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Effects of Drought or Heat Stress Alone and in Combination on Kentucky Bluegrass.

KENTUCKY BLUEGRASS is a cool-season grass widely used for home lawns and commercial landscapes in temperate climates. The optimum temperature for shoot growth of Cool-season grasses is 15 to 23 [degrees] C (Beard, 1973). However, temperatures in the transitional zone often approach 30 [degress] C or higher during summer months. In conjunction with heat stress, drought often lasts for prolonged periods. Drought or heat stress alone causes a severe decline in turf quality of cool-season grasses (Wehner and Watschke, 1981; Huang et al., 1998a,b). Physiological changes associated with performance of cool-season grasses in response to drought or heat vary with species or cultivars (Aronson et al., 1987; Howard and Watschke, 1991; Huang et al., 1998a,b). Drought stress reduced root dry weight, leaf water potential, evapotranspiration, and photochemical efficiency in tall rescue (Festuca arundinacea L.), Kentucky bluegrass, and perennial ryegrass (Lolium perenne L.) (Aronson et al., 1987; Carrow, 1996; Perdomo et al., 1996; Huang et al., 1998a). Heat stress alone caused reductions in photosynthetic rate, chlorophyll content, cell membrane stability, and carbohydrate accumulation in many species, including creeping bentgrass (Agrostis palustris L.), Kentucky bluegrass, and perennial ryegrass (Wehner and Watschke, 1984; White et al., 1988; Howard and Watschke, 1991; Huang et al., 1998b).

Heat and drought stresses often occur simultaneously during summer months, limiting plant growth. Simultaneous heat and drought stresses reduced the rates of [CO.sub.2] uptake and [O.sub.2] evolution in bean (Phaseolus vulgaris L.) (Yordanov et al., 1997); considerably inhibited leaf growth in sorghum [Sorghum bicolor (L) Moench.] (Kaigama, 1986); increased cell membrane permeability of perennial ryegrass (Chen et al., 1988); and reduced leaf water content, water potential,and osmotic potential in wheat (Triticum aestivum L.) (Shah, 1992).

Drought stress also often occurs prior to heat stress in the summer either because of irrigation or insufficient precipitation. Several studies have reported that prior exposure of plants to water deficit (drought preconditioning) reduces osmotic potential and enhances stomatal opening and photosynthetic rate during subsequent drought and, thus, increases plant resistance to the stress (Thomas et al., 1976; Bennett and Sullivan, 1981; Abrams, 1988). Wehner and Watschke (1981) reported that infrequent irrigation increased heat tolerance in Kentucky bluegrass, perennial ryegrass, and annual bluegrass (Poa annua L.). Becwar et al. (1983), however, found that drought preconditioning did not increase in vitro heat tolerance in four turfgrass species. How drought and heat stresses interact and influence growth of cool-season turfgrasses is not well understood. Knowledge of this interaction would help to identify physiological factors involved in drought and heat tolerances to improve summer performance of cool-season turf grasses.

Two experiments were conducted to investigate the interactive effects of drought and heat stresses on Kentucky bluegrass. The objective of the first experiment was to compare the effects of drought, heat, or the combined stresses on turf growth and physiological activities of Kentucky bluegrass. The objective of the second experiment was to determine the effects of drought preconditioning on Kentucky bluegrass responses to subsequent heat stress.


Experiment 1

Sods (10 cm in diameter and approximately 1 cm thick) of Kentucky bluegrass (cv. Mystic) were collected from field plots at the Rocky Ford Turfgrass Research Center, Kansas State University, Manhattan, KS, in September 1998. Sods were washed free of soil and then planted in polyvinyl chloride (PVC) tubes (10 cm in diam, 60 cm long) filled with a mixture of sand and topsoil (fine, montmorillonitic, mesic, aquic arquidolls) (1:2, v:v) in the greenhouse for 30 d and then transferred to growth chambers with a temperature of 20 [degrees] C/15 [degrees] C (day/night), 14-h photoperiod, and photosynthetically active radiation of 600 [micro]mol [m.sup.-2] [s.sup.-1]. Grasses were maintained in growth chambers for 15 d before drought and heat treatments were imposed. Controlled-release fertilizer (17N-6P-10K) was topdressed twice prior to dry-down to provide a total of 17 g N [m.sup.-2]. Turf was hand-clipped twice weekly at about a 4-cm height.

The experiment included two temperatures and two soil moisture treatments. Temperature treatments were optimum temperature (20 [degress] C/15 [degress] C, day/night) and heat stress (35 [degrees] C/ 30 [degrees] C). Soil moisture treatments included (i) well-watered control, grasses in eight containers (replicates) irrigated every other day until water drained from the bottom of the PVC tubes and (ii) drought stress, irrigation withheld until permanent leaf wilting occurred (leaves were no longer rehydrated at night or early mornings) for plants in eight containers. The treatments combining soil moisture and temperature were as follow: Control (well watered, optimum temperature); drought (D--unwatered, optimum temperature); heat (H--high temperature, well watered); and the combined drought and heat (H+D). Plants in four of drought-stress containers were rewatered when permanent leaf wilting occurred in drought or H+D treatment and allowed to recover. The potential of growth recovery was evaluated for drought-stressed and drought-and-heat stressed plants.

Soil moisture in the 0- to 20- and 40- to 60-em soil layers was monitored to determine soil dryness during drought stress by time domain reflectometry (Soil Moisture Equipment Corp., Santa Barbara, CA). The field capacity of the sand and topsoil mix was 30% [+ or -] 2% (mean of four replications [+ or -] standard error), which was measured in four PVC tubes with the TDR when drainage ceased following watering the soil to complete saturation.

Several physiological measurements were made at various times during the experiment. Canopy photosynthetic rate ([P.sub.n]) was measured with an LI-6400 portable gas exchange system (Li-COR Inc., Lincoln, NE), which was expressed as [CO.sub.2] uptake per unit turf canopy area. Leaf photochemical efficiency expressed as chlorophyll fluorescence (Fv/Fm ratio) was determined on five randomly selected leaves in each container with a fluorescence induction monitor (Dynamax, Houston, TX).

Plants in four containers in drought stress and well-watered treatments under optimum temperature and heat conditions were harvested when leaves of drought-stressed plants permanently wilted and when volumetric soil moisture dropped to 5% (about the minimal level, 17% of field capacity) in the 0- to 20-cm layer of soil in the drying treatment. At harvest, roots were separated from shoots; collected separately from 0- to 20-, 20- to 40-, and 40- to 60-cm soil layers and washed free of soil. Root dry weight was determined after samples were dried in an oven at 85 [degrees] C for 3 d.

Experiment 2

Growth conditions were the same as described above, except the tubes were 40 cm long. Plants in eight containers were well watered (non-reconditioning). Plants in another eight containers were subjected to two cycles of soil drying and rewatering (drought preconditioning) before being exposed to heat stress. When volumetric soil moisture reached about 5% in each drying period, grasses were rewatered and turf quality was allowed to recover to the same level as the well-watered plants. The first cycle of drought stress lasted for 17 d, and the second cycle for 28 d. After the second cycle of rewatering, drought preconditioned and non-preconditioned plants in eight containers were exposed to 35/30 [degrees] C (day/night) in growth chambers for 35 d. Plants in four containers in heat, preconditioned treatment or heat, non-preconditioned treatment were well-watered, and plants in the other four containers were not watered to induce drought stress.

The [P.sub.n] and Fv/Fm were measured as described in Exp. 1. Leaf water potential ([[Psi].sub.m]) was measured with a thermocouple psychrometer (Decagon Devices Inc., Pullman, WA). Osmotic potential ([[Psi].sub.[Pi]]) was measured with a vapor pressure osmometer (Wescor, Inc., Logan, UT) after cell saps was expressed from frozen leaves with a hydraulic press. Turgor pressure ([[Psi].sub.p]) was determined by calculating the difference between [[Psi].sub.w] and [[Psi].sub.[Pi]]. Stomatal conductance ([g.sub.s]) and transpiration rate were measured with a steady state porometer (LI-COR 1600, Inc., Lincoln, NE). At the end of the experiment, roots were separated from shoots and washed free of soil. Total root length was measured with an image analysis system (Decagon Devices, Inc., Pullman, WA). Root dry weight was determined as described in Exp. 1.

The experiments involved two factors (temperature and soil moisture) arranged in a split- plot design (temperature as main plot and soil moisture as sub-plot) with repeated measurements in four replicates (containers) (Kempthorne, 1952). Each temperature regime was replicated four times in four growth chambers. Soil moisture treatments were arranged randomly within each temperature regime. Plants in four replicates were randomly sampled from four growth chambers for each measurement. Analysis of variance was based on the general linear model procedure of the Statistical Analysis System (SAS) (SAS Institute Inc., Cary, NC). Interactions among temperature, soil moisture, and treatment duration occurred for all parameters examined. Effects of temperature and soil moisture treatments were analyzed by comparing them with the control at a given measurement time. Least significance difference (LSD) at a 0.05 probability level was used to detect the differences between treatment means.


Experiment 1

Soil water content in the 0- to 20- and 40- to 60-cm layers decreased rapidly under turf subjected to drought and heat stresses (Fig. 1). Water content declined to about 5%, which was about the minimum level of soil moisture, in the 0- to 20-cm layer after 12 d of the combined drought and heat stress (H+D), but not until 25 d after drought alone. At 12 d of drought stress, soil water content declined to about 10% in the 0 to 20-cm layer and 20% in the 40- to 60-cm layer. At 12 d of H+D, the corresponding figures were 5 and 13%, respectively.


Canopy [P.sub.n] declined significantly below the control level beginning at 3 d under heat stress, and at 6 d under drought conditions (Fig. 2A). The decline in [P.sub.n] was more severe under heat than under drought during the first 10 d of treatment. However, no significant differences in [P.sub.n] were observed between heat and drought at 12 and 20 d. The combination of heat and drought caused more rapid and severe reduction in [P.sub.n] than either stress alone. Canopy [P.sub.n] dropped to about 0 at 10 d of H+D, 20 d of drought, and 34 d of heat stress. The [P.sub.n] of drought-stressed plants increased after rewatering; it recovered to the control level at 14 d of rewatering. However, [P.sub.n] of drought- and heat-stressed continued to decline and never recovered after rewatering.


Leaf photochemical efficiency decreased to below level of the control at 20 d of drought and 34 d of heat stress (Fig. 2B). The reduction in Fv/Fm was more severe under drought than heat at 20 d of treatment. The H+D stress caused an earlier and more dramatic decline in Fv/Fm than either stress alone, dropping below the control level after 9 d of treatment. The Fv/Fm of plants subjected to drought alone returned to the control level after 14 d of rewatering. For plants exposed to H+D, Fv/Fm recovered to some extent, but did not return to the control level 28 d after rewatering.

The majority (85%) of roots of both control and stressed plants were distributed in the top 0 to 20 cm of soil (Fig. 3). Drought, heat, or H+D significantly reduced root dry weight in that layer, compared with the control. The reduction in root dry weight caused by heat alone did not differ significantly from the H+D stress. Root dry weights in 20 to 40 and 40 to 60 cm of soil did not differ between treatments.


Experiment 2

Canopy [P.sub.n] for both drought-preconditioned and nonpreconditioned plants decreased from their initial levels when they were subjected to heat or H+D (Fig. 4A). However, drought-preconditioned turf when exposed to heat alone had a higher canopy [P.sub.n] than non-preconditioned turf, starting 9 d of treatment. No differences in [P.sub.n] were observed between drought-preconditioned and non-preconditioned plants when exposed to H+D and drought stresses. Drought preconditioning had no effect on Fv/Fm during subsequent stress from heat alone or H+D (Fig. 4B).


Drought-preconditioned plants had significantly higher [[Psi].sub.w] and [[Psi].sub.p] than non-preconditioned plants at 28 d of heat stress (Table 1). They also had a higher [[Psi].sub.w], [g.sub.s], and transpiration rate than non-preconditioned plants at 35 d of heat stress. No differences in [[Psi].sub.p] were observed between preconditioned and non-preconditions plants at 35 d of heat stress.

Table 1. Leaf water potential ([[Psi].sub.w]), turgor pressure ([[Psi].sub.p]), stomatal conductance ([g.sub.s]), and transpiration rate (trans.) of drought-preconditioned and non-drought preconditioned Kentucky bluegrass exposed to 28 and 35 d of heat stress.
                             28-d Heat stress

Treatment            [[Psi].sub.w]      [[Psi].sub.p]

Preconditioned       -1.65a([dagger])       0.21a
Non-preconditioned   -2.02b                 0.03b

                             35-d Heat stress

Treatment            [[Psi].sub.w]   [[Psi].sub.p]

                                -- MPa --

Preconditioned          -1.76a           0.11a
Non-preconditioned      -2.13b          -0.02a

                      35-d Heat stress

Treatment            [g.sub.s]   trans.

                     mmol [m.sup.-2] [s.sup.-1]

Preconditioned        31.58a     1.42a
Non-preconditioned    18.84b     0.85b

([dagger]) Means followed by the same letters within a column were not significantly different on the basis of LSD test (P = 0.05).

Root length density expressed as root length per unit volume soil and dry weight of drought-preconditioned plants were about two times those of the non-preconditioned plants when exposed to 35 d of heat stress (Table 2). Both variables did not differ significantly between preconditioned and non-preconditioned plants when exposed to the combined stresses (data not shown).

Table 2. Root length density expressed as root length per unit soil volume and dry weight of drought-preconditioned and non-drought preconditioned Kentucky bluegrass exposed to 35 d of heat stress.
Treatment            Root length density   Root dry weight

                       cm [cm.sup.-3]             g

Preconditioned             3.25([dagger])       1.05a
Non-preconditioned         1.77b                0.52b

([dagger]) Means followed by the same letters within a column were not significantly different on the basis of LSD test (P = 0.05).


Heat stress had a more detrimental effect on canopy [P.sub.n] than drought during the early periods of treatment. However, prolonged periods of drought could be more detrimental than heat stress. The decline in canopy [P.sub.n] is closely related to turf quality decline under conditions of heat or drought stress (Huang et al., 1998b; Huang and Fry, 1998). Studies have demonstrated that photosynthesis of plants in general is particularly sensitive to heat stress with increased photoinhibition of photosystem II (Weis and Berry, 1988; Georgieva and Yordanov, 1993), but is relatively resistant to water deficits (Kaiser, 1987; Cornic and Briantais, 1991). Drought stress alone did not completely damage the photosynthetic apparatus, as evidenced by the quick recovery in [P.sub.n] after rewatering. However, drought combined with heat caused rapid leaf desiccation and permanent damage to the photosynthetic system. The strong interaction between drought and heat suggested that these two stresses have additive effects on [P.sub.n] in cool-season turfgrass species when they occur simultaneously. Ludlow et al. (1990) reported that the combined stresses of heat and drought caused a dramatic decline in grain yield of sorghum in the field.

Reduction in leaf Fv/Fm under drought and heat alone occurred at 10 to 12 d and 30 d later than that of [P.sub.n], respectively, suggesting that [P.sub.n] was more sensitive to these individual stresses. Under the combined drought and heat stresses, the reduction in Fv/Fm was correlated with that of [P.sub.n]. The interaction of drought and heat caused more rapid and severe damage to the photochemical efficiency of PSII than either stress alone, similar to the effects on [P.sub.n]. Unlike canopy [P.sub.n], Fv/Fm of some leaves exposed to the combined stresses recovered to some extent after rewatering, although they did not reach to level of the control. The discrepancy between the responses of Fv/Fm of individual leaves and [P.sub.n] to drought or heat and rewatering following exposure to stresses suggested that factors other than Fv/Fm contributed to the decline in canopy [P.sub.n] during the early periods of stress and during recovery for Kentucky bluegrass.

An extensive, deep root system is an important characteristic of drought resistant plants (Sheffer et al., 1987; Marcum et al., 1995; Carrow, 1996; Huang and Fry, 1998). Such a root system facilitates water uptake and, in turn, affects plant tolerance to drought and heat. These stresses and their combined influence reduced root growth in the surface soil layer. A strong combination effect of drought and heat on root dry weight was observed, but the reduction was similar to that from heat stress alone. The more severe inhibition of root dry weight by heat stress than drought could have been related to the more significant reduction in [P.sub.n] during the early periods of treatment.

Drought preconditioning enhanced canopy [P.sub.n] during subsequent heat stress. This could have been associated with improved root growth and water relations. Drought-preconditioned plants developed more extensive root systems than non-preconditioned plants, which could facilitate water uptake under heat stress. Drought-preconditioned plants had significantly higher [g.sub.s] and transpiration rate under heat, which could lead to greater cooling effects than in non-preconditioned plants. High [g.sub.s] and transpiration rate are related positively to seedling survival under high temperatures (Kolb and Robberecht, 1996). Our results also demonstrated that drought-preconditioned grasses maintained higher [[Psi].sub.w] and [[Psi].sub.p] during subsequent heat stress, which could have been due to solute accumulation during drought. Ashraf et al. (1994) found that a heat-tolerant cultivar of cotton (Gossypium hirsutum L.) accumulated significant amounts of organic osmotica such as soluble sugar and proline under heat stress. Drought stress also might induce accumulation of solutes in cells, which would enhance turgor maintenance during heat stress. This deserves further investigation.

Our results agree with those of Wehner and Watschke (1981) who also reported drought preconditioning increased heat tolerance of turfgrass by observing plant recovery after a 30 min exposure to 47 [degrees] C. However, our results refuted those of Becwar et al. (1983) who found that drought did not enhance tolerance to a short-term heat shock (48 [degrees] C) as determined in excised leaves.

In summary, heat and drought stresses significantly reduced canopy [P.sub.n], Fv/Fm, and water uptake, which would result in a decline in turf quality during hot and dry summers. The detrimental effects of the combined stresses were significantly greater than those of either stress alone. However, drought preconditioning increased tolerance of Kentucky bluegrass to subsequent heat stress, which suggested that water deficit or infrequent irrigation during spring could be used to encourage root growth and enhance hardiness to drought or heat stress during summer.

Abbreviations: Fv/Fm, photochemical efficiency or chlorophyll fluorescence; [[Psi].sub.w], leaf water potential; [[Psi].sub.[Pi]], osmotic potential; [[Psi].sub.p], turgor pressure; [g.sub.s], stomatal conductance; LSD, least significance difference; [P.sub.n], canopy photosynthetic rate.


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Yiwei Jiang and Bingru Huang(*)

Dep. of Horticulture, Forestry, and Recreation Resources, 2021 Throckmorton Plant Sciences Center, Kansas State University, Manhattan, KS 66506. Contribution No. 99-508-5 from the Kansas Agricultural Experiment Station. Received 29 July 1999. (*) Corresponding author (

Published in Crop Sci. 40:1358-1362 (2000).
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Author:Jiang, Yiwei; Huang, Bingru
Publication:Crop Science
Article Type:Statistical Data Included
Geographic Code:1USA
Date:Sep 1, 2000
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