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What causes sugar ends in potatoes?

Published in Potato Quality Defects 3 mins read

Sugar ends in potatoes are primarily caused by environmental stress, particularly heat or moisture stress, experienced by the potato plant during critical stages of tuber development.

Understanding Sugar Ends

Sugar ends are a quality defect in potatoes, especially noticeable after frying. They appear as dark, often translucent, discolored areas at the stem end (proximal end) of the potato. This discoloration results from an excessive accumulation of reducing sugars (glucose and fructose) in this part of the tuber. When these sugars are subjected to high heat during processing (like French fry production or chip making), they caramelize rapidly, leading to undesirable dark spots and a sweet, sometimes burnt, taste.

The Root Cause: Stress During Tuber Development

The most significant factor contributing to sugar end development is the plant's susceptibility to heat or moisture stress during key growth phases.

  • Critical Periods of Sensitivity: Tubers are particularly vulnerable to these stresses during two crucial stages:

    • Tuber Initiation: This is when the stolons (underground stems) begin to swell and form small tubers.
    • Bulking Phase: This is the period of rapid tuber growth and starch accumulation. The reference specifically highlights that tubers are "particularly sensitive to stress during the early bulking phase."
  • Mechanism of Stress Impact:
    When potato plants undergo stress, especially during these sensitive periods, their metabolic processes are disrupted. The plant's ability to convert sugars into starch within the tuber is impaired. Instead of efficiently storing energy as starch throughout the tuber, sugars accumulate, particularly at the stem end. This accumulation of reducing sugars is the direct precursor to sugar end formation.

Factors Increasing Susceptibility

Several cultural or management practices can inadvertently increase a potato plant's susceptibility to these detrimental stresses:

  • Inadequate or Inconsistent Irrigation:

    • Drought Stress: Insufficient water supply, especially during tuber initiation and early bulking, is a major contributor.
    • Waterlogging (Excessive Moisture): While less common than drought, overly wet soil conditions can also stress the plant by limiting oxygen to roots and disrupting nutrient uptake.
  • High Air and Soil Temperatures: Prolonged periods of excessively high temperatures, particularly during the critical tuber development stages, put immense stress on the potato plant, directly impeding starch synthesis.

  • Nutrient Imbalances: Improper fertilization, especially deficiencies or excesses of certain nutrients (e.g., potassium), can weaken the plant's resilience to environmental stresses.

  • Soil Compaction: Poor soil structure can hinder root development and water/nutrient absorption, indirectly increasing stress susceptibility.

Preventing Sugar Ends

Managing potato crops to minimize sugar end defects involves proactive strategies focused on reducing environmental stress:

  • Optimized Irrigation Management: Implementing precise irrigation schedules to ensure consistent moisture availability, avoiding both drought and waterlogged conditions, particularly during tuber initiation and early bulking.
  • Variety Selection: Choosing potato varieties known for their heat tolerance and resistance to sugar end formation in the local climate.
  • Proper Planting Timing: Adjusting planting dates to avoid peak heat periods during critical tuber development stages.
  • Soil Health Maintenance: Improving soil structure, organic matter content, and drainage to create a more stable environment for root growth and water uptake.
  • Balanced Nutrition: Ensuring adequate and balanced nutrient supply to enhance plant vigor and stress tolerance.

Understanding the direct link between environmental stress, especially heat and moisture stress during specific growth stages, and sugar end development is crucial for growers aiming to produce high-quality potatoes.