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Understanding Drought Stress in Kentucky Bluegrass: Insights for Turf Professionals

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Kentucky bluegrass is a popular choice for golf courses due to its lush appearance and resilience. However, prolonged periods of drought can challenge even the hardiest of grass varieties. A 2019 study by Zhang et al. from Virginia Tech sheds light on how drought stress affects Kentucky bluegrass, providing valuable insights for golf course superintendents and turf professionals.



Research Overview

Scientists conducted an in-depth study to understand Kentucky bluegrass's physiological and hormonal changes under drought conditions. The study involved two groups of grass: one group was well-watered, while the other experienced drought stress. Over a period of several weeks, the researchers measured various indicators of grass health.


Key Findings


Turf Health Indicators

Drought stress significantly impacts Kentucky bluegrass's visual quality and physiological functions. Key indicators such as turf quality, electrolyte leakage, chlorophyll content, photosynthetic rate, and stomatal conductance showed marked differences between well-watered and drought-stressed grass.



Turf Quality

  • Initially, both well-watered and drought-stressed grass maintained similar appearances. However, within a week, the drought-stressed grass began to deteriorate.

  • By the end of the study, the drought-stressed grass exhibited significantly lower turf quality compared to the well-watered grass.

Plant Health

  • Electrolyte Leakage (EL): Increased in drought-stressed grass, indicating higher cell membrane damage. This was a clear sign of stress response at the cellular level.

  • Chlorophyll Content: Chlorophyll levels, essential for photosynthesis, dropped significantly in the drought-stressed grass.




Plant Activity

  • Photosynthetic Rate (Pn): The photosynthetic rate dropped in drought-stressed grass as stomata closed to conserve water, limiting carbon dioxide intake necessary for photosynthesis.

  • Stomatal Conductance (gs): Stomatal conductance, which measures the exchange of gases through the stomata, also decreased significantly, further impacting the grass's ability to photosynthesize effectively.


Hormonal Changes

The hormonal balance in Kentucky bluegrass shifts significantly under drought stress, influencing the plant’s ability to cope with adverse conditions. Increased levels of stress hormones and decreased growth hormones highlight the grass’s physiological adaptations to drought.




Abscisic Acid (ABA)

  • ABA levels increased under drought conditions. This hormone helps the plant manage stress by closing stomata to conserve water. However, this also limits carbon dioxide intake, reducing photosynthesis efficiency.

Cytokinins (ZR and iPA) and Auxins (IAA):

  • These growth-promoting hormones decreased during drought. Cytokinins and auxins are vital for cell division, growth, and root development. Their reduction hampers the grass's ability to maintain its health and recover from stress.

Gibberellin A4 (GA4):

  • Levels do not show significant changes under drought stress, indicating that GA4 is not as responsive to drought conditions in this context.




Correlation Data

The study's findings are summarized in Table 1, showing the correlations between various aspects of turf health and hormone levels.

Table 1 Highlights:

Impact of Drought on Turf Quality:

  • Drought stress significantly reduces turf quality in Kentucky bluegrass. This decline is associated with reduced photosynthetic activity, lower chlorophyll content, and increased cell membrane damage as indicated by higher electrolyte leakage.

Hormonal Responses to Drought:

  • Drought stress leads to increased levels of abscisic acid (ABA), a hormone associated with stress responses. In contrast, levels of growth-promoting hormones such as cytokinins (ZR and iPA) and indole-3-acetic acid (IAA) decrease under drought conditions.

  • The ratio of ABA to cytokinins (ABA/CK) increases significantly during drought, indicating a shift towards stress response mechanisms at the expense of growth processes.

Photosynthetic Rate and Stomatal Conductance:

  • The reduction in photosynthetic rate and stomatal conductance under drought stress is linked to the hormonal changes, particularly the increase in ABA, which promotes stomatal closure to conserve water but also limits CO2 uptake for photosynthesis.

Role of Cytokinins and Auxins:

  • Higher levels of cytokinins and auxins (IAA) are associated with better drought tolerance, as these hormones support growth and delay senescence. The decline in these hormones under drought stress correlates with reduced turf quality and physiological function.


Conclusion

The impact of drought on Kentucky bluegrass is multifaceted, involving complex hormonal interactions that affect turf quality, plant health, and photosynthetic efficiency. Understanding the role of hormones in drought response provides a pathway to more sustainable and effective turf management strategies in the face of increasing environmental stresses.


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