Inherent and acquired resistance to fruit sunburn and poor colour in various apple/pear cultivars

The objective of this project was to identify the anatomical, biochemical and physiological characteristics that offer photoprotection in apple and pear peel. We wanted to characterise the geographical variation, variation between cultivars, and variation between fruit maturities in these characteristics, and establish what the threshold conditions are for photothermal stress for different cultivars and whether these change through the season. In addition, we also aimed to study the contribution of water availability, crop load and fruit flesh characteristics to sunburn.

Various trials were conducted and the word limitation of the abstract doesn’t allow the detailed description of all methodology (Please refer to theses and publications for detail).

Previously sun-exposed peels of apple fruits were resistant to photodamage under high UV-B dosage throughout fruit development. However, the shaded peels of mature fruits incurred photodamage under UV-B stress.

Photodamage induced by heat and PAR stress during fruit development was not well correlated to fruit pigments, phenolic levels as well as fruit peel anatomical characteristics. In addition, repeated heat and PAR stress up to 9 hours did not induce any fruit sunburn symptoms. The poor correlation and inability to induce sunburn symptoms possibly implies that more factors are involved in fruit sunburn manifestation.  Sunburn symptomology does not necessarily relate to the severity of the photodamage that fruit peel of a particular cultivar underwent.  We did find variation in the dependency of different cultivars on the xanthophyll cycle for photoprotection under heat and PAR stress.

Sunburn increased with an increase in moisture stress. Mulching had no effect on plant water status, but it enhanced plant photochemistry, reducing fruit surface temperature (FST) and sunburn.

Chemical and textural characteristics of exposed, but non-burned ‘Cripps’ Pink’ apples were compared with that of fruit with induced and naturally occurring sunburn. Flesh firmness, TSS and TA of induced and naturally occurring sunburnt fruit did not differ from each other, but were significantly different from the non-burnt fruit. Based on this, we concluded that sunburn induces textural and compositional changes in sunburnt fruit rather than some fruit being predisposed to develop sunburn due to their chemical composition.

We found clear evidence that anthocyanin accumulation masks sunburn browning in blushed and red cultivars, but it does not seem to increase the susceptibility or protect these cultivars from sunburn damage.

Heat stress appears to cause the highest damage to fruit photosystems, while UV-B and PAR enhances the effect of heat stress. Our data cast doubt on a primary role of UV-B in sunburn development in adapted, sun-exposed apple peel.

Our results also indicate that the level of sunburn symptomology does not necessarily relate to the level of stress that the peel of a particular cultivar experiences.  Hence, cultivar expression of sunburn symptoms and cultivar photodamage sensitivity is not necessarily correlated.

The incidence of sunburn browning is likely to be underestimated in red and blushed cultivars, with compounding effects on comparative studies of cultivar sunburn susceptibility and sunburn physiology.

The increase in sunburn incidence towards harvest in apples is not due to an increase in TSS.  Higher TSS and sunburn is correlated in sun-exposed apples but the relationship is not causal.

Water stress increases sunburn in apples due to an increase in fruit surface temperature.