Microdust impact on leaf gas exchange parameters in oak species of Northern Zagros forests, west of Iran

[Research]

Microdust impact on leaf gas exchange parameters in oak species of Northern Zagros forests, west of Iran

Moradi A.¹*, Taheri Abkenar K.¹, Afshar Mohammadian M.², Shabanian N.³

1.Department of Forestry, Faculty of Natural Resources, University of Guilan, Sowmeh sara, Guilan,Iran     

2. Department of Biology, University of Guilan, Rasht, Iran           

3. Department of Forestry, Faculty of Natural Resources, University of Kurdistan, Sanandaj, Iran

* Corresponding author’s E-mail: aiuobmoradi60@gmail.com             (Received: April 18. 2018 Accepted: Aug. 27. 2018)

In recent years, the microdust phenomenon has greatly changed in concentration,  duration and continuity as well as the frequency of occurrence in comparison with dust storms in the past which has caused a great deal of concern. Microdust is one of the most devastating factors in the environment threatening all animal and plant species. Regarding to the microdust impending threat, its ecological and economic impacts on scarce species is critical. Zagros forests act as an intrinsic filter for microdust in the western region of Iran. This study investigates the effect of microdust on oak, the most important tree in the Zagros forests. So that, three-year old seedlings of three oak species (Quercus branti, Q. libni and Q. infectoria) were exposed to microdust under natural conditions during spring and summer 2016. We examined the rate of photosynthesis, stomatal conductance, transpiration, internal CO₂, mesophyll conductance, water use efficiency in control and treated plants. The results indicated that microdust had a significant impact on the examined parameters of the three oak species (P≤0.01). The gas exchange and photosynthetic rates of the treated plants were significantly reduced.  In Q. infectoria, microdust had the greatest impact on photosynthesis, stomatal conductance, leaf internal CO₂, transpiration and mesophyll conductance. Accordingly, microdust had a substantial influence on photosynthesis and mesophyll conductance in Q. brantii as well as the leaf internal CO₂ and mesophyll conductance in Q. libani. Therefore, based on these findings, it can be concluded that microdust can disrupt the physiological activities of the examined species. Hence, continuous - exposure to microdust will accelerate the process of destruction of these forests.

INTRODUCTION

Iran, situated in an arid and semi-arid zone, is frequently exposed to local and synoptic dust flux (Rasooli et al. 2011). Studies in recent years have shown that microdust is greatly varied in concentration, duration and continuity as well as the frequency of this occurrence compared to dust storms in the past and so it has caused huge concern. Prolonged drought, reduction of rainfall and relative humidity along with destructive human activities such as war, misusages of water resources and elimination of canebrake has led to drying of lagoons and lakes in Syria and Iraq which has in turn resulted in an increase in microdust. In the west and south-west of Iran, this phenomenon has had more destructive effects and impact on ecology, economy and the health of inhabitants especially in frontier cities and provinces in a short period of time (Shojaii 2011). Zagros forests as the most extensive ecosystem in Iran is composed of various forest communities at different altitudes and latitudes indicating high ecological significance of these forests. These forests are known as major natural filters acting as the lung in the western areas of Iran. Unfortunately, considerable parts of these forests have been lost in recent years and as a consequence decreased the natural barriers in the way of microdust. In fact, microdust is amongst the most destructive factors causing various ecological and economic problems and affecting all animal and plant species. Clearly, destroying vegetation of lagoons is one of the main reasons for the occurrence of dust storms. Eliminating trees and shrubs would multiply the calamity of this occurrence because forests act as filter and absorb vast parts of dust. According to Hiltron (1990) dust - covered leaves receive less light for photosynthesis which in turn affects the reduction of leaf stomatal conductance influence, plant biomass and the rate of photosynthesis. Stomatal conductance is influenced by environmental factors, position at the canopy and the age of the leaves. Moreover, leaves covered with dust absorb more solar radiation that increase leaf temperature (Wijayratne et al. 2009).

Other factors such as sandblasting result in the loss of plant leaves which decrease photosynthetic activities and production of grains or fruits. Dust also impacts soil chemistry and might deteriorate other stresses such as drought or pathogen (Farmer 1993). Studies by Singh & Rao (1981) and Chaturvedi et al. (2012) examined the effect of industrial particles on development and physiological parameters of crop.

In addition, Chaturvedi et al. (2012) conducted an in-depth study of the chemical composition of dust, its particulate size, age of plants and deposition rate. A study by Darley (1996) found that photosynthesis in green beans is reduced (73%) by cement dust load and Cook et al. (1981) observed that the rate of the photosynthesis of apple trees reduced up to 90% by ash. Gas exchanges are an effective factor in plant growth (Si­ Sakht & Zolfaqari 1993). Nanos & Ilias (2007) investigated the reduced stomatal conductance and transpiration rate of olive trees as a result of cement dust. They found that cement dust diminished leaf total chlorophyll content and chlorophyll a/ chlorophyll b ratio leading to a lessening of photosynthetic rate and quantum yield. Similarly, Shukla et al. (1990) concluded that chlorophyll content of plants lessened after exposure to cement dust due to the high alkalinity of cement dust. They found that reduced cross sectional area of stomata and interception in the gas exchanges lead to reduced photosynthesis. Despite the outstanding role of forests in microdust absorbance and air filtration, few studies have been conducted to examine the rate of dust absorbance by various forest species. These valuable resources in the Zagros habitat have been facing the problem of microdust in recent years. Hence, we examined the effects of dust on some eco-physiological parameters in three oak species including Quercus Brantii Lindl., Q. libni Oliv. and Q. infectoria Oliv.

MATERIALS AND METHODS

To assess the effects of microdust, three-year old seedlings of oak trees including Quercus branti, Q. libani and Q. infectoria were used. The potted seedlings from Rixlan nursery, Department of Natural Resources from Mariwan city in Iran were transferred to the study region in Chenare district of Mariwan forests on March 4, 2015. This region is on the path of dust movement and positioned at longitude range of 616862 to 621940E and at latitude range of 3940705 to 3947100N in 38s zone in the UTM coordinate system with heights of 1450 and 2100 m above sea level, respectively (Fig. 1). Seedlings were divided into two groups of 15 pots. Five potted seedlings of each species were placed in individual groups. One group was assigned as the dust treatment and the other as control which was washed regularly to remove dust. The seedlings were irrigated once every 3 days from April 6 through   July 6 2015. At irrigation, the leaves of control pots were washed to remove dust while water was only poured in the pots with no leaf contact in the dust treatment pots. To have accuracy in the research procedure, the direction of wind on the pots was marked. IRGA (Infrared Gas Analyser), LCI (ADC Company) were used to measure the photosynthetic and transpiration rate, stomatal conductance and the concentration of leaf internal CO₂. All measurements were conducted at 11 AM with light intensity of 1400-1600 µm m-² s^-1. To measure these parameters, the third leaf from the branch top (in the same direction) was selected to put in the leaf chamber for 45 seconds (Rohi & Siose Marde 2009).

To determine the water use efficiency (WUE), the photosynthetic rate was divided by the value of stomatal conductance (cf.), and mesophyll conductance was estimated by dividing the photosynthetic rate by the value of leaf internal CO₂ concentration (Rohi & Siose Marde 2009).

After checking the normality of the data and residuals (by Kolmogorov-Smirnov test), data was analyzed as a factorial project including two- factors, type of the species (Q. branti, Q. libni and Q. infectoria) and microdusts at two levels (one containing micro dust and the other without micro dust as control). Then, Duncan test was performed to study the difference between means when the assumption of equality of variances was conformed. All analyses were conducted using SPSS software, version 23.

Fig. 1. Location map of the study area.

RESULTS

As shown in Table 1, in general, type of the species did not significantly affect the photosynthetic rates of control plants ( ). In other words, no significant difference was observed regarding the photosynthetic rates of the examined species. However, microdust deposition influenced the photosynthetic rates of the treated species significantly (P ≤ 0.01). In addition, the mutual effects of microdust and species on photosynthetic rates was not statistically significant; this means that each factor (species and microdust) influenced the photosynthetic

rates independently. According to the results, photosynthesis, stomatal conductance, leaf internal CO₂, mesophyll conductance and water use efficiency were not significantly different amongst the examined species. However, microdust deposition on the leaves of these species significantly influenced the examined parameters (except for transpiration, P = 0.29). To check the variance of means, the Duncan test was used. Table 2 demonstrates a summary of the obtained variables of gas exchange parameters. The results in Table 2 show the statistically- significant influence of microdust on gas exchange rates in the examined species treated by microdust. In Q. infectoria, microdust had the most significant impact on photosynthesis, stomatal conductance, leaf internal CO₂, transpiration and mesophyll conductance. Accordingly, microdust had a substantial influence on the photosynthesis and mesophyll conductance in Q. brantii as well as on the leaf internal CO₂ and mesophyll conductance in Q. libani. However, microdust did not greatly affect the water use efficiency of the examined species compared to the control group.The means comparison charts of investigated parameters in both dust treated and control plants are shown in Fig. 2.

Fig. 2. Comparison of the gas exchange parameters in the examined species under dust effect. A: photosynthesis (µmol CO₂ m^-2 s^-1), B: stomatal conductance (mmol H₂O m^-2 s^-1) C: transpiration (mmol H₂O m^-2 s^-1) D: Leaf internal CO₂ (mmol) E: Mesophyll conductance (mmol CO₂ m^-2 s^-1) F: Water use efficiency (µmol CO₂ mol^-1 H₂O).

Table 1. The results of analyzed data obtained from the LCI using the ANOVA and Duncan test. ANOVA test on gas exchange data of the examined species treated with microdusts.

                        ns: not significant*: significant at 5%.         

                        **: significant at 1%.

DISCUSSION

According to the results of the present study, there was no significant difference between the three examined oak species concerning to the gas exchange parameters in control plants unlike treatments with microdust. In general, photosynthetic reduction is one of the main reasons for reducing plant growth. Limiting factors in photosynthesis are categorized into two groups: a) stomatal factors leading to reducing CO2 diffusion in intercellular space as a result of decreased stomatal conductance and b) non-stomatal factors limiting photosynthesis by direct effect of water shortage on biochemical process of carbon

production (Sio-se Marde et al. 2005). Fischer et al. (1998) have reported that the main limiting factor in photosynthesis is the decreased mesophyll conductance.

They believe that a fundamental factor in photosynthesis reducing, is the closure of stomata under drought conditions leading to a decreased stomatal conductance, and as a consequence, reduced photosynthetic rate. In fact, stomatal limitations reduce the rate of photosynthesis and CO₂ concentrations in intercellular space of leaves and in turn, lead to decreased biomass of plants (Lawlor & Cornic 2002). According to previous studies, stomatal factors are limiting ones for photosynthesis (Rohi & Siose Marde 2009). In the present study, results illustrated that stomatal pores are closed by microdust and thus tend to limit gas exchange rates through stomata. In relation to photosynthesis, as exhibited in Fig. 2, trees exposed to microdust had a lower photosynthetic rate compared to control trees in the three examined species. In fact, microdust caused decreased photosynthesis. Microdust could close the stomatal pores. So, closed stomata concerning to the reduced light absorption due to microdust deposition on the leaves might also cause disorders in gas exchanges, hence decreased photosynthetic rate as well as the rate of vegetative products or biomass. Deposition of smaller particle sizes leads to stronger reduced photosynthesis compared to coarse particles (Hirano et al. 1990). This effect is seemingly due to the closer lining of dust particles on leaf surface leading to a greater shading effect of photosynthetically active radiation (PAR). The rate of reduction in photosynthesis is directly related to stomatal closure and also decreased light absorption by the covered leaves with microdust. This is the most universal problem in plants exposed to microdust. Takashi (1995) reported that plants exposed to enormous resources of microdust are brought into hazard such as chronic reduced photosynthesis and consequently reduced plant growth. Thus, it was concluded that a diameter of one-millimeter coverage of ash on leaves reduces the photosynthetic rate by 90%. In a lesser diameter, the rate of reducing varies from 25% through 33%. Of course, this effect depends on the condition and the type of the plants. Microdust deposition on leaf area in addition to causing a reduced photosynthesis leads to hasty leaf senescence and therefore a delay in plant development as well as decrease in whole plant function (Arvin et al. 2014).

Similar to our results, Takashi (1995) also found that microdust reduced stomatal conductance. Stomata plays a main role in three vital activities: photosynthesis, transpiration and respiration in plants. Williams & Ricks (1974) pointed out that closure of stomata by little particles such as dust in addition to the reduced photosynthesis and transpiration will decrease the rate of respiration, while increase the stomatal resistance of plants at night.  However, the level of the negative impact of dust depends on both the size of dust particles and the microstructure of the plants.

A decreased leaf internal CO₂ concentration stimulates stomata to be opened (Mojtahedi & Lesani 2008).

Table 2. Variables of gas exchange parameters of the studied species treated with microdusts.

Similar Roman letters beside means of any parameter indicates no difference at 5% level between attributes.