Heat Stress
 
Optimum temperature for growth
 
 
The upper limit for higher plant growth is normally considered to be about 45 to 50° C. Some higher plants can survive a short-term exposure to temperatures of 60 to 65° C.
 
Heat stress rarely occurs in field conditions in the absence of water stress. Desiccation and pests often injure the turf during periods is sustained high temperature.
 
Warm-season turf grasses possess excellent heat tolerance while heat stress is a primary factor restricting use of cool-season turf in many areas of the USA.
 
What happens as temperatures exceeds optimum growth ranges?

Direct high temperature stress ( acute high temperature stress)
Indirect high temperature stress (chronic high temperature stress)
 
Importance of water - transpirational cooling is the most efficient way to cool plants. Therefore, often difficult to separate the effects of heat stress from drought stress since they often occur together
 
Increased transpiration due to rising temperatures results in stomatal closure, thus reducing transpiration cooling and increasing leaf temperature.
 
Syringing for the purpose of moderate conditions in the northeastern USA. Duff and Beard (1966) observed a canopy temperature reduction of 1 to 2° C. which lasted 2 hours following application of 6.4 mm of water at noon.
 
In North Carolina, DiPaola (1984) found bentgrass canopy temperatures in the absence of wilt were not alter one hour after syringing regardless of the water volume or timing.
 
 
Direct high temperature stress ( acute high temperature stress)
 
Short-term exposures to high temperatures resulting in plant injury or death.
 
System affected - studies have revealed that photosystem II is most sensitive to heat injury and is probably the system responsible for heat injury effects in plants.
It is generally believed that photosystem II is the most heat-sensititve system in plants. Short exposures 10-30 minutes at temperatures above 40° C (104° F) can cause plant injury or death.
 
 
 
Unlike freezing stress, the duration of heat stress is a critical factor in the degree of plant injury sustained.
 
Heat hardening (exposure to 39° C. for 10 days) of Kentucky bluegrass increased his killing temperature by 1° C., yet extended a high temperature (50° C.) exposure time needed for injury by more than one hour (Wallner et al., 1982).
 
The duration and temperature of heat kill are related as shown below
 
T = a - b log D

T= the killing temperature, degrees C.

D=exposure duration for kill, minutes

a=67.5

b=10.2
 
Wallner et al. (1982) found that heat killing time more clearly agreed with previously known drought resistance of turfgrasses than did the killing temperature.
 
Photosynthesis
 
Creeping bentgrass photosynthetic rate when grown at 20/10 °C night/day temperatures peaked at 30° C. at 81 µL O2 cm-2 h-1.
 
Once exposed to 40° C., the photosynthetic rate of creeping bentgrass decreased to 61 µL O2 cm-2 h-1.
 
Watschke et al. (1972) found that those cultivars best adapted to high temperature conditions also had the highest photosynthetic rates.
 
Carbohydrate reserves
 
Heat tolerance of grasses has long been linked to the level of carbohydrate reserves within the plant.
 
Bentgrass and Kentucky bluegrass performance under higher temperature conditions was best when night temperatures were cool.
 
Researchers have found depressed carbohydrate reserves for Kentucky bluegrass grown under high temperatures.
 
This response led these authors to conclude the that prolonged periods of heat stress could result in a depletion of carbohydrate reserves.
 
However, Duff and Beard (1974) working with creeping bentgrass demonstrated that carbohydrate levels can increase by better than 50% under high temperature stress conditions despite shoot growth reductions of 36% for the turf.
 
Protein synthesis
 
Net proteins synthesis in cool-season turfgrasses appears to be a heat liable process.
 
Wehner and Watschke (1984) found the incorporation of radio labeled leucine was reduced by 69% in plants previously exposed to 43° C. compare with those held at 27° C.
 
Label incorporation at 45° C. was only 10% of that for control turf at 27° C.
 
Although more widely studied in animals, plants synthesize heat shock proteins in response to high temperature stress.
 
 
Induction of heat shock proteins at 40° C. is quite rapid.
 
Transcriptional level increases in certain heat shock proteins genes has been observed in as little as 3 to 5 minutes of heat shock.
 
In corn, heat shock protein synthesis occurs following only 15 minutes at 41° C..
 
Heat shock proteins are thought to somehow enhance the heat tolerance of plant tissue, but their identity is largely unknown.
 
All corn tissues synthesize heat shock proteins during heat shock, with each exception of germinating pollen.
 
The physiological role of the heat shock proteins is currently unclear, particularly for turfgrass species.
 
Cooper and Ho (1987) postulated potential role for heat shock proteins in the maintenance of plant membranes during heat stress.
 
Altschuler and Mascarenhas (1982) suggested that heat shock proteins might be needed for recovery from sudden heat shock. These proteins clearly exist in grasses and may well contribute to improve heat tolerance of the plant.
 
Indirect high temperature stress (chronic high temperature stress)
 
extended periods of high temperature stress
systems affected - ? Perhaps the same photosystem II.
 
Many researchers have tried to link carbohydrate reserves to high temp stress. Some successfully, however, other studies have shown opposite effects.
 
For example, Duff and Beard compared bentgrass grown at 40/30° C (104/86° F) temperature regime to bent grown at a 20/10 (68/50) temp regime.
 
High temperature grown bentgrass had increased carbohydrate levels of 50% compared to bent at 20/10 levels. However, high temp had a reduction in shoot growth of 36% compared to cool regime.
 
Watschke found a reduction in carbos from 12.7 to 9.8% (dry wgt basis) when taking plants from a 10/10 regime to a 35/20 regime (97/68).
 
 
Xu and Huang (2000) conducted a study in a growth chamber to examine the impact of increases in temperature to both roots and shoots of creeping bentgrass. They designed the growth chamber to heat the roots and shoots separately.
 
for treatments were set up:
shoots
roots
20° C 20° C
35° C 20° C
20° C 35° C
35° C 35° C
 
 
Canopy Photosynthetic Rate
 
 
Photochemical Efficiency
 
 
Root fresh weight
 
 
Root number
 
 
In a very recent study, Wang et al. (2004) examined the effect of extended photoperiod on creeping bentgrass grown under high temperature.
 
All treatments were grown at 20/15° C for 14 days prior to light treatments. During the light treatments all pots were held at 33/28° C for different periods of light.
 
 
Shoot extension rate
 
 
Root fresh weight
 
 
Root activity
 
 
 

Protein synthesis slows dramatically at elevated temperatures (other then heat shock proteins)

27 C (81 F) = 100%
43 C (109 F) = 69%
45 C (113 F) = 10%
 
Since proteins are enzymes, the same problem that occurs with photosystem II may occur in protein synthesis, if synthesis occurs in membranes, then membrane dysfunction may reduce protein synthesis. Alternatively, proteins do denature (unfold ) near this temperature.
 

Heat shock proteins - inducible proteins that form in response to exposure to high temperatures. Function probably as protectants to prevent proteins and enzymes from denaturing.

 
Cultivar differences
 
Can be important. Observation in field is difficult to tell because of complex interactions with drought, rooting depth, etc.
 
In a 2001 paper, Xu and Huang present a study examining morphological and physiological characteristics of two creeping bentgrass cultivars under heat stress.
 
They chose L-93 as the heat tolerant cultivar and Pencross as the heat susceptible cultivar.
 
Turfs were grown under either a 20/15° C or 35/30° C (heat stress) régime for 64 days with a 14 hour photoperiod.
 
Turf quality
 
 
Photosynthetic rate
 
 
RuBP carboxylase activity
 
 
Shoot density
 
 
Root numbers
 
 
The authors concluded that the newer variety, L-93, possibly had a higher transpirational rate due to the greater root system and higher density reducing the effects of higher temperatures.
 
Perdomo et al., (1996) examined physiological changes in Kentucky bluegrass during summer heat stress. They looked at two cultivars Midnight (heat tolerance) and Nugget (heat susceptible).
 
 
Stomatal resistance
 
 
Is generally thought that heat tolerant cultivars would have a low stomatal resistance to allow greater transpiration and therefore cooling. A suitable root system is also needed to support this water loss.
 
Seasonal changes
 
Wehner et al. (1985) found that Adelphi Kentucky bluegrass heat tolerance, measured as recovery weight, increased from May through July and then decrease from August through October.
 
 
Heat tolerance for a given day was best predicted when based on mean daylength and low temperatures for the preceding 2 day period.
 
Annual bluegrass heat tolerance in the field was found to increase from mid-spring to late spring and then remain relatively high through late summer (Martin and Wehner, 1987).
 
 
Unlike Kentucky bluegrass, the best regression equation for annual bluegrass heat tolerance included mean daily maximum temperature for the 2 preceding days and mean total precipitation for the preceding 2 to 4 days.
 

Cultural management

Strategy - must have transpirational cooling

1) reduce N levels - no brainer

2) irrigation plus syringing to allow evaporative cooling

3) raise cutting height

4) reduce clipping frequency

5) increase air movement - fans

 
Summary
 
 
Warm-season species more tolerant than cool-season species. Generally, there is a direct relationship with drought tolerance.