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Carbohydrate metabolism in two apple genotypes that differ in malate accumulation
1Swiss Federal Research Station for Horticulture, Wädenswil CH-8820, Switzerland. j.berueter@bluwin.ch
Journal of Plant Physiology
|October 27, 2004
Summary
Apple malic acid content varies by genotype, impacting fruit metabolism. Low-acid apples accumulate less malate due to restricted cellular uptake, altering carbon partitioning towards carbohydrates like starch and sucrose.
Area of Science:
- Plant Physiology
- Biochemistry
- Fruit Metabolism
Background:
- The apple variety 'Usterapfel' exhibits two genotypes with distinct malic acid levels.
- Malic acid is a key organic acid in apple fruit, influencing taste and metabolism.
Purpose of the Study:
- To investigate the physiological and biochemical basis for differing malic acid content in apple fruit.
- To understand the impact of malic acid accumulation on carbon partitioning and carbohydrate metabolism.
Main Methods:
- Enzyme activity assays for malate metabolism enzymes (PEPcarboxylase, malate dehydrogenase, NADP malic enzyme).
- Radioisotope labeling ([14C]glucose and [14C]malate) to trace metabolic pathways and uptake.
- Analysis of organic acid, ATP, hexosephosphate, and carbohydrate (starch, hemicellulose, sucrose) concentrations.
Main Results:
- Low-acid (LA) apples showed significantly lower malate accumulation, not due to synthesis differences but restricted malate uptake into parenchyma cells.
- Carbon partitioning differed: high-acid (HA) apples compensated for vacuolar malate storage with increased glycolytic flux; LA apples directed carbon towards gluconeogenesis and carbohydrate synthesis (starch, sucrose).
- LA apples exhibited higher PEPcarboxykinase activity, increased ATP and hexosephosphate levels, and enhanced activities of enzymes involved in carbohydrate synthesis and glucose metabolism.
Conclusions:
- Reduced malate accumulation in LA apples is primarily due to impaired malate transport/storage in vacuoles.
- This difference significantly alters carbon flow, favoring carbohydrate synthesis in LA apples.
- Phosphoglucomutase activity plays a regulatory role in partitioning carbon between malate and carbohydrate polymers.
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