By Bingru Huang and Yan Xu
High temperature is a primary factor causing summer bentgrass
decline. One of the typical symptoms of summer bentgrass decline
is leaf senescence, which is characterized by loss of chlorophyll
and photosynthetic activities in leaves. Cool-season turfgrass
species, such as creeping bentgrass (
Agrostis stolonifera
), are sensitive to heat stress and quickly lose color and suffer
from a series of physiological injuries when exposed to
temperatures above 30°C (86°F). Leaf senescence was observed
after 20 days at 30°C and only 8 days at 35°C (95°F) for
Penncross creeping bentgrass.
1,2
 | |
| One approach to understand mechanisms of
plant tolerance to stresses has been to examine plants
adapted to extremely stressful environments. Several
cool-season grass species have recently been identified
growing in geothermally heated areas in Yellowstone National
Park. One of the two predominant grass species in thermal
areas is
Agrostis scabra
(thermal rough bentgrass). |
Phytohormones are major biochemical factors that regulate leaf
senescence. Ethylene, abscisic acid (ABA), and cytokinins are
three major phytohormones that mediate signaling events involved
in leaf senescence, but the mechanisms of heat-induced leaf
senescence in turfgrass are largely unknown. Identification of
physiological or metabolic factors associated with leaf
senescence has practical value for developing practices that
promote healthy turf during the summer, and it is important for
revealing basic mechanisms of turfgrass heat tolerance.
Recently, a cool-season grass species,
Agrostis scabra
(thermal rough bentgrass), has been identified growing in
geothermally heated areas in Yellowstone National Park.
6
It survives and even thrives in chronically hot soils with
temperatures up to 45°C (113°F).
23
Our studies demonstrated that when exposed to 35°C, thermal
bentgrass exhibited much better heat tolerance than creeping
bentgrass, exhibiting less leaf senescence, higher photosynthesis
activity, more efficient carbon utilization, and better root
growth.
4,5
This study was designed to determine whether superior heat
tolerance in the thermal bentgrass was associated with metabolic
factors regulating heat-induced leaf senescence, specifically
changes in the three major senescence-related hormones (ethylene,
ABA, and cytokinins). Turf quality and the content of two
pigments (chlorophyll and carotenoid) were measured to evaluate
the degree of heat tolerance and leaf senescence. Quantitative
changes in ethylene, ABA, and two major forms of cytokinins
during heat stress were determined to examine their relationship
with heat-induced leaf senescence.
EVALUATION OF HEAT-INDUCED LEAF SENESCENCE AND HORMONE
PRODUCTION
Creeping bentgrass (cv. Penncross) plugs were collected from
field plots at Hort Farm II, Rutgers University, N.J. Plants of
A. scabra
, originally collected from geothermally heated areas in
Yellowstone National Park, Wyoming, were propagated in a
greenhouse at Rutgers University. Both species were planted in
plastic pots (15 cm diameter by 20 cm deep) filled with
sterilized sand and fertilized weekly with full-strength
Hoagland's solution. Plants of both species were exposed to
35°C/ 30°C (day/night, high temperature) or 20°C/15°C (68°F
day/59°F night, optimum temperature) for 35 days in
controlled-environment growth chambers.
 |
| Soil temperature at a 2-inch depth was
approximately 113°F at a thermal site in Yellowstone National
Park (A), where thermal
Agrostis scabra
plants grow and the plant still possesses healthy roots and
leaves. Heat-sensitive creeping bentgrass (B) is compared to
heat-tolerant thermal
A. scabra
(C), where both species were exposed to elevated air/soil
temperatures in a growth chamber. |
Turf quality was evaluated based on color, density, and
uniformity of the grass canopy using a scale of 0 to 9, with 9
representing fully green, dense turf canopy and 0 representing
completely dead plants. Leaf chlorophyll and carotenoid were
extracted from fresh leaves. Ethylene production of leaves was
determined using a gas chromatograph. ABA and two forms of
cytokinin (trans-zeatin/zeatin riboside and isopentenyl
adenosine) were quantified by an indirect competitive
enzyme-linked immunosorbent assay.
RELATIONSHIP BETWEEN HORMONE ACCUMULATION AND HEAT-INDUCED
LEAF SENESCENCE
Heat stress caused decline in turf quality in both bentgrass
species, but the decline occurred three weeks later in the
thermal bentgrass than creeping bentgrass. Chlorophyll and
carotenoid content of the thermal bentgrass exposed to heat
stress were maintained at the optimum temperature level for
approximately 14 days without any significant decrease until 21
and 28 days, respectively. The decline in turf quality,
chlorophyll, and carotenoid content was less severe for the
thermal bentgrass than creeping bentgrass. The thermal bentgrass
exhibited delayed and less severe leaf senescence under heat
stress. Previous studies on root response to high temperatures
for these two species also found that the thermal bentgrass
exhibited higher tolerance to high soil temperature than creeping
bentgrass, with smaller decreases in root growth rate, cell
membrane stability, maximum root length, and nitrate uptake.
4,5
The ethylene production rate of both bentgrass species increased
significantly under heat stress, when there was a 20% decline in
chlorophyll content. Leaf ABA content also increased under heat
stress for both species. However, the increased production of
ethylene and ABA in the thermal bentgrass occurred 14 days later
than that in creeping bentgrass. This delay of ethylene or ABA
accumulation in the thermal bentgrass was consistent with the
delay of leaf senescence as manifested by decline in turf quality
and chlorophyll and carotenoid contents.
| |  |
| Researchers at Rutgers University are
using thermal rough bentgrass (
Agrostis scabra
) plants collected from geothermal sites at Yellowstone
National Park (left) to identify high-temperature tolerance
genes. The goal is to identify the mechanisms in an effort to
improve heat tolerance of other creeping bentgrass
varieties. |
The production of both forms of cytokinins (Z/ZR and IPA)
consistently decreased under heat stress in both bentgrass
species. In terms of species variation, the decreases of both
forms of cytokinins were delayed for 7 days and were less severe
after 35 days of heat stress in the thermal bentgrass than in
creeping bentgrass. This suggests that maintenance of a higher
level of endogenous cytokinin for a longer period of time may
contribute to better heat tolerance.
We performed a correlation analysis between hormone
accumulation and leaf senescence to determine whether changes in
hormone production during heat stress are associated with
heat-induced leaf senescence, and to determine which hormone is
more important in controlling leaf senescence. The results
suggested that endogenous ethylene and ABA production was
negatively correlated and cytokinin production was positively
correlated with turf performance under heat stress.
PRACTICAL IMPLICATIONS
The results in this study suggest that approaches that can
increase endogenous cytokinin levels or suppress ethylene
production may lead to improved heat tolerance and delayed foliar
senescence. Exogenous spray of cytokinin, or its derivatives, may
be one possible method. Liu et al.
3
reported that applications of 1 and 10 mM zeatin riboside to the
rootzone of creeping bentgrass increased cytokinin content in
leaves and roots and mitigated heat stress injury in both shoots
and roots.
Endogenous cytokinin levels may also be increased by
transgenic approaches, introducing favorable genes. In another
study, we transformed creeping bentgrass plants with a gene
controlling cytokinin synthesis and found that transgenic plants
exhibited superior heat tolerance compared to non-transgenic
plants. This demonstrated that heat tolerance was associated with
the maintenance of cytokinin production and leaf chlorophyll
content during heat stress (unpublished data).
Conversely, since ethylene production was negatively
correlated with heat-induced senescence, delayed leaf senescence
may also be achieved by transgenic approaches or using ethylene
inhibitors. In a recent study, we sprayed an ethylene inhibitor
to the canopy of creeping bentgrass exposed to 35°C and found
that treated turf maintained greener and higher photosynthetic
activity for a longer period of time compared to untreated
turf.
Our studies suggest that foliar application of cytokinins or
ethylene inhibitors may be useful to suppress or delay leaf
senescence and ultimately improve turfgrass performance during
summer months. A field study is in progress at Rutgers University
to test the effectiveness of exogenous application of cytokinins
and ethylene inhibitors as well as biostimulants in preventing
summer bentgrass decline.
REFERENCES
1. Huang, B., and H. Gao. 2000. Growth and carbohydrate
metabolism of creeping bentgrass cultivars in response to
increasing temperatures.
Crop Sci.
40:1115-1120.
2. Huang, B., X. Liu, and J. D. Fry. 1998. Shoot physiological
responses of two bentgrass cultivars to high temperature and poor
soil aeration.
Crop Sci.
38:1219-1224.
3. Liu, X., B. Huang, and G. Banowetz. 2002. Cytokinin effects
on creeping bentgrass responses to heat stress: I. Shoot and root
growth.
Crop Sci.
42:457-465.
4. Lyons, E., J. Pote, M. DaCosta, and B. Huang. 2006.
Whole-plant carbon relations and root respiration associated with
Agrostis
grass responses to high soil temperatures.
Environ. Exp. Bot.
(in press).
5. Rachmilevitch, S., H. Lambers, and B. R. Huang. 2006. Root
respiratory characteristics associated with plant adaptation to
high soil temperature for geothermal and turf-type
Agrostis
species.
J. Exp. Bot.
57:623-631.
6. Stout, R. G., and T. S. Al-Niemi. 2002. Heat-tolerant
flowering plants of active geothermal areas in Yellowstone
National Park.
Ann. Bot.
90:259-267.
7. Tercek, M. T., D. P. Hauber, and S. P. Darwin. 2003.
Genetic and historical relationships among geothermally adapted
Agrostis
(bentgrass) of North America and Kamchatka: evidence for a
previously unrecognized, thermally adapted taxon.
Amer. J. Bot.
90:1306-1312.
ACKNOWLEDGEMENT
We would like to thank the United States Golf
Association's Turfgrass and Environmental Research Program
and the Rutgers Center for Turfgrass Science for funding this
project.
Editor's Note:
For the original publication of this paper, visit USGA
Turfgrass and Environmental Research Online (
http://usgatero.msu.edu
).
Bingru Huang, Ph.D., professor; and Yan Xu, graduate
research assistant; Dept. of Plant Sciences, Cook College,
Rutgers University, New Brunswick, N.J.