عنوان مقاله [English]
Introduction: Mediterranean climate conditions induce several stresses that plants have to cope with, especially during summer months when high temperature and radiation levels along with low water availability in the soil prevail for long periods. Variation in physiological traits such as photosynthesis and plant water status and their association with morphological characters can play an important role in the adaptability of the species to environmental constraints. The previous studies show that scorching weather not only affects the rate of gas exchange, but also results in diurnal changes in activity. Thus, the impact of environmental stresses on plants growing in these conditions should be assessed by examining the evolution of their diurnal variations on leaf gas exchange. Aromatic plants represent a renewable source of valuable compounds that can be used in food, perfumery, and pharmaceutical industry. Among these plants, sweet basil (Ocimumbasilicum), holy basil (Ocimum sanctum), lemon balm (Melissa officinalisL.) and catnip (Nepetacataria) are very important for different industries. Studies on environmental physiology of medicinal plants are relatively scarce and very few information is available concerning the physiological basis of medicinal plant response to heat stress that is one of the most important factors limiting production of medicinal plants in Khuzestan province.
Material and methods: In order to evaluate the diurnal fluctuation of gas exchange of mentioned plants, an experiment was carried out in 2013 at research farm of Horticultural Science, Shahid Chamran University (31°20'N latitude and 48°40'E longitude and 22.5m mean sea level), Ahvaz (Iran), a site characterized by a semidry and scorching weather during late spring and summer. The experiment was arranged based on randomized complete block design (RCBD) with three replications and 4×8 factorial scheme (Four plants including lemon balm, catnip, holy basil and basil; and eight times of evaluation 7:00,9:00, 10:00, 11:00, 12:00, 13:00, 17:00 and 20:00 h). Land preparation consisted of disking and the formation of raised beds (15cm high and 45cm wide across the top) using a press-pan-type bed shaper. The plants were arranged on two rows on each bed, with 20 cm in-row and 40 cm between-row spacing. The plants were irrigated weekly as needed. Gas exchange parameters were investigated from June 9-11at end of vegetative phase under natural environmental conditions. The parameters of gas exchange were measured on the 5th and 6th nearly full expanded leaves between the hours of 07:00 and 20:00 during bright sunlight on clear and cloudless days. Determination of leaf net photosynthesis rate (Pn), stomatal conductance (gs) and transpiration (E) was made with Infra-red gas analyzer (LCA4, ADC Co. Ltd., Hoddesdon, UK).Instantaneous water use efficiency (WUEinst) and apparent quantum yield(AQY) were calculated as Pn/E andPn/PPFD ratios, respectively.
Result and discussion: The result showed that plant type had significant effect on all measured traits as well as record time. Interaction between plant type and record time were significant for PPFD, leaf temperature and net photosynthesis. The highest Pnof Lemon balm (8.97 µmol CO2 m-2 s-1), catnip (11.2 µmol CO2 m-2 s-1) and sweet basil (13.75 µmol CO2 m-2 s-1) were recorded at 9:00 when the photosynthetic photon flux density (PPFD) was 1488, 1598 and 1645 µmol photon m-2 s-1, respectively. Holy basil showed highest Pn (15.47 µmol CO2 m-2 s-1) at 10:00 when PPFD was 1821 µmol photon m-2 s-1.High irradiances caused photoinhibition of the four plants and it seems the four plants reach to light saturation point about 1500 µmol photon m-2 s-1.The midday depression of photosynthesis likely resulted primarily from long periods of high PPFD, limitation in stomatal conductance and high temperature. Catnip was more sensitive to high irradiance. The Pn had positive and significant correlation with gs in four plants. The stomatal conductance was also positively correlated with E in four plants. The plants represented double peak curve for WUE. The first and second peaks appeared at 9:00 and 17:00, respectively. The four plants also showed highest AQY at 7:00. There were significant difference between four plants for leaf temperature, gs, Pn, WUE and AQY. Lemon balm showed lower leaf temperature than other plants due to its high gs. The highest amounts of Pn, WUE and AQY were observed in holy basil.
Conclusion: In regard to Pn, WUE and AQY, it seems holy basil and sweet basil can tolerate weather condition of Ahvaz.