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HEAT AND LIGHT STRESS/SUNBURN

Schrader, L.

[email protected]

Professor Emeritus, Washington State University, Wenatchee, WA

KEYWORDS Apple; skin disorders; sunburn necrosis; sunburn browning; photo-oxidative sunburn; fruit surface temperature (FST); Fuji stain; lenticel marking; sunburn scald in Granny Smith; water core in Honeycrisp ABSTRACT Several fruit skin disorders are induced by heat and/or light stress in apples (Malus x domestica Borkh.). Some disorders appear on fruit prior to harvest, whereas others do not appear until after harvest and cold storage. Fruit do not utilize much light energy, so excess light energy is converted to heat injury. On a hot day, the fruit surface temperature (FST) of the sun-exposed side of apple can be 20 to 30 °F above air temperature. Sunburn is usually the major cause of cullage. It appears before harvest and its incidence often provides an early signal that other disorders will appear later. Three types of apple sunburn have been characterized (Figure 1). One type (sunburn

necrosis) is caused by heat alone. A high FST of ~126 °F results in thermal death followed by necrosis. A second type (sunburn browning) occurs with high FST (115 to 120 °F) and damaging UV-B radiation. A third type (photooxidative sunburn) appears to be caused by visible light alone and occurs on green peel (non-acclimated to light) that is suddenly exposed to full sunlight so that photobleaching occurs first, followed by necrosis. This third type results from photooxidative damage and can occur at much lower FST and without UV-B radiation. Another disorder that results from heat and light

Necrosis Browning Type 3

Figure 1: Three types of fruit skin disorders induced by heat and/or light stress

stress is lenticel marking. Its incidence increases in fruit that have more severe sunburn. Fuji stain is a skin disorder that appears only after a period of cold storage. This stain disorder appears primarily in sunburned fruit, and its incidence rises sharply as the severity of sunburn increases. We have evidence that UV-B radiation is involved; incidence of stain is higher in orchards with excess Nitrogen. Another disorder that is enhanced by heat stress is bitter pit (especially in Jonagold). Although Calcium deficiency is reported to cause bitter pit, we observed increased bitter pit in Jonagold apples that were exposed to high temperatures and water stress as they neared maturity. Color of the peel did not develop normally, but had a blotchy appearance. High temperatures near maturity also increased water core in ‘Honeycrisp’. This physiological disorder is associated with internal moisture stress, and high temperatures cause premature conversion of starch to sugar and pronounced leakage from cells into intercellular spaces. Sunburn scald in ‘Granny Smith’ develops during cold storage on the sun-exposed side of fruit that showed sunburn browning earlier in the season. If sunburn browning is prevented, the skin disorders listed above seldom appear. This suggests that management practices are needed to reduce absorption of UV-B radiation and to keep FST below critical temperatures at which sunburn browning occurs. Such practices include overhead evaporative cooling; sunscreen such as RAYNOX; sun blockers such as Eclipse or Surround WP; and photo-selective netting.

DR. LARRY SCHRADER

PROFESSOR EMERITUS

WSU TREE FRUIT RESEARCH &

EXTENSION CENTER

WENATCHEE, WA USA

HEAT AND LIGHT STRESS/SUNBURN

WSU FRUIT SCHOOL, WENATCHEE, WA

NOVEMBER 18, 2015

Schrader-2015©

MANY DISORDERS IN APPLES ARE

INDUCED BY HEAT AND/OR LIGHT

STRESS.

SOME DISORDERS WILL APPEAR

BEFORE HARVEST AND OTHERS APPEAR

LATER IN COLD STORAGE

DEFINE STRESS ENVIRONMENT

INTRODUCTION

Schrader-2015©

1. MAX. AIR TEMP OF 95 TO 109 ºF

2. HIGH SOLAR RADIATION—PFD >2200 µmol

m-2 s-1

3. RELATIVE HUMIDITY OF <20%

4. LITTLE OR NO WIND

RESULT OF THIS ENVIRONMENT:

HIGH FRUIT SURFACE TEMPERATURE

(FST) THAT INDUCES SEVERAL FRUIT

SKIN DISORDERSSCHRADER-2015©

IF SOLAR (LIGHT) ENERGY IS NOT USED,

THE EXCESS ENERGY IS CONVERTED TO

HEAT ENERGY.

LEAVES USE LIGHT ENERGY FOR

PHOTOSYNTHESIS ETC., AND CAN

DISSIPATE EXCESS ENERGY THROUGH

TRANSPIRATION. LEAF TEMPERATURE =

AIR TEMPERATURE

WHY DOES FRUIT SURFACE

TEMPERATURE (FST) INCREASE IN THIS

STRESS ENVIRONMENT?

SCHRADER-2015©

FRUIT, HOWEVER, USE LITTLE ENERGY

FOR PHOTOSYNTHESIS, AND POSSESS

LITTLE ABILITY FOR DISSIPATING EXCESS

ENERGY.

THUS, HEAT ENERGY ACCRUES AND

INCREASES FST TO TEMPERATURE AT

WHICH A SKIN DISORDER IS INDUCED.

SCHRADER-2015©

SCHRADER-2015©

25

30

35

40

45

50

55Te

mp

era

ture

(oC

)

No EC FST Air T

104 °F

95 °F

122 °F

113 °F

SCHRADER-2015©

APPLE SKIN DISORDERS

SEVERAL SKIN DISORDERS CAUSED BY

LIGHT AND/OR HEAT STRESS

SOME DISORDERS APPEAR BEFORE

HARVEST & SOME APPEAR LATER

1. SUNBURN

2. LENTICEL MARKING

APPEAR BEFORE HARVEST:

SCHRADER-2015©

3. ‘HONEYCRISP’

WATERCORE

4. ‘JONAGOLD’

BITTER PIT

DISORDERS

ENHANCED BY

HEAT STRESS

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5. ‘FUJI’ STAIN 6. “SUNBURN SCALD”

OF ‘GRANNY SMITH’

DISORDERS APPEAR AFTER COLD STORAGE

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SUNBURN USUALLY #1 SOURCE OF

CULLAGE—OFTEN AT LEAST 10% OF

CROP LOST WITHOUT PROTECTANTS

APPLES IN WASHINGTON STATE VALUED

AT $2 BILLION (U.S.); SO 10% LOSS COSTS

GROWERS ~$200 MILLION/YEAR

CONFUSION EXISTS ABOUT

TYPES AND CAUSES OF

SUNBURN IN FRUITS—MANY

HAVE USED DETACHED

FRUITS TO STUDY SUNBURN

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SUNBURN AS INDICATOR

SUNBURN DAMAGE CAN SERVE AS

INDICATOR THAT OTHER HEAT AND LIGHT-

INDUCED DISORDERS WILL APPEAR

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1. SUNBURN NECROSIS

WE IDENTIFIED AND CHARACTERIZED THREE

TYPES OF SUNBURN & THEIR CAUSES:

2. SUNBURN BROWNING

3. PHOTOOXIDATIVE SUNBURN

SCHRADER-2015©

WHAT CAUSES SUNBURN NECROSIS?

SUNBURN NECROSIS OCCURS WHEN FRUIT

SURFACE TEMPERATURE (FST) REACHES

~126 ºF ± 2 ºF FOR ONLY 10 MIN UNDER WASH-

INGTON STATE ENVIRONMENTS.

THERMAL DEATH OCCURS;

CELL MEMBRANES BECOME

LEAKY; & NECROSIS APPEARS.

SCHRADER-2015©

SUNBURN BROWNING

SUNLIGHT REQUIRED.

BLOCKING DAMAGING UV-B

SUNBURN BROWNING OCCURS AT FST OF 115

TO 120 ºF (VARIETY-DEPENDENT) IN WASH.

STATE ENVIRONMENTS. UV-B RADIATION ALSO

INVOLVED. PINK LADY MOST TOLERANT;

CAMEO AND HONEYCRISP MOST SUSCEPTIBLE

RADIATION & REDUCING FRUIT SURFACE

TEMP. (FST) DECREASES SUNBURN.

SO TWO CAUSAL FACTORS: HIGH FST;

DAMAGING UV-B RAYSSCHRADER-2015©

Sb-0 Sb-1 Sb-2 Sb-3 Sb-4 Sb-5

NONE NECROSIS

1Schrader and McFerson System

SIX CLASSES OF SUNBURN1

SCHRADER-2015©

PHOTOOXIDATIVE SUNBURN

• OCCURS ON “NON-ACCLIMATED” APPLES

SUDDENLY EXPOSED TO FULL SUNLIGHT

• INITIAL DAMAGE—BLEACHING OR WHITENING OF

APPLE SURFACE (PEEL) APPEARS WITHIN 24 HR

OF EXPOSURE TO FULL SUNLIGHT.

• BLEACHED AREA TURNS BROWN

WITH CONTINUED EXPOSURE TO

SUNLIGHT; CELLS BECOME LEAKY

• TEMPERATURE- INDEPENDENT

• VISIBLE LIGHT—CAUSES PHOTOOXIDATIVE

DAMAGE (FREE RADICALS etc.)

SCHRADER-2015©

THREE TYPES OF SUNBURN:

NECROSIS, BROWNING, & PHOTO-

OXIDATIVE

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HEAT ONLY HEAT + LIGHT LIGHT ONLY

DOES SUNBURN BROWNING AFFECT

INTERNAL FRUIT QUALITY, OR IS SUNBURN

“SKIN-DEEP?”

CONDUCTED A SYSTEMATIC STUDY OF

FRUIT QUALITY IN ‘GALA’ (also FUJI,

GRANNIES, GOLDEN DELICIOUS, &

JONAGOLD) WITH DIFFERENT GRADES OF

SUNBURN BROWNING (Sb-0 TO Sb-4)

SCHRADER-2015©

EFFECT OF SUNBURN BROWNING

ON FRUIT QUALITY OF APPLES

1. FLESH FIRMNESS

2. SOLUBLE SOLIDS CONCENTRATION (SSC)

3. TITRATABLE ACIDITY (TA)

4. RATIO OF SSC TO TA

SCHRADER-2015©

CONCLUSIONS—FRUIT QUALITY

1. AS SEVERITY OF SUNBURN INCREASED:

A. FLESH FIRMNESS INCREASED

B. SOLUBLE SOLIDS INCREASED

C. TITRATABLE ACIDITY DECREASED

D. RATIO OF SSC TO TA INCREASED!!

2. EXTENDED STORAGE—TA DECREASED

RAPIDLY; RATIO OF SSC:TA ROSE MARKEDLY

SCHRADER-2015©

SCHRADER-2015©

CONCLUSIONS—FRUIT QUALITY

3. AS SUNBURN SEVERITY INCREASED, STARCH

DECREASED ON SHADE & SHOULDER SIDE

4. SUNBURN IS MORE THAN SKIN DEEP—

AFFECTS THE CORTEX TOO

HOW CAN SUNBURN BROWNING BE

REDUCED?

1. WHAT ABOUT FST?

EVAPORATIVE COOLING (EC) IS MOST

EFFECTIVE WAY TO DECREASE FST, BUT

INSTALLATION IS EXPENSIVE & WATER USE IS

HIGH. PROVIDES NO UV PROTECTION.

2. WHAT ABOUT UV RADIATION?

BLOCK ONLY PART OF DAMAGING UV RAYS,

AS UV-B IS REQUIRED FOR “RED COLOR”

PIGMENT (i.e. ANTHOCYANIN) DEVELOPMENT

SCHRADER-2015©

TOOLS FOR SUPPRESSING SUNBURN

1) APPLY A SUNSCREEN (i.e. RAYNOX®)

2) APPLY A SUNBLOCKER (PARTICLE FILM)—

SURROUND WP, ECLIPSE; PURSHADE

3) ACTIVATE EVAPORATIVE COOLING (EC)

5) PHOTO-SELECTIVE NETTING & BAGGING

6) GOOD HORTICULTURAL PRACTICES—

PROPER PRUNING, THINNING, IRRIGATION ETC.

Schrader-2015©

4) BMP: COMBINE RAYNOX® AND EC

THANKS FOR YOUR

ATTENTION

QUESTIONS?

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