Effect of Drought, Salinity, and Defoliation on Growth Characteristics of Some Medicinal Plants of Iran

Abstract

ABSTRACT The effects of water stress, salinity, and defoliation on growth of four medicinal plants—Shiraz thyme (Zataria multiflora), kakooti (Ziziphora clinopodioides), garden thyme (Thymus vulgaris), and cat thyme (Teucrium polium)—were evaluated in greenhouse experiments. Water stress and defoliation were studied using a completely randomized factorial design with three replications and four levels of water potential (−0.03, −0.5, −1.0, and −1.5 megapascals) and three levels of defoliation (0%, 25%, 50%). Salinity and defoliation were studied in a split plot with a completely randomized block design and three replications using three levels of salinity (0, 5, 10 ds.m−1) in the main plots and a combination of three levels of defoliation (0%, 25%, 50%) for four plant species allocated in subplots. Measurements on relative chlorophyll content (SPAD readings), stomatal resistance, temperature within the plant community, leaf-to-stem ratio, specific leaf weight, and root-to-top ratio indicated drought stress increased chlorophyll content in kakooti, specific leaf weight in Shiraz thyme, and kakooti and temperature in Shiraz thyme and cat thyme as compared with controls. Defoliation, however, reduced chlorophyll in Shiraz thyme, kakooti, and cat thyme, specific leaf weight in Shiraz thyme, kakooti and garden thyme, and leaf-to-stem ratio in Shiraz thyme as compared with controls. Increased salinity levels reduced chlorophyll content and increased stomatal resistance in all species. A significant negative correlation was noted between salinity and leaf-to-stem ratio and between drought stress and leaf-to-stem ratio for all species. With increased defoliation intensity, the root-to-top ratio for Shiraz thyme and garden thyme decreased but was increased in kakooti and cat thyme. A significant positive correlation was observed between the root-to-top ratio and drought levels. Kakooti was the most drought-resistant, and garden thyme had lowest resistance to drought and defoliation stresses but was the most resistant to salinity.