Physiology of Turfgrass Drought Response. Daniel C. Bowman

Transcription

1 Physiology of Turfgrass Drought Response Daniel C. Bowman

2 Anyone who can solve the problem of water will be worthy of two Nobel prizes one for peace and one for science. John F. Kennedy

3 Turf from Space

4 Let s Examine n Why plants need water n How plants get water n How plants lose water n Drought n Differences between grasses n Where to focus for conservation

5 Rule #1 n There is no silver bullet, no magic elixir, and no high-tech turf that will survive without water.

6 Why Do Plants Need Water? n A small amount of water is needed to build new tissues n The vast majority of water the plant absorbs from the soil is actually lost as water vapor from the leaves, to the atmosphere, by the process of transpiration.

7 Transpiration occurs through pores in the leaf (stomates), and is crucial because it cools the leaf.

8 Plant Water n If not for transpirational cooling, a leaf in the sun could reach a lethal temperature of 120 o F during midsummer. Fortunately, transpiration keeps leaf temperatures much cooler, usually below 90 o.

9 Rule #2 n Turfgrass water use and drought response are tied to the soil environment. Thus, we have to know a little about soils

10 Water In The Soil n Once water has entered the soil, it fills the spaces between the soil particles (pores). Water continues to move downward, under the force of gravity, through the macropores. Eventually they drain completely, and are refilled with air. Micropores, on the other hand, retain water against the force of gravity. This water represents the reservoir for plant absorption

11 Water In The Soil n Micropores aren t all one, uniform size. They range from relatively large to very tiny. The force with which water is held in the pores is related to the pore size. Larger pores have just enough force to hold the water against gravity s force, but not enough force to resist the force of roots to obtain water. The small pores hold on to their water very tightly, much more tightly than the force of gravity, and often more tightly than the force of the root to extract water from the soil.

12 Available vs. unavailable water n This means that only some of the micropores give up their water to the plant. Some retain their water even though the plant may be wilting from drought.

13 Saturation n During and immediately after a rain or irrigation, all the soil pores, large and small, are filled with water. This is termed saturation, and is a very temporary situation in most soils since drainage due to gravity starts to suck water from the big pores almost immediately

14 Field Capacity n After drainage has removed the water from the macropores, the soil is at field capacity, with water occupying perhaps 30-35% of the total volume. This represents the maximum amount of plant available water, plus unavailable water. But the soil doesn t stay at field capacity very long.

15 Permanent Wilting Point n Absorption by roots starts to deplete the water from the large micropores, and the soil begins to dry out. Roots then go after water held in medium-sized micropores. After a while, the only water left is in the smallest micropores, which the plant can t get, and it starts to wilt. When the plant can no longer recover, even if irrigated, the soil is considered to be at the permanent wilting point, which may occur when soil water is around 10-15% of the total volume.

16 Available Water n The difference between field capacity and permanent wilting point is the amount of available water.

17 Soil Textural Class Affects How Much Water is Held by Soil Water Holding Capacity (inches per foot of soil) Soil Texture Total Unavailable Available Sand Sandy Loam Loam Silt Loam Clay Loam Clay

18 Available Soil Water n n As a rule of thumb, most well-watered soils (except sands) contain approximately inches of available water per foot of soil, and roughly the same amount of unavailable water A shallow root system is like a small cup, whereas a deep root system is like a tall glass. More water is held in the tall glass and more water is available to a deep root system

19 H 2 O Absorbed from a Loam

20 H 2 O Absorbed from a Sand

21 Plant Water Rule #1 n We all know that water runs downhill. It flows from a position of high energy (the top of the hill) to a position of lower energy (the bottom of the hill). Sometimes there aren t any actual hills involved, but water can still exist in high energy and low energy states. This is the case in the soil/root environment.

22 Water Moves from Soil into Root n Soil water in a moist soil is at a fairly high energy level. By contrast the water in the root is fairly low energy. Thus, there is a natural tendency for water to flow downhill from the soil and into the root. And that s just what the plant wants.

23 Water Moves from Higher to Lower Energy Shoot Air Well-Watered Conditions -6 bars Root Soil -4 bars

24 Water Moves from Roots to Leaves n The second rule about water is that it is sticky. It sticks to itself, which is called cohesion. Consider what happens when you suck water through a straw, even a real long one. The water is pulled up against the force of gravity, in a continuous column n This ability to pull long columns of water up, against gravity, is fundamental to getting water up a plant, from roots to leaves

25 The Stomatal Cavity H 2 O

26 Evaporation Pulls Water Up Through the Plant Evaporative Tug Air Well-Watered Conditions -6 bars Root Soil -4 bars

27 Stomates n Also function to let CO 2 in, so that photosynthesis can proceed. When the stomates close, photosynthesis slows to a crawl. This is the dilemma most plants face every day capturing energy at the expense of water, or conserving water at the expense of photosynthesis

28 Stomates Open CO 2 H 2 O

29 Drought Stomates Close CO 2 H 2 O

30 Warm vs. Cool Season Species n Under drought, both close stomates during the day to reduce water loss. This also limits the amount of CO 2 that is taken in and converted to sugars. However, warm season species have evolved different photosynthetic mechanisms to get around this. They are able to maintain good CO 2 conversion to sugars, even with the stomates closed. This gives them the advantage during the summer.

31 Drought Stress Response n Drought occurs when plant demand for water exceeds supply n Drought stress is the deviation from normality that the turf experiences n Drought response is how the turf copes with the stress

32 Drought Symptoms n Curling of leaves in some species n Gray or blue color develops n Footprinting n Wilting n Death

33 Short term vs. Long term Response n Many plants experience daily drought during summer afternoons, even with good soil moisture. It is a temporary condition and the plants fully recover during the night n But long term drought is a different situation. It involves drying soil and a more permanent change in the water status of the plant. The plant can adapt for a period of time, but eventually the stress exceeds the response.

34 Response n Some species have the ability to withstand drought stress. They do this by adjusting their cell contents to maintain turgor pressure or going dormant for survival. n Dormancy is a very effective strategy, but not all grasses are equally adept. Those with rhizomes are generally best, bunch grasses the worst. Tall fescue is in between.

35 Warm-season turfgrasses vary

36 Tall fescue drought recovery (day 56) 34 days 41 days 48 days 24 days 55 days

37 Differences Between Turfgrasses? n Warm season grasses use somewhat less water than cool season grasses under identical conditions. Also seasonal effects. n Most grasses use large amounts of water when it is plentiful differences aren t very important n Differences become more important under deficit irrigation, where water is supplied below the maximum requirement of the turf

38 Texas A&M Drought Study The SAWS Drought Study in 2006 evaluated 25 warm-season grasses on either a 4-inch soil depth or the native, unrestricted soil

39 28 days drought 39 days drought 46 days drought 28 days recovery

40 How Much Water Does Turf Use? n It depends primarily on the environment, and less so on the turf species, management practices, and soil moisture. Environmental factors that control water loss are: Temperature Wind speed Relative Humidity Light Intensity (Radiation)

41 ET Rates for Common Turfgrasses (compiled from various researchers) Range of Evapotranspiration Species (mm/day) (inches/week) Tall Fescue Perennial Ryegrass St. Augustinegrass Creeping Bentgrass Centipedegrass Bermudagrass Zoysiagrass Kentucky Bluegrass

42 Drought-tolerant Buffalograss 25% 40% n Uses ~60% as much water as tall fescue n Stays green with ~35% n n n Survives as dormant grass for a season Heat and cold tolerant So why isn t it used in NC?

43 How is ET Determined? n There are a number of methods used to estimate how much water a turf requires at any given time, under any given environment. The most common method uses weather data to calculate a standard value. This value is referred to as ETp, or potential evapotranspiration, and it is used as a starting point to estimate actual ET for any given species.

44 Automated Weather Stations Both measure: -Temperature - Wind - Rel. Humidity - Light

45 Estimating Turf ET from ETp n Reference ETp is used to calculate ET for any given turf. It is multiplied by a conversion factor which is specific for each turf. n The Crop Coefficient (Kc) for a given turf reflects the unique characteristics of that species. n ETp x Kc = estimated ET n Kc for tall fescue = 0.9 +/- n Kc for bermudagrass = 0.7 +/-

46 Atmometer n Another method is the Atmometer, a simple, inexpensive device that mimics a leaf canopy to estimate ET.

47 Soil Moisture Sensors Rainbird Water Watcher Irrometer Echo Acclima

48 Soil Moisture Sensors n New sensors that measure soil moisture are now available, and they look promising. They consist of buried rings or cables and have the advantage that soil moisture is measured and averaged over a much larger soil volume than with older sensor technologies.

49 Slit and Sensor

50

51 Taking a Reading

52 Irrigating Between the Lines Field Capacity Refill Level Perm. Wilt Pt.

53 Deficit Irrigation n Most turf species will use large amounts of water when it is available. However, most will survive on less. Intentionally irrigating with less than ET is termed deficit irrigation. It is not uncommon to irrigate a turf at 70-80% of estimated ET, or even lower where survival rather than appearance is the goal.

54 Focusing Conservation Efforts (turf doesn t waste water, people waste water) n n Obsessing about which grass uses less water is largely a waste of time. In the bigger picture, they re all pretty much the same* Water savings can be realized by: Efficient irrigation system design, installation, and maintenance Water audits and adjustments of existing systems Educating the end user, and encouraging them to use their systems properly Practicing deficit irrigation Deep cultivation and preparation of soils prior to planting Scaling back planted areas and expectations