Schreb.) under salinity stress using leaf spectrum · Salinity is one of the major abiotic factors that adversely affect turfgrass quality. Tall fescue has a moderate to high salinity

170 E u r o p e a n J o u r n a l o f H o r t i c u l t u r a l S c i e n c e

Assessing leaf senescence in tall fescue (Festuca arundinacea Schreb.) under salinity stress using leaf spectrumY. Gao and D. LiDepartment of Plant Sciences, North Dakota State University, Fargo, ND, USA

SummarySalinity is one of the major abiotic factors that

adversely affect turfgrass quality. One of the symp-toms of salinity injury is accelerated leaf senescence. Detecting such injury may guide to a better mainte-nance of turfgrass. The objective of this study was to assess leaf senescence in tall fescue under salin-ity stress using reflectance leaf spectrum. Two tall fescue cultivars, ‘Tar Heel II’ and ‘Wolfpack’ were watered to field capacity at the 4-leaf stage with a Hoagland solution that contained 16 g L-1 of NaCl and CaCl2 (1:1 w/w). Compared to control, production of Malondialdehyde (MDA) in the 4th leaf of salinity treated plants increased starting at 1 week after treatment (WAT). Chlorophyll a (Chla) content in the 4th leaf of salt treatment was lower than the un-treated control starting at two WAT. No difference of Chlb content in the 4th leaf was detected until 3 WAT. Salt treatments resulted in larger wetting angle on the adaxial side of the 4th and 5th leaves compared to the control, which may be contributed to epicu-ticular wax accumulation as confirmed by scanning electron microscopy (SEM) of the leaf surface. The senescent leaves also showed higher levels of degra-dation of grana and thylakoids in the chloroplasts than the control. As a result, senescent leaves had lower quantum yield of photosystem II compared to the control. Three hyperspectral reflectance indi-ces were tested in the study, mSR750/705 [(R750-R445)/(R750+R445)], mND750/705 [(R750-R705)/(R750+R705-2R445)], and SI710/760 [R710/R760], where R is the relative reflec-tance at a given wave length. All were shown to be very sensitive to detect senescing due to salinity stress, which may be used in turfgrass management.

Keywordschlorophyll, chloroplast, epicuticular wax, salt tolerance, turfgrass, vegetation index

Europ. J. Hort. Sci. 80(4), 170–176 | ISSN 1611-4426 print, 1611-4434 online | http://dx.doi.org/10.17660/eJHS.2015/80.4.4 | © ISHS 2015

Original article

IntroductionSalinity is one of the major abiotic factors that adversely

affect turfgrass quality. Tall fescue has a moderate to high salinity tolerance (Beard, 1973; Marcum, 2006). It was re-ported that tall fescue seed germination was not affected by moderate salinity at 3 dS m-1 (Johnson et al., 2007). Horst and Beadle (1984) reported large variations of germination rates among 16 tall fescue varieties evaluated at 7.5 to 15 g L-1 of NaCl and CaCl2. In a non-repeated experiment, Wipff and Rose-Fricker (2003) screened 59 varieties and exper-imental lines at mature stage using 17 and 24 g L-1 of NaCl, and a preliminary result showed that ‘Tar Heel II’ and ‘Pure

German Society for Horticultural Science

Significance of this studyWhat is already known on this subject?• Leaf longevity is a very important component in turf-

grass quality. Salinity stress can cause physiological drought, nutrient imbalance, and ion toxicity in turf-grass.

What are the new findings?• Salinity stress reduces the life span and functional-

ity of leaves. The stress levels can be sensed using reflectance spectrum before visible symptoms are developed.

What is the expected impact on horticulture?• A turfgrass manager can use reflectance spectrum

of turfgrass canopy to detect the severity of salinity stress and adjust the management practices such as irrigation, nutrient levels, and mowing height and frequency.

Gold’ had the highest survival rate, while ‘Wolfpack’ had the lowest survival rate.

Salinity stress often leads to accelerated leaf senes-cence which is normally considered age-dependent. Leaf senescence is defined as a series of degenerative changes that lead to death (Nooden, 1980). Under normal condi-tions, leaf senescence happens when the leaf reaches ma-turity. The process of leaf senescence is a succession of physiological and molecular events in three stages: initi-ation, degeneration, and terminal phase (Yoshida, 2003). Both biotic and abiotic stress can induce and/or accelerate senescence (Ghanem et al., 2008). Premature senescence caused by salinity starts with osmotic compartmentation and metabolic changes (Munns, 2002), and is probably regulated by hormonal signals (Ghanem et al., 2008). The first phase (minutes to hours) of growth response to sa-linity stress is identical to those of water stress caused by drought. Later, as excessive amounts of salt enter the plant and eventually rise to toxic levels in the older transpiring leaves, senescence occurs. This second phase is signified by a reduction of assimilates and limitation of growth, and is salt-specific (Gao et al., 2012). This phase separates species and genotypes that differ in the ability to tolerate salinity (Munns, 2002; Munns and Tester, 2008).

During the senescence process, the leaves are subjected to the most physiological and biochemical changes. The loss of leaf turgor, discoloration, thickening of cuticle, ac-cumulation of wax, and chlorosis are some of the primary results from senescence (Richardson et al., 2005). At the cellular level, leaf senescence includes disassembling of chloroplasts and other organelles (Benjamin et al., 1999).

V o l u m e 8 0 | I s s u e 4 | A u g u s t 2 0 1 5 171

Gao and Li | Assessing leaf senescence in tall fescue (Festuca arundinacea Schreb.)

The biochemical process consists of hydroxylation of chlo-rophyll (Chl), proteins, lipids, nucleic acids, and carbohy-drates (Nooden et al., 1997; Quirino et al., 2000). All known plant hormones have been reported to play an active role in the senescence process (Hung and Kao, 2003; Carimi et al., 2004). The levels of many reactive oxygen species in-crease at certain stages of senescence (Niewiadomska et al., 2009). Over all, leaf senescence is most often quantified by proteolysis, Chl concentration and fluorescence (Lutts et al., 1996).

Previous research on salt tolerance in tall fescue fo-cused only on survival rate and yield reduction (Wipff and Rose-Fricker, 2003; Johnson et al., 2007). Limited re-search has been conducted to understand the physiological responses of tall fescue to salinity. To understand the life span of tall fescue leaves under salinity stress is important in determining a mowing and fertilization program. Gao and Li (2012) reported that single leaf spectrum is closely related to physiological properties in response to salinity stress in tall fescue. The objective of this study was to in-vestigate potential use of leaf spectrum to assess leaf se-nescence in tall fescue under salinity stress.

Materials and methodsThe experiment was conducted in a greenhouse at

North Dakota State University (Fargo, ND) in 2009 and re-peated in 2010. Two tall fescue cultivars, ‘Tar Heel II’ and ‘Wolfpack’, were seeded in plastic containers measuring 4 cm in diameter and 20 cm in depth with silicon sand as growth medium. The plants were maintained at an average temperature of 25/15°C (day/night), 14-h photoperiod, and under natural light supplemented with metal halide lamps to provide a minimum midday photosynthetic active radi-ation (PAR) of 400 µmol m-2 s-1. The growth medium was maintained at field capacity at about 12% volumetric wa-ter content by watering twice daily until the seeds germi-nated, when the watering was reduced to once daily. The plant materials were fertilized with half strength Hoagland solution (Hothem et al., 2003) at 10 ml per container twice a week. At the 2-leaf stage, the plants were thinned to one plant per container.

At the 4th leaf stage, full strength Hoagland solution was applied to the sand medium at 20 ml per container every other day and the experimental treatments were initiated. The salt treatment was applied by adding 16 g L-1 of NaCl and CaCl2 to the full strength Hoagland solution to reach an electrical conductivity (EC) of about 25 dS m-1 measured with an EC meter (Model 1054, VWR Scientific, Radnor, PA). Plants that received Hoagland solution only were included as control. Each experimental unit contained 45 contain-ers to allow for multiple destructive sampling over time. On the day prior to the initiation of treatments and weekly thereafter for four weeks, the following measurements were taken from the 4th to the youngest mature leaves on the primary shoot of the plant.

Quantum yield of photosystem II (PS II) from three plants in each experimental unit was measured using a portable chlorophyll fluorometer MINI-PAM (Heinz Walz GmbH, Effeltrich, Germany) with the fiberoptics placed 6 mm from the leaf surface at 60° angle using the leaf-clip. The reflectance spectrum from 350 to 1,000 nm also were collected from the same leaves used for quantum yield measurement based on a method by Gao and Li (2012). Briefly, a leaf was clipped on a black background with the adaxial side facing up in a leaf chamber that was illumi-

nated by a high intensity halogen light (Warner-Lambert Tech. Inc., Buffalo, NY) from the top. The reflectance spec-trum was collected with an S2000-TR temperature-regu-lated fiber optic spectrometer (OceanOptics Inc., Dunelin, FL) which has a miniature fiberoptics inserted in the leaf chamber at 60° to the surface plane. Three reflectance in-dices, mSR750/705 (Sims and Gamon, 2002), mND750/705 (Sims and Gamon, 2002), and SI710/760 (Carter, 1994), were calcu-lated from the relative reflective spectra using the follow-ing equation:

2

Both biotic and abiotic stress can induce and/or accelerate senescence (Ghanem et al., 2008). Premature senes-cence caused by salinity starts with osmotic compartmentation and metabolic changes (Munns, 2002), and is prob-ably regulated by hormonal signals (Ghanem et al., 2008). The first phase (minutes to hours) of growth response to salinity stress is identical to those of water stress caused by drought. Later, as excessive amounts of salt enter the plant and eventually rise to toxic levels in the older transpiring leaves, senescence occurs. This second phase is signified by a reduction of assimilates and limitation of growth, and is salt-specific (Gao et al., 2012). This phase separates species and genotypes that differ in the ability to tolerate salinity (Munns, 2002; Munns and Tester, 2008).

During the senescence process, the leaves are subjected to the most physiological and biochemical changes. The loss of leaf turgor, discoloration, thickening of cuticle, accumulation of wax, and chlorosis are some of the primary results from senescence (Richardson et al., 2005). At the cellular level, leaf senescence includes disassem-bling of chloroplasts and other organelles (Benjamin et al., 1999). The biochemical process consists of hydroxyla-tion of chlorophyll (Chl), proteins, lipids, nucleic acids, and carbohydrates (Nooden et al., 1997; Quirino et al., 2000). All known plant hormones have been reported to play an active role in the senescence process (Hung and Kao, 2003; Carimi et al., 2004). The levels of many reactive oxygen species increase at certain stages of senescence (Niewiadomska et al., 2009). Over all, leaf senescence is most often quantified by proteolysis, Chl concentration and fluorescence (Lutts et al., 1996).

Previous research on salt tolerance in tall fescue focused only on survival rate and yield reduction (Wipff and Rose-Fricker, 2003; Johnson et al., 2007). Limited research has been conducted to understand the physiological responses of tall fescue to salinity. To understand the life span of tall fescue leaves under salinity stress is im-portant in determining a mowing and fertilization program. Gao and Li (2012) reported that single leaf spectrum is closely related to physiological properties in response to salinity stress in tall fescue. The objective of this study was to investigate potential use of leaf spectrum to assess leaf senescence in tall fescue under salinity stress.

Materials and methods

The experiment was conducted in a greenhouse at North Dakota State University (Fargo, ND) in 2009 and repeated in 2010. Two tall fescue cultivars, ‘Tar Heel II’ and ‘Wolfpack’, were seeded in plastic containers measur-ing 4 cm in diameter and 20 cm in depth with silicon sand as growth medium. The plants were maintained at an average temperature of 25/15°C (day/night), 14-h photoperiod, and under natural light supplemented with metal halide lamps to provide a minimum midday photosynthetic active radiation (PAR) of 400 µmol m-2 s-1. The growth medium was maintained at field capacity at about 12% volumetric water content by watering twice daily until the seeds germinated, when the watering was reduced to once daily. The plant materials were fertilized with half strength Hoagland solution (Hothem et al., 2003) at 10 ml per container twice a week. At the 2-leaf stage, the plants were thinned to one plant per container.

At the 4th leaf stage, full strength Hoagland solution was applied to the sand medium at 20 ml per container every other day and the experimental treatments were initiated. The salt treatment was applied by adding 16 g L-1 of NaCl and CaCl2 to the full strength Hoagland solution to reach an electrical conductivity (EC) of about 25 dS m-1 measured with an EC meter (Model 1054, VWR Scientific, Radnor, PA). Plants that received Hoagland solution only were included as control. Each experimental unit contained 45 containers to allow for multiple destructive sampling over time. On the day prior to the initiation of treatments and weekly thereafter for four weeks, the following measurements were taken from the 4th to the youngest mature leaves on the primary shoot of the plant.

Quantum yield of photosystem II (PS II) from three plants in each experimental unit was measured using a portable chlorophyll fluorometer MINI-PAM (Heinz Walz GmbH, Effeltrich, Germany.) with the fiberoptics placed 6 mm from the leaf surface at 60° angle using the leaf-clip. The reflectance spectrum from 350 to 1000 nm also were collected from the same leaves used for quantum yield measurement based on a method by Gao and Li (2012). Briefly, a leaf was clipped on a black background with the adaxial side facing up in a leaf chamber that was illuminated by a high intensity halogen light (Warner-lambert Tech. Inc., Buffalo, NY) from the top. The reflectance spectrum was collected with an S2000-TR temperature-regulated fiber optic spectrometer (OceanOptics Inc., Dunelin, FL) which has a miniature fiberoptics inserted in the leaf chamber at 60° to the surface plane. Three re-flectance indices, mSR750/705 (Sims and Gamon, 2002), mND750/705 (Sims and Gamon, 2002), and SI710/760 (Carter, 1994), were calculated from the relative reflective spectra using the following equation:

445705445750705750 RRRRmSR 445705750705750705750 2RRRRRmND

760710760710 RRSI where R is the relative reflectance at a given wave length.

where R is the relative reflectance at a given wave length.After the collection of reflectance spectrum, the 4th leaf

from six plants were excised, flash frozen in liquid nitro-gen, and kept under -80°C for the measurements of Chl and MDA content. Malondialdehyde content of the 4th leaves was determined by the thiobarbituric acid (TBA) reaction. The leaf samples were ground and extracted with 1 ml 5% trichloroacetic acid. The extract was centrifuged immedi-ately at 3,000 gn for 10 min. Then 0.5 ml of the superna-tant was pipetted to a new centrifuge tube, and mixed with equal volume of 0.67% TBA. The mixture was incubated in a water bath at 100°C for 30 min. The light absorbance of the mixtures at the wavelengths of 450, 532 and 600 nm was read using a Beckman DU 640 spectrophotometer (Beckman Instruments Inc., Fullerton, CA). The MDA con-tent was calculated using an extinction coefficient of 155 mmol L-1 cm-1 (Heath and Packer, 1968).

For Chl measurement, approximately 0.03 g of each leaf sample was ground, weighed, and placed in a centrifuge tube. A volume of 2 mL of 80% acetone was added to each sample. The tubes then were capped and kept in darkness at 4°C overnight. The extraction was centrifuged at 4°C at 3,000 gn for 10 min, and 50 µL of the extraction were di-luted by adding 950 µL of 80% acetone. After thorough mixing, the absorbance of the diluted extraction was mea-sured with a Beckman DU 640 spectrophotometer (Beck-man Instruments Inc., Fullerton, CA) at the wavelengths of 470, 646.8, and 663.2 nm. The chlorophyll a (Chla) and chlorophyll b (Chlb) contents were determined following the method and equations by Lichtenthaler (1987).

Leaf epicuticular wax accumulation was estimated by measuring the leaf surface water wetting angle of the 4th and 5th leaf at the fourth week of treatment. The wet-ting angles were measured at room temperature (20°C) using a pipette to deliver a droplet of 5 µL distilled water onto the center of the abaxial and adaxial surface (Cape, 1983), and an image was taken with a digital camera and analyzed using ImageJ software (National Institutes of Health, USA) (Figure 1). The status of leaf surface wax also was confirmed by SEM. Fresh samples were cut into small squares, fixed with 2.5% glutaraldehyde in 0.1 M sodium phosphate buffer, pH 7.35 (Tousimis Research Corporation, Rockville, MD), dehydrated using a graded alcohol series. The fractured pieces were then critical point dried using an autosamdri-810 critical point drier (Tousimis Research Corporation, Rockville, MD) with liquid carbon dioxide as the transitional fluid. The small fractured pieces were

(1)

(2)

(3)



attached to aluminum mounts by silver paint and coated with gold using a Balzers SCD 030 sputter coater (BAL-TEC RMC, Tucson, AZ). Images were obtained using a JEOL JSM-6490LV Scanning Electron Microscope (JEOL Inc., Peabody, MA).

Half of the samples collected for SEM were processed for transmission electron microscopy (TEM). The speci-mens were fixed in 2.5% glutaraldehyde in 0.1 M sodium phosphate buffer, pH 7.35 (Tousimis Research Corporation, Rockville, MD) for at least two hours in a refrigerator at 4°C. They were rinsed with the sodium phosphate buffer and then placed in 2% osmium tetroxide in buffer for two hours at room temperature. Following dehydration in a graded acetone series, samples were embedded in Epon-Araldite-DDSA with a DMP-30 accelerator and sectioned at 60 nm thickness on a RMC MTXL ultramicrotome (Boeck-eler Instruments, Tucson, AZ). Sections on grids were stained with lead citrate for 2.5 minutes and dried before observation on a JEOL JEM-100CX II electron microscope (JEOL Inc., Peabody, MA).

The factorial combination of two varieties and two levels of salts constituted the experimental units, which

were arranged in a randomized complete block design with three replicates. The treatment units were blocked by potential temperature gradient in the greenhouse. The data were subjected to analysis of variance (ANOVA) using mixed procedures in SAS 9.2 (SAS, 2008) with replication blocks treated as a random variable. Treatment means were separated using Fisher protected least significant dif-ference (LSD) at 0.05 probability level.

Results and discussionThe two years data were homogenous based on F-test

and therefore were reported together. There was no variety difference or interaction between variety and stress treat-ment. Therefore, the results were presented with data av-eraged across varieties. The lack of difference between two varieties differed from the preliminary results reported by Wipff and Rose-Fricker (2003). One of the reasons for the difference may be the stages of maturity of plants, where Wipff and Rose-Fricker used mature plants. However, more study is needed to confirm this factor because the study by Wipff and Rose-Fricker was not replicated.

Salt treated plants showed lower Chla content than the untreated control as early as two weeks after treatment (WAT) (Table 1). Salt treat-ment showed no change of Chlb content com-pared to the control until 3 WAT (Table 1). As a result, salt treatment had a lower Chla/Chlb ratio than the control starting at 3 WAT (Table 1). Compared to the control, lipid per-oxidation (as indicated by excessive produc-tion of MDA) increased in the salinity treat-ments at 1 WAT (Table 2). This indicated that lipid peroxidation was initiated prior to that of Chl hydroxylation in the process of senescence. Similar results were reported in perennial ryegrass (Lolium perenne L.) (Hu et al., 2012) and Kentucky bluegrass (Poa pratensis L.) and creeping bentgrass (Agros-tis stolonifera L.) (Yang et al., 2012).

The wetting angle on leaf surface by wa-ter is often used as an indirect estimation of epicuticular wax accumulation on the leaf surface (Bolger et al., 2005). Salt treatments resulted in larger wetting angle on the ad-

Table 1. Chlorophyll a (Chla) and b (Chlb) content of the 4th leaves below apical meristem on main shoot of two tall fescue cultivars, ‘Tar Heel II’ and ‘Wolfpack’, watered with full strength Hoagland solution with or without 16 g L-1 NaCl and CaCl2 at the onset of 4th leaf seedling stage.

Treatment0 WAT† 1 WAT 2 WAT 3 WAT 4 WAT

mg g-1 dry weightChla

Control 9.1 9.8a‡ 9.2a 7.2a 5.5aSalts 9.1a 7.2b 4.3b 1.8b

Chlb

Control 3.2 3.5a 3.4a 3.0a 2.5aSalts 3.3a 3.0a 2.3b 1.3b

Chla/ Chlb

Control 2.8 2.8a 2.7a 2.4a 2.2aSalts 2.8a 2.4a 1.9b 1.4b

† WAT, week after treatment.‡ Numbers within one comparing column followed by same letters are not significantly

different at the 0.05 probability levels.

Figure 1. Illustration of the measurement of leaf surface water wetting angle θ in tall fescue (Festuca arundinacea Schreb.).



axial side of the 4th and 5th leaves compared to the control (Table 3). This indicates that the epicuticular wax accumulation on the leaf sur-face is not only an indicator of leaf senescence but also an adaptation to the salinity stress as shown in the 5th leaf with adaxial wetting an-gles averaged over varieties for salt treatment and control were 136° and 117°, respectively. There was no difference in wetting angles on the abaxial side of the leaves between salt treatment and the control, both were about 58° (Table 3). The hypothesized epicuticular wax accumulation on the leaf surface under salinity stress was confirmed by SEM images of the leaf surface (Figure 2), which showed denser wax layer under salinity stress. The accumulation of epicuticular wax, as a means to conserve water by leaves under drought or other stress, was reported in other grasses (Garcia et al., 2002; Bolger et al., 2005; Rich-ardson et al., 2005; Jefferson, 2008; Honour et al., 2009).

Leaf senescence in the cellular ultra-struc-ture (Figure 3) from the TEM showed that the control plants had higher levels of integrity of grana and thylakoids in the chloroplasts. This corroborates the evidence shown above for lipid peroxidation and Chl hydroxylation.

Table 2. Malondialdehyde (MDA) content of the 4th leaves of two tall fes-cue cultivars, ‘Tar Heel II’ and ‘Wolfpack’, watered with full strength Hoagland solution with or without 16 g L-1 NaCl and CaCl2 at the onset of 4th leaf seedling stage.

Treatment0 WAT† 1 WAT 2 WAT 3 WAT 4 WAT

nmol g-1 FWControl 5.16 5.58b‡ 5.91b 7.38b 9.53bSalts 8.87a 40.60a 71.08a 74.38a

† WAT, week after treatment.‡ Numbers within one column followed by same letters are not significantly different at

the 0.05 probability levels.

Table 3. Surface water wetting angles on the 4th and 5th leaves of the main tiller of two tall fescue cultivars, ‘Tar Heel II’ and ‘Wolfpack’, four weeks after irrigated with full strength Hoagland solution alone or containing 16 g L-1 NaCl and CaCl2 at the onset of 4th leaf seedling stage.

Treatment 4th leaf 5th leafAdaxial Abaxial Adaxial Abaxial

Control 124.1b† 57.8a 117.0b 58.0aSalts 149.1a 58.2a 136.0a 58.9a

† Numbers within a column followed by same letters are not significantly different at the 0.05 probability levels.

Figure 2. Epicuticular wax accumulation on the abaxial (top panel) and adaxial (bottom panel) leaf surfaces in tall fescue (Festuca arundinacea Schreb.) under (a) natural aging, (b) salinity treatment with full strength Hoagland solution contain-ing 16 g L-1 NaCl and CaCl2. Samples were taken from the 5th leaf 4 weeks after the initiation of treatment.



The grana and thylakoids in the cells of salt treated plants showed higher levels of deterioration (lower clarity) in thy-lakoid membranes, which was also reported by Yamane et al. (2008).

The vegetation indices showed different stress levels among the 4th, 5th, and 6th fully developed leaves. Greater stress was associated with high SI710/760, or low mSR750/705 and mND750/705. Salinity stress was significantly higher in

treated leaves than the control (Figure 4). Older leaves (4th and 5th) were more severely affected as shown from SI710/760, mSR750/705 and mND750/705 (Figure 4). The trend also was confirmed by the quantum yield data, where a low quantum yield indicated higher stress levels (Figure 4). However, quantum yield measurement also was different in the 6th leaves between the salinity treatment and con-trol. This indicates that senescence was accelerated by sa-

Figure 3. Thylakoids (indicated by arrows) in the leave mesophyll cells of tall fescue ‘Tar Heel II’ (Festuca arundinacea Schreb.) under (a) natural aging, (b) salinity treatment with full strength Hoagland solution containing 16 g L-1 NaCl and CaCl2 at the onset of 4th leaf seedling stage. Samples were taken from the 5th leaf 4 weeks after the initiation of treatment.

Figure 4. Three reflectance indices, mSR750/705, mND750/705, and SI710/760, calculated from the relative reflective spectra, and quantum yield of the 4th to 6th leaves of two tall fescue (Festuca arundinacea Schreb.) varieties, ‘Tar Heel II’ and ‘Wolfpack’, watered with full strength Hoagland solution with or without 16 g L-1 NaCl and CaCl2 the onset of 4th leaf seedling stage. Data were averaged from two varieties. Samples were taken at 4 weeks after the initiation of treatment.



linity stress in all leaves with older leaves more severe and losing photosynthesis ability faster. Similar results caused by salinity stress were reported previously (Munns, 2002; Ghanem et al., 2008; Hajlaoui et al., 2010).

The hyperspectral reflectance indices are highly cor-related with MDA levels, with correlation coefficients to mSR750/705 and mND750/705 were -0.78 and -0.83, respectively. The correlation coefficient between MDA level and SI710/760 was 0.79. On the other hand, the correlation coefficient be-tween MDA and total Chl was -0.76. The results suggested that these hyperspectral reflectance indices are reliable indicators of the plant stress levels.

In general, tall fescue showed unique morphological and physiological changes due to salinity induced senescence as compared to natural senescence. The concentration of Chla and Chlb in salinity stressed mature leaves decreased compared to the control. The ratio of Chla/Chlb decreased in salinity stress over the time course of senescence com-pared to the control. Salinity stress increased MDA levels in the mature leaves. Leaf epicuticular wax accumulation on adaxial side increased in salinity treatment compared to the control and chloroplast integrity was affected by salinity stress. Ultimately, senescent leaves showed lower photosynthesis ability as indicated by lower quantum yield parameters. Hyperspectral reflectance indices were shown very sensitive to detect senescing due to salinity stress, which may be used in turfgrass management as a quick and reliable means of assessing levels of salinity stress and leaf senescence.

AcknowledgmentsThe research was funded by the Hatch Project ND01558.

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Received: Aug. 6, 2014Accepted: Mar. 19, 2015

Addresses of authors: Yang Gao and Deying Li*Department of Plant Sciences, North Dakota State University, Fargo, ND 58108, USA* Corresponding author; E-mail: [email protected]

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Schreb.) under salinity stress using leaf spectrum · Salinity is one of the major abiotic factors that adversely affect turfgrass quality. Tall fescue has a moderate to high salinity