No abstract

Antioxidants production is amongst the physiological responses of plants to protect their tissues from oxidative damages caused by chilling and freezing stresses. Indeed, cold tolerance of plants is related to production of reactive oxygen species (ROS) scavengers to combat oxidative stress. In this study, two citrus cultivars including Citrus sinensis 'Siavaraz' and Citrus limon 'Lisbon' grown at the north of Iran were examined to compare antioxidants changes including total flavonoid, total phenol and total antioxidant capacities (TAC) in the flavedo tissue of the fruits under various low temperature treatments of control (15 ºC), 3, 0, -3 and -6 ºC. The results indicated that total flavonoid content and TAC in Citrus limon 'Lisbon' was significantly higher than in Citrus sinensis 'Siavaraz'. During the treatments, both cultivars showed a significant increase in the flavonoid content. Meanwhile, total phenol content significantly increased from 15 to -6 ºC in Citrus limon 'Lisbon' and from 15 to 0 ºC in Citrus sinensis 'Siavaraz'. These results suggest that the biosynthesis of phenolics and flavonoids in flavedo of citrus fruit in response to low temperature might be correlated with environmental adaptation rather than antioxidant response.

To assess the effects of IAA and Kinetin plant growth regulators in order to improve the drought tolerance in rice seedlings (Oryza sativa L.), a factorial experiment was carried out based on complete randomized design with three replications. The experimental factors included different rice genotypes ['Gharib', 'Khazar',], drought stress from 1 to 4 code of the Vergara coding system and control (normal irrigation) and growth regulators in three levels (IAA and Kinetin through foliar spraying and non-application as control). The results indicated, under normal irrigation condition together with IAA application, 'IR83750' rice genotype had the highest number of tillers and leaf greenness, with mean of 18.27 and 49.46, respectively. The highest amount of leaf relative water content (95.11%) was related to 'Sepidrood'. Under drought stress condition, the highest electrolyte leakage (36.59%) was observed in 'Gharib'. In drought condition, the highest leaf drying score was related to 'Gharib' in both years, but the highest score of leaf rolling index (9) was observed in 'Gharib' and 'Khazar'. The present findings showed that drought stress had harmful effects in all examined genotypes and the impact in susceptible genotypes ('Gharib' and 'Khazar') was more than 'IR83750' and 'Sepidrood'. Application of growth regulators (IAA and Kin) improved growth conditions for of all genotypes. Therefore, using tolerant genotypes along with growth regulators can improve the rice growth traits.

Dust is one of the most devastating factors for the environment threatening all animal and plant species. In many regions, the ecological and economic impact of microdust on scarce species is critical. In the western region of Iran, the Zagros forests have been exposed to dust storms for many years. In this study, the effect of dust on oak trees, the most important trees of Zagros forests, is investigated. For this purpose, 3-year-old seedlings of three species of oak trees under natural conditions were exposed to dust during spring and summer months. Seedlings were divided into two groups; one group was assigned as dust treatment and the other as control that the control group washed regularly to remove dust. Anatomical characteristics of leaves and dust deposits on leaves during the study period were examined by scanning electron microscope (SEM). The rate of photosynthesis and gas exchange in control and treated plants was examined by IRGA, LCI. SEM images showed that stomata structure, trichome density, and epicuticular waxes of leaves are different in all three species. This difference in micromorphology of species influences the effects of dust deposited on the leaves. A comparison of leaf species images in control and dust treatment showed that in dust treatment the percentage of stomata blocked by dust in three species (per unit area) of Quercus infectoria, Q. libni, and Q. brantii were 61/6, 48/4, and 38/1%, respectively. The results of leaf gas exchange investigation indicated that stomatal occlusion by dust had a negative impact on the examined parameters of three oak species (P ≤ 0.01). Thus, gas exchange and photosynthetic rates of the treated species were significantly reduced. The results of both parts of the study showed the vulnerability of the three species to dust as Q. infectoria > Q. libni > Q. brantii. Therefore, based on these findings, dust can disrupt the physiological activities of the studied species and the continuation of the exposure to dust will accelerate the process of destruction of these forests.

The hypothesis that stomatal wax plugs in Agathis robusta (C. Moore ex F. Muell.) F. Muell. (Araucariaceae) protect leaves against fungal invasion by preventing hyphae entering the stomatal pore was tested. Leaves with intact wax plugs and leaves from which wax plugs had been experimentally removed were inoculated with either Botrytis cinerea (De Bary) Whetzel or Alternaria solani (Ellis & G. Martin) L.R. Jones & Grout. Interactions of fungal hyphae with stomata were subsequently analysed by scanning electron microscopy. Wax plugs blocked the penetration of fungal hyphae into stomata of A. robusta in 100% of encounters. In contrast, hyphae readily penetrated stomata of leaves from which wax plugs had been removed or where the wax plugs were damaged. On rare occasions, hyphae were also observed to penetrate directly through the cuticle. Florin rings around the external surface of stomata also acted to deflect fungal hyphae from the stomatal opening. Formation and degradation of wax plugs was also examined to assess the period over which they offer protection from fungal invasion. Wax plugs formed in young leaves in spring and were complete by summer. There was subsequent degradation of wax plugs in the following winter, and they then regenerated again in spring. However, regeneration of wax plugs appeared to decline after the leaves reached 2 years of age. The data are discussed with reference to the adaptive significance of wax plugs in conifers and other plants.