Feeling the Heat
Decline in turf quality for creeping bentgrass during summer is a common problem on many golf courses, especially in warm climatic regions. One of the primary causes of summer bentgrass decline is high soil temperature. The optimum soil temperature is 10°C to 18°C for the growth of cool-season turfgrasses. However, soil temperatures often reach above 25°C during summer months in many areas. High soil temperature is more detrimental than high air temperature in causing growth and physiological inhibition of shoots and roots in creeping bentgrass. Summer bentgrass quality decline under high soil temperature conditions is largely because of limited root growth and root death during summer. Root growth is more sensitive to high temperature than shoots because of their low optimum temperature requirement, and therefore root growth tends to decline before shoot growth is affected. Our studies found that decreased root biomass and increased root mortality occurred prior to decline in turf quality and shoot growth in response to high soil temperature, suggesting that the adverse effects of high soil temperature on shoot growth could be initiated by the decline in root growth and viability. Root growth inhibition reduce the supply of water, nutrients and root-produced hormones such as cytokinins to the shoots, which is probably a major factor leading to the decline in turf quality.
Root growth is positively associated with the performance of creeping bentgrass during summer because an extensive root system facilitates water uptake, increases transpirational cooling and promotes heat avoidance. In a 2001 study, we found that heat-tolerant ‘L-93’ bentgrass maintained a higher root-to-tiller number ratio than Penncross, suggesting that L-93 had more roots to provide water, nutrients and hormones to support each individual tiller. This suggests that maintaining a large root system relative to shoots would help bentgrass plants survive heat stress. Healthy roots are essential for maintaining high-quality turfgrass during the summer months.
HIGH TEMPERATURE AND BENTGRASS ROOT GROWTH
Root growth of creeping bentgrass exhibits seasonal patterns, with slowing root growth and weakening rooting ability during summer. Summer root decline of creeping bentgrass is associated with decreased new root production and increased root mortality, but to a greater extent with increased root mortality (see Figure 1 and Figure 2 on page 40). Root mortality of creeping bentgrass ranges from 40 to 60 percent during midsummer, which means about half of the roots produced prior to the summer die during the hot periods. However, limited numbers of new roots are still produced during summer months. New root production does not cease during summer, but is minimal during warmest periods (see Figure 1 on page 40). Midsummer decline of root production and increased root mortality are closely related to increases in soil temperatures.
Root death under high temperature is believed to be associated with carbohydrate starvation among other physiological changes. High soil temperatures generally increase the respiration rate in roots, which might cause carbohydrate starvation and root dieback. Our recent studies have shown that the increase in root respiration in creeping bentgrass was mainly because of increases in maintenance respiration with increasing temperatures, which may result in insufficient carbohydrate levels needed for growth respiration and, thus, decreased root growth under high temperature conditions. Carbon allocation to roots also decreases under heat stress. Carbohydrate content in roots decreases more in roots than in shoots of creeping bentgrass when exposed to high temperatures. These results suggested that roots may have lower priority than shoots when carbohydrates become limited during heat stress. Limited carbon allocated to roots and low carbohydrate storage in roots may result in the limited growth and death of roots that occur under high temperature conditions in creeping bentgrass.
ROOT FACTORS ASSOCIATED WITH TURF QUALITY DECLINE
The death of roots initiates the decline of shoot growth, which might result from high temperature-labile processes in the roots. Among the three major functions of roots for turfgrasses, hormone synthesis may be the most sensitive, although water and nutrient uptake are also limited. We recently reported that cytokinin content of creeping bentgrass showed the most severe and rapid decline in response to high soil temperature at the earliest time of the stress period, followed by decreases in nutrient content, and water content. Cytokinins are predominantly produced in roots, and are involved in shoot and leaf formation, cell division, chlorophyll biosynthesis and leaf senescence. The early dysfunction of cytokinin metabolism may be a major factor leading to shoot growth inhibition and leaf senescence under high soil temperature conditions. Our earlier studies found that cytokinin levels in both leaves and roots of creeping bentgrass decreased at high soil temperature, but increased when cytokinin was applied to the root zone. Applying cytokinin (10 µmol) to the root zone improved turf quality of plants exposed to high soil temperatures (95 °F) to the level similar to that exposed to optimum temperatures. These results suggest that the inhibition of cytokinin biosynthesis in roots could contribute to decline in shoot and root growth, and cytokinin application to the root-zone could help alleviate summer bentgrass decline.
Nutrient uptake of roots is also limited under heat stress. Our recent studies showed that among the three major nutrient elements (nitrogen-N, potassium-K, and phosphorus-P), K accumulation was the most sensitive to changes in root temperature compared to N and P, and high soil temperatures reduced K content earlier and more severely than for N and P contents. Low K content may lead to stomatal closure, and thus result in decreased photosynthetic rate and reduced transpirational cooling under heat stress. Application of K significantly alleviates heat injury in creeping bentgrass.
Decline in leaf water content has been observed with increasing soil temperature, but does not reach to the physiologically detrimental level. When roots of creeping bentgrass were exposed to high soil temperature (95 °F), no leaf wilting was observed within 30 days of heat stress and only slight wilting was observed at 40 days of stress. Our results suggest that water uptake of roots may also be inhibited under heat stress, but water deficit may not be a major factor involved in heat injury in creeping bentgrass.
PROMOTING HEALTHY ROOTS
Development of deep, extensive root systems (big root mass or length) is often emphasized as an important factor in plant growth and adaptation to environmental stresses. Persistent production of new roots and maintenance of low root mortality could contribute to creeping bentgrass adaptation to summer stress. As discussed above, high soil temperature initially inhibits root growth and then shoot growth decline. Root growth decline or death under high temperature conditions could contribute to limited carbohydrate supply under heat stress. Decline in shoot and root growth of creeping bentgrass induced by high soil temperatures is related to the limitation of water and nutrient status, as well as the interruption of cytokinin metabolism of the root system. However, cytokinin metabolism in roots is most sensitive to high soil temperature, followed by the changes in nutrient and water content.
Based on the physiological causes of root growth decline and factors affecting root functions, several cultural practices could be developed to prevent or alleviate heat stress injury in creeping bentgrass:
Keep mowing height at highest level allowed. It is generally accepted that high mowing promotes carbohydrate production in leaves and increases carbohydrate supply to roots. Maintaining higher cutting increases root longevity during summer months.
Lowering soil temperature to reduce root respiratory consumption and increase carbohydrate availability in roots. When soil temperature was lowered to 75°F, creeping bentgrass cultivars grown at high air temperature (95°F) maintained the same turf quality as those grown under optimum air and soil temperature conditions. The results clearly demonstrated that reducing soil temperature is an effective means to prevent summer bentgrass decline. Studies have shown that the combined use of fans and syringing (cooling via application of a light spray of water) is effective in reducing soil temperature to below the injurious level. Soil cooling with fan and syringing simultaneously is more effective than using fan or syringing alone. Soil cooling could also be achieved using sub-surface air cooling.
Manipulation of cytokinin metabolism of roots could prevent further heat injuries in shoots or improve heat tolerance for creeping bentgrass. This could be achieved by exogenous application of cytokinins or through modification of endogenous level of cytokinins. Applying cytokinin (10 µmol) to the root zone has been found to be effective in improving root growth and turf quality of creeping bentgrass exposed to high soil temperatures.
Increasing N, P and K fertilization, particularly K, may alleviate heat stress injury in creeping bentgrass. Seasonal growth habits of roots should be a consideration in developing effective turfgrass management practices, particularly the timing of fertilization that is critical in the culture of creeping bentgrass.
Bingru Huang is an associate professor with the Department of Plant Biology and Pathology at Rutgers University (New Brunswick, N.J.).
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