Zhang Cui-ting, Xie Fu-chun, Yin Hang, Zhang Gao-yun, Guo Zhi-xin, Zuo Yang, Zhao Wei, Shah Saud, and Chen Ya-jun*

1 College of Horticulture and Landscape Architecture, Northeast Agricultural University, Harbin 150030, China

2 College of Animal Sciences and Technology, Northeast Agricultural University, Harbin 150030, China

Abstract: In order to develop the resources of native turfgrass, the morphological traits and drought resistance of native Siberian bluegrass (Poa sibirica, abbreviated as PS) was evaluated using the introduced Kentucky bluegrass 'Midnight' (Poa pratensis, abbreviated as PP) as a control. Two water schemes were imposed to plants in this pot culture study in greenhouse. One was with drought stress persistent limiting water supply for 20 days, the other was re-hydrated until 14 days after drought. The leaf shape, turf color, water status and cell plasma membrane permeability were evaluated. Similar changing trends with these parameters were shown for both species, and there were not significant differences with most evaluations during drought and re-water periods. The values leaf width and length of PS were higher while leaf color intensity was slightly lower than that of PP, but the greenness of PS leaf was still visually acceptable. There were not significant differences with cell membrane stability between the two species. In comparison, the native wild species PS possessed the potential for to be domesticated into a new cultivar for turf industry.

Key words: turfgrass, drought, morphological characteristics, water status, cell membrane permeability

Introduction

As an important complement of urban landscape, turfgrass has been widely used in urban greening. Currently, the turf coverage is as one of the important indicators for modern cities (Liuet al., 2018). Compared with other landscape plants, turfgrass has the advantages of quick effect and simple planting, it has irreplaceable roles of landscape and ecology. It is necessary for turfgrass to require a large quantity of water to maintain a better landscape, which is a vital environmental issue that constrains the quality of the turfgrass. According to Wuet al. (2005), generally the water consumption of the turfgrass can reach 3-8 mm per day, and even some can reach 12 mm per day, the cool-season turfgrass requires more water than other kinds of turfgrasses. Moreover, different degrees of water stress not only to decrease the growth of turfgrass, but also affect the turf performance such as turf quality (Gaoet al., 2006). In recent years, as the water consumption in agriculture and industrial production has increased, water resources have limited the turfgrasses development which has also aggravated the degree of drought on turf fields (Wu, 2017). Harbin City is situated in the northeastern part of China, and the climate of this region belongs to the northern temperate continental monsoon climate. The annual precipitation is between 500-600 mm, with the characteristics of large spring dryness, warm and humid summer, rapidly cooling in autumn, then cold and dry in winter (Donget al., 2019).

Along with the improvement of urban municipal construction, the turfgrass has become an important plant material in urban greening and beautification, the proportion of turf coverage in urban greening is recently increased in this region. However, main turfgrass varieties in Harbin City rely on the introduced Kentucky bluegrass which is almost from the US (Wanget al., 2016), it is urgently to need the germplasm of cool-season turfgrass with water saving attributes for reducing water supply. Therefore, converting native grasses to turfgrass and selecting their drought-tolerant merits are crucial for local turfgrass management from water saving perspective.

Siberian bluegrass (Poa sibirica) is a native wild species of the genusPoaand it is discovered in the Changbai Mountains area of Jilin Province, with shorter rhizomes and fresh green color of leaf, which has potential for turf use in landscape. In this experiment, native Siberian bluegrass was evaluated on turf performance and drought resistance compared to Kentucky bluegrass. Some morphological and physiological indexes of the two bluegrasses were assessed by pot study in greenhouse. Hopefully, this research would provide theoretical and practical basis for developing new cool-season germplasm of native turfgrass with water saving characteristics.

Materials and Methods

Plant materials

Siberian bluegrass (Poa sibiricaabbreviated as PS) collected from Changbai Mountain of Jilin Province in the northeast of China (127°52′55″E, 42°15′18″N, 2 150 m altitude) was used in this study, and the control was the introduced Kentucky bluegrass cultivar named 'Midnight' (Poa pratensis, abbreviated as PP).

Methods

Each species was planted in a plastic tray (70 cm×30 cm×10 cm) in greenhouse at the Horticultural Experimental Station of Northeast Agricultural University, Harbin City, China. After two leaves of the seedlings were emergent, they were transplanted into a PVC pot (16 cm in diameter and 40 cm in height). The bottom of each PVC pot was covered with double-layer gauze (under the premise of ensuring water permeability and preventing the cultivation substrate from falling off), and each pot was filled with 5.2 kg of the mixed peat, vermiculite and decomposed loam with the proportioning ratio of 2 : 1 : 1, and pH 7.0. The total 90 plants were transplanted evenly in each labeled pot. During maintenance period, watering once every 2 days, trimming to 10 cm once a week, and applying 0.2 g fertilizer (N : P : K=17 : 6 : 10) every 15 days for pot cultivated seedlings. The environmental conditions in green-house were with (25±2)℃/(15±2)℃ day/night,(700±10) μmol ? m-2? s-1light illumination and 75%±2%relative humidity. After 60 days cultivation, continuous drought stress was imposed to the seedlings of the two species until 20 days. Measurements were performed with four replicates on every 5 days during drought periods. After 20 days of drought stress treatment, the corresponding materials were re-hydrated immediately at 7: 00 a.m. on the next day, and the same measurements were carried out on 7 and 14 days after re-watering. The following indicators were examined. Soil water content was detected at 20 cm depth in soil by a TDR200 (USA, SPECTRUM). The leaf width and length were measured using a ruler (minimum scale of 0.01 cm). Leaf color intensity was measured using a SPAD502 chlorophyll meter (Konica Minolta, Japan). The measurements of leaf relative water content, leaf electrolyte leakage (EL) and malondialdehyde (MDA) content were with reference to the methods of Zou (2000). Relative water content (%)=(fresh weight-dry weight)/(saturated weightdry weight)×100%. Relative electrolyte leakage (%)=primary value/final value×100%. MDA extraction was followed the method of Hao (2002). Briefly, 0.2 g leaves were grinded with 10 mL of 10% trichloroacetic acid (TCA), then centrifuged at 4 000 r ? min-1for 10 min. The 2 mL of supernatant plused 2 mL of 0.6% thiobarbituric acid (TBA) was centrifuged for 15 min. The final top supernatant was used to inspect the spectro-photometric values at 532, 600 and 450 nm by a visible spectrophotometer (Model 721, China). MDA content (μmol ? g-1)=CMDA? ×N? ×W-1, in whichCMDA=6.45×(A532-A600)-0.56A450,Nwas the extraction volume (mL), andWwas the leaf fresh weight (g).Areprsented spectro-photometric value at 532, 600 and 450 nm, espectively.

Data analysis

The ANOVA variance of data was analyzed by SPSS (Statistical Product and Service Solutions, Version 19.0). GraphPad Prism 7.00 software was used for describing figures.

Results

Morphological characteristics and turf color of PS and PP under drought stress

Under normal condition (day 0), the width and length of leaves for the two species were distinctly different. The width of PS leaves varied between 2.30 and 2.40 mm, and the length of PS leaves varied between 33.50 and 35.48 cm. While, the width of PP leaves varied between 1.60 and 1.90 mm, and the length of PP leaves varied between 28.6 and 32.8 cm. In comparison, the leaves of PS showed a little bit wider and longer than the leaves of PP. However, under different levels of drought stress, the width and length of leaves for the two species changed in a similar trend that increased first during light drought stress and decreased later along with the increased drought intensity, again increased after re-hydrated treatment (Fig. 1a and b). The values of leaf width and length of PS had been performed higher than those of PP during the two water schemes treated. Leaf color intensity was usually positive to turf quality. While in this study, during the stress period of 10-20 days, the color values of the leaves of the two bluegrasses showed a sharp decline, PS and PP decreased by 39.20% and 34.20%, respectively. After re-watered, the color values of the leaves of the two bluegrasses increased. The changing trends of leaf color intensity for the two species were opposite in corresponding to the leaf width and length, whether under control condition or stressed period. The values of leaf color intensity of PS had been lowed than those of PP, although the leaf color of PS was visually acceptable during light drought stress (Fig. 1c).

Changes of water status of PS and PP under drought conditions

With the increase of drought stress, the changing trends of soil water contents of the two kinds of bluegrass were basically the same, indicating that PS had a similar water evaporation rate in contrasted to PP.

Although soil water contents decreased much with the drought stress period extend, there were no significant differences between the two species. After 20 days of drought stress, soil water contents of PS and PP decreased by 88.94% and 90.17%, respectively, and all returned to the control levels after re-hydration to 7 days (Fig. 2a).

With the drought stress intensity increased, the relative water contents of leaves decreased for both species. In the early drought stress period, the changing trends declined slowly from day 0 to day 10, but from day 15 to day 20, both species showed a sharp decline (Fig. 2b).

Comparatively, the leaf water content in PS was a little bit higher than that in the control species PP during drought stress, suggesting that the leaf water content was the species-specific difference, and it was hard to say that the drought resistance of PS was relatively stronger than that of PP. After re-watering, the leaf relative water contents of the two species were similar to those of the leaves on day 0.

Changes of cell membrane permeability of PS and PP under drought stress

As shown in Fig. 3, the leaf electrical conductivity and MDA content of the two kinds of bluegrass increased along with the increasing of drought stress and recovered after re-hydration. When the drought stress was during 0-5 days, the leaf electrical conductivity and MDA content of the two kinds of bluegrass did not change notably. With the stress time increased, the degree of the damage to the cell plasma membrane increased distinctly indicated by the significantly increased leaf electrical conductivity (Fig. 3a) and MDA content (Fig. 3b).

In contrast, the electrical conductivity and the accumulation of MDA content in PS appeared more aggressive than those in PP, especially on the 20th day of drought stress, suggesting that the extent of the damage to cell membrane in PS was a little bit more serious than that in PP.

Fig. 1 Changes in leaf width (a), leaf length (b) and turf color (c) of PS and PP under drought stress and re-watering conditions

Fig. 2 Changes of soil water content (a) and leaf relative water content (b) of two species under drought stress and re-watering conditions

Fig. 3 Changes in leaf electrical conductivity (a) and MDA content (b) of PS and PP under drought stress and re-hydration conditions

However, from the perspective of the whole drought process, there was no significant difference in the degree of membrane damage between the two species.

Discussion

Morphological attributes and turf color intensity

Turfgrass leaf was an important indicator for assessing turf quality in practical turf management, also reflected the ability of turfgrass to tolerant to the drought environment and was closely related to the physiological status of plants (Saudet al., 2017). Because plant leaf morphology was extremely sensitive and environmentally malleable, it could adapt to the damage caused by environmental stress through changes in leaf morphology (Liet al., 2012). When plants were exposed to water stress, the leaf area index decreased to reduce water dispersion loss which would affect the exchange of matter and energy between plants and the surrounding environment (Vogel, 2009). The results in this study showed that with the persistence of drought stress, the leaf length and leaf width of the two species showed a slow downward trend, that was, the leaf area reduced under water stress, and the water dispersion reduced, thereby improving their drought resistance. The length and width of the two bluegrass leaves showed an upward trend with the re-water treatment, indicating that both species had strong self-recovering ability in response to drought. In addition, compared to PP, longer and wider leaf of PS entailed the beneficial for its light absorption and energy storage, which might also improve drought resistance.

Turf color was considered as an important parameter for evaluating the quality of turf (Turgeon, 2012). Turfgrass with uniformly dark green leaf color meant having a unique ornamental value for application. However, many abiotic and biotic factors influenced leaf color, thus reduced the turf quality (Saudet al.,2016). The normal physiological synthesis and metabolism of chlorophyll maintained the leaf greenness of plants (Liet al., 2008). The results of this experiment showed that before the water stress (day 0), the leaf color of PP was higher than that of PS, indicating that the possible reason for this phenomenon was much more chlorophyll in PP leaves than in PS. However, the chlorophyll content in each species didn't measure, although the leaf color intensity in this study could also as an indicator for inferring to it. Actually, as a measure of plant resistance to various stresses, the metabolism of chlorophyll was well worth studying, especially for turfgrass.

Water status and cell membrane stability

Soil water content could reflect the evapotranspiration rate of plants. Lower evapotranspiration rates could save water that was considered to be a good attribute for turgrass management, due to the economic issues (Chenet al., 2018). Turfgrass maintained higher leaf water content during drought might serve to the cell membrane stability thus increasing drought resistance (Xuet al., 2011). Cell membrane stability played a critical role in maintaining cell turgor and physiological functions, especially during plant facing drought stress, and electrical conductivity and MDA content had been widely used to examine cell water status and membrane stability (Zhanget al., 2008). MDA was an indicative substance that could cause cell membrane peroxidation, and increase cell membrane permeability and relative cytoplasmic conductivity (Xuet al., 2011). Drought stress had a great influence on the cell membrane of plants. When the leaves were arid and dehydrated, the structure of the cell membrane was damaged, and the membrane protein structure was degenerated, and the fatty acid composition of the membrane was modified. These changes would increase the permeability of the cell membrane. When the permeability of the cell membrane increased, the membrane binding ability decreased, the extracellular cytoplasm and other electrolytes in the membrane would destroy the normal behavioral work of the physiological activities of the plant (Farooqet al.,2009). In this study, there were no differences in soil water and leaf water content for both species during the two water treatment schemes. Moreover, except for the 20 days of drought stress, all the results of electrical conductivity and MDA content from the two species with the rest evaluation days showed no notably differences, indicating that the water physiological attribution and the cell membrane stability with the species PS were close to the control species PP which was a popular variety of Kentucky bluegrass and had been widely used in the world (Saudet al., 2017). This research revealed that the native wild species PS had the potential to develop a new cultivar for application in turf industry.

Conclusions

In summary, the plants growth characteristics, turf quality performance, water status in soil and plants, and cell plasma membrane permeability were evaluated for both native species PS and the introduced common turfgrass species PP. Drought stress and re-watering caused similar change patterns for all the parameters in the two species. There were not significant differences with most evaluations for each measurement. The values of leaf width and length of PS were higher than those of PP. While the leaf color intensity was slightly lower than that of PP, but the leaf appearance still was visually acceptable. During the drought period, there were no significant differences in the extent of membrane damage between the two species. In comparison, the native wild species PS had the potential for developing a new cultivar for turf industry.