocytic type. Leaves of all species were hypostomatic, with the lower leaf side possessing more stomata than the upper one, where stomata are limited or even absent (in C. vulgare). Generally, altitude was positively correlated with stomatal density, since leaves of high-altitude plants had more stomata than those of low-altitude plants. In O. vulgare, August leaves at 200m contained dark phenolics in all of their tissues. At the same altitude, such substances were similarly found in M. spicata (epidermal cells) and C. vulgare (mesophyll cells) August leaves. June and October leaves at 200m contained phenolics only in their epidermis. Raising altitude resulted in a significant decrease of phenolics, which were located only in the epidermis or were totally absent (C. vulgare). The size of the chloroplasts and the contained starch grains was also affected by the seasonal and altitudinal gradients. The presence of small mesophyll chloroplasts with a few or no starch grains was a common characteristic for autumn 208 /Fleaves of the highest elevation (1760m). This feature is in agreement with the lower chlorophyll content and the lower photochemical efficiency of PSII (Fv/Fm) of these leaves. Drought stress predominating at low altitude during summer, reflected similar low values of chlorophyll content and Fvm. The highest photochemical efficiency of PSII appeared in the mid-altitude populations where environmental conditions were more favourable. O. vulgare exhibited the highest values of essential oil content at the lowest altitude population (200m). June leaves of 200m yielded the highest amount of essential oil (5,6%). Carvacrol, the main component of the oil of this population gradually decreased up to October, where p-cymene was the major component of the oil. In all of the months, however, the sum of the four biosynthetically correlated compounds of oregano oil (carvacrol+thymol+p-cymene+γ-terpinene) was very high (87,1-95,6%). The sum of these compounds was lower in the essential oil of the 950m population (35,4-48,8%), where carvacrol was the main component of the oil as well. The sum of the four related compounds was even lower (8,8-12,4%) in the essential oil from the high-altitude population, which was composed mainly of sesquiterpenes. At the same altitude, plants with a faint lavender smell were spotted, and were found to have a high linalool content (72,6%).
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