temperatures greater than 30ºC and can be lethal above 45ºC. Fortunately, even with global warming, the UK is unlikely to experience temperatures at the upper end of this range. But, it is not just the severe temperatures that can be injurious. Prolonged periods in which temperatures are above the optimum range can cause significant damage. Plants exposed to heat stress show various physiological and metabolic responses. Photosynthesis captures light energy and carbon dioxide to manufacture carbohydrates. These are used to produce new leaves, roots, stems, stolons and rhizomes. Simultaneously, respiration consumes some of the carbohydrates in generating energy to support the growth and development of the plant parts. When temperatures are within the optimal range, photosynthesis proceeds at a greater rate than respiration and the plant can accumulate carbohydrate reserves. However, under high temperatures, the rate of photosynthesis decreases whilst the respiration rate increases. This leads to a depletion of carbohydrate reserves and inhibition to growth. The situation is especially marked at low mowing heights where leaves are removed and, consequently, the plant's photosynthetic potential is diminished. The decline in carbohydrate availability resulting from heat stress causes a decrease in root growth and tillering.
Species and Cultivar Differences
Heat tolerance varies amongst species and cultivars. For example, during prolonged periods when daytime temperatures are 35ºC or higher, perennial ryegrass is more resistant than fine fescues. At these temperatures, the quality of fine fescue turf quickly declines, whilst ryegrass and smooth-stalked meadowgrass are more persistent.
Annual ecotypes of annual meadowgrass avoid heat stress by completing their life cycle in late spring, having produced large quantities of seed to perpetuate the species. Perennial ecotypes, on the other hand, display a range of tolerances with some clones being more tolerant than others. Some, in fact,
can
be as tolerant as browntop bent and Chewing's fescue. As temperatures rise to 35ºC and beyond, there is a decrease in the synthesis of normal proteins and an increase in protein degradation. Fortunately however, the production of other proteins increases. These are called heat shock proteins (HSP) and are rapidly synthesised in response to temperatures reaching about 35ºC. Some of these HSPs can be synthesised within minutes of the temperature reaching such a chronic level. Production of HSPs may offer an explanation for differences in heat tolerance among cultivars of creeping bentgrass and other species. Rooting depth can also influence heat tolerance of species and cultivars. Exploitation of soil moisture deep in the soil profile can maintain a continuous water supply for transpiration. The cooling effect of transpiration can keep leaves 3 to 10ºC cooler than the air temperature. However, stomata (i.e. pores on the leaf surfaces) must be open for cooling to occur and nutrient uptake may have a significant role to play in this process. High potassium levels in leaves and
roots will facilitate stomatal opening and water uptake by roots, thus increasing transpirational cooling and avoiding heat stress. Heat stress can be particularly damaging on sunny days, with no wind, and inadequate soil water to meet transpirational needs. Decline in turfgrass quality under these conditions is most noticeable on south-facing slopes and localised dry patches. On occasions in the UK, such areas can reach temperatures in excess of 45ºC and can be lethal. Under similar weather conditions, but where there is adequate soil moisture, turfgrass canopy temperatures can be 7 to 10ºC higher than the ambient air temperature. A slight breeze of about 8 kph (5 mph) can reduce the canopy temperature by 4 to 7ºC. Hence the need to ensure adequate air flow over the surface of the turf. Tree and shrub removal may be required around sheltered golf greens to preserve adequate air movement. Within many enclosed soccer stadiums, being concrete and steel structures and lacking any air movement, very high temperatures can develop. In these cases, installation of mechanical fans could improve air circulation and reduce leaf canopy temperatures.
Soil Temperatures
Soil temperature has been found to be more critical than air temperature in influencing
growth. Whilst roots are tolerant of soil temperatures below the optimum range of 10ºC to 18ºC and will continue to grow until temperatures drop to 0ºC, they are less tolerant of temperatures above the optimal range. Once soil temperatures rise above 21ºC, roots are adversely affected. At 27ºC root initiation ceases and at temperatures higher than 30ºC root growth will stop and they begin to lose their ability to function. Above 35ºC, root hairs die, roots turn brown and become dysfunctional. If steps are not taken to moderate soil temperatures before this stage is reached, rapid deterioration of the turf occurs. This was clearly evident on many occasions during the summer of 2006, often to the embarrassment of the turf managers.
The methods by which roots respond to high soil temperatures and regulate shoot growth are not fully understood but are thought to be linked to a decline in cytokinin production in the roots. Cytokinin is a group of plant hormones that play an important role in preventing yellowing and death of leaves during hot weather. Applications of cytokinin to the root system can reduce leaf senescence and enhance heat resistance. Some seaweed (generally kelp) and humic acid extracts contain cytokinin and may help improve drought and heat resistance.
Pre-conditioning the turf to heat stress
Turfgrass management practices prior to the onset of high temperatures can greatly influence the turf’s survival capacity. A primary aim of management must be to maximise rooting depth. Ensuring adequate soil drainage and aeration is a vital contribution to rooting. Wet or compacted soils lack sufficient oxygen for turfgrass root respiration and can result in significant root loss during periods of high soil temperatures. Microbial respiration consumes a large proportion of the soil oxygen when temperatures are high. And wet or compacted soils can very quickly become anaerobic. Organic matter management is also crucial to ensure adequate soil aeration, maximise infiltration rates, and avoid saturation at the soil surface. Coring, scarifying and light, frequent topdressing are essential practices of turfgrass management. Fertiliser applications will also influence the heat tolerance of turfgrasses. As discussed earlier, potassium plays an important role in regulating stomatal opening. Springtime applications will enhance drought and heat resistance. On the
HEAT IS ON
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