TUBER PHYSIOLOGY IMPORTANCE OF THE TUBER PERIDERM. Tuber Physiology & Physiological Diseases. Anatomy of a Tuber. Tuber Periderm

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1 Tuber Physiology & Physiological Diseases Potato Post Harvest Management TUBER PHYSIOLOGY EYE Anatomy of a Tuber BUD END SKIN / PERIDERM CORTEX VASCULAR RING PERIMEDULLARY ZONE MEDULLARY RAYS branch to each eye or bud LENTICELS PITH PHELLEM (CORK) PHELLOGEN (CORK CAMBIUM) PHELODERM SCALE LEAF BUD IMPORTANCE OF THE TUBER PERIDERM STEM END APICAL BUD TUBER Tuber Periderm An intact skin (a.k.a. periderm) represents an effective barrier Reduced water loss Reduced respiration Resistance to infection Tuber Periderm Proper top killing prior to harvest is critical to triggering skin set Careful handling during all harvest and postharvest processes can reduce skinning/bruising/wounding 1

2 Tuber Periderm Wound Healing Damage that occurs during harvest/post harvest processes will heal 1. Tissues are Injured BROKEN / CRUSHED LAYER OF CELLS Damaged tissues (cuts, bruises and abrasions) will start to form wound tissues very rapidly Wound tissue formation follows a set process STORAGE CELLS INTERCELLULAR SPACES 2. Intercellular Bridging (1 8 hours) (1 14 days) LIGNIN/PECTIN PRODUCED TO FORM CROSS LINKS IN INTERCELLULAR SPACES BELOW Temporary barrier to inhibit entry of pathogens SUBERIN 2

3 Inhibits H2O loss and fungal infection PHELLOGEN LAYER = LONG RECTANGULAR CELLS FORMED SUBERIZED LAYER 4. Formation of Phellogen Layer (3 21 days) Tuber Temperature ( C) Rate of Suberization Initial Suberization (Days) Periderm Complete (Days) < transfer/harvest storage/ Wound Healing RATE OF HEALING INFLUENCED BY: TEMPERATURE OPTIMUM = 15 18C SLOWER AT LOWER TEMPERATURES RELATIVE HUMIDITY BETWEEN 70 AND 95% PHYSIOLOGICAL AGE OF TUBER YOUNGER = FASTER AIR MUST BE WELL VENTILATED RESPIRATION 3

4 Respiration Conversion of starch into H 2 O, CO 2 and heat Results in reduction of dry matter content and weight loss RELATIVE RATE OF RESPIRATION HARVEST DURING STORAGE ONCE SPROUTING BEGINS HIGH DECREASES INCREASES APPROXIMATE DRY MATTER LOSSES/MONTH 1 2% 0.8% 1 5% Factors Affecting Tuber Respiration FACTORS AFFECTING RATE OF RESPIRATION PHYSIOLOGICAL AGE SPROUTING VS. DORMANCY SKIN DAMAGE / HEALING Increases with age Maturity influences skin permeability Lowest during dormancy Cutting doubles respiration rate Significant bruising increases rate by 30 50% Factors Affecting Tuber Respiration FACTORS AFFECTING RATE OF RESPIRATION SKIN PERMEABILITY STORAGE TEMPERATURE Immature tubers have highly permeable skins compared to physiologically mature tubers Rates increase as temperature increases (2x for every 10C) Increases below 5C WATER LOSS TEMPERATURE Water Loss FACTORS AFFECTING WATER LOSS RELATIVE HUMIDITY Cooler = slower Big differences between tubers and air = changes rate of loss Bigger difference between tubers and surrounding air affects rate of loss RATE OF AIR MOVEMENT AROUND TUBERS PERMEABILITY OF THE SKIN Water Loss FACTORS AFFECTING WATER LOSS Greater rate of air movement = increased moisture loss Immature tubers lose water x faster than mature tubers (with a less permeable skin) 4

5 FACTORS AFFECTING DISEASE TOLERANCE OF STORED TUBERS Factors Affecting Disease Tolerance in Stored Tubers Skin set proper top killing = critical Maturity of harvested tubers Duration of time in field after top killing Influences potential for infection Tuber temperature at harvest Influences bruising/wounding Factors Affecting Disease Tolerance in Stored Tubers Damage minimize during harvest and postharvest handling Wound healing proper curing Storage temperature Lower temperature slows disease development Ventilation Influences humidity, temperature (hot spots), etc. PHYSIOLOGICAL DISEASES (A.K.A PHYSIOLOGIAL CONDITIONS THAT AFFECT TUBER STORABILITY AND MARKETABILITY) Enlarged Lenticels Cause = tubers exposed to very wet conditions (in field or storage) Oxygen deprivation Tissues below lenticels (pores/stomates) swell and burst through the lenticel Results in a corky mass around the lenticel Enlarged Lenticels Impact: Increases susceptibility to pathogens Reduced marketability 5

6 Low Temperature Injury Occurs in storage or field Wide range of injury Frozen tubers to chilling injury Symptoms: Grey/brown patches of tissue Brown discoloration of the vascular ring Dark coloured processed potatoes; grey boiled potatoes Freezing / Chilling Injury Symptoms variable depending on the temperature, exposure period, and potato cultivar Externally Tuber appears wrinkled and flabby Internally Tissues turn bluish grey to black Chilling / Freezing Injury Chilling damage Occurs at temperatures below 38 F Appears as diffuse smoky grey areas inside the tubers Freezing injury Occurs at temperatures below 29 F Tissues are soft and watery. Tissues tend to disintegrate and eventually dry out Bruising, Cracking, Malformations Blackspot Bruising Individual cells are ruptured below the skin without breaking the skin Bruises develop within a couple of days Are only visible when potatoes are peeled Bruising, Cracking, Malformations Shatter Bruising Thin cracks or splits occur in the flesh of the tuber, particularly in larger tubers Thumbnail cracks Occur when cold tubers are handled in storage A form of shatter bruising Bruising/Cracking/Malformations Pressure Bruising Develops in storage Flattened or depressed area on tuber Usually results from tuber dehydration (H 2 O loss) Low soil moisture before harvest Inadequate humidification of ventilation air in storage 6

7 Wet or Muddy Tubers Extra moisture and soil to deal with Drying Washing Etc. Can contribute to rots in storage Black heart Insufficient oxygen for tubers during growth or storage Causes: Too high or too low temperatures in storage result in blackheart development Tubers harvested from low, wet areas Internal Sprouting Sprouts grow into the tuber, with small tubers formed internally Causes: Storage at high temperatures (above 16 C) causes physiological aging Low concentrations of sprout inhibitors Other Physiological Disorders Black heart Brown centre/hollow heart Internal brown spot/heat necrosis Stem end/vascular discoloration QUESTIONS??? Rob Spencer, BSA, MSc, P.Ag. Commercial Horticulture Specialist Alberta Ag Info Centre 310 FARM 7