Anatomical determinants of fruit firmness in blueberries: A tissue-level analysis

Fruit firmness is an important quality characteristic in blueberries (Vaccinium corymbosum L.) because it determines the sensory quality of fresh fruit, its postharvest shelf-life, its ability for long-distance transport, and the feasibility of mechanical harvest.Fruit firmness is genetically determined, but several factors, including the time of harvest, Ca nutrition, pre-and postharvest ambient temperature, and other management practices, also affect gene expression.Little is known about the effect of the anatomy of the different fruit tissues on their firmness.We investigated the anatomy of the different tissues of the fruit in three blueberry cultivars with contrasting firmness: 'Jewell' with 146.6 29.2 g mm -1 mechanical compression firmness; 'Flicker' with 185.2 16.6 g mm -1 , and 'Sweetcrisp' with 210 62.1 g mm -1 .The tissues analyzed were the epidermis, hypodermis, outer mesocarp, and middle mesocarp of fruits collected at harvest time.These tissues were then fixed, embedded in paraffin, stained, sectioned, and mounted for microscopic observation.Statistical differences were found among cultivars concerning epidermal cell size, with 'Jewell' having the largest epidermal cell size and 'Sweetcrisp' the smallest.'Flicker' showed larger hypodermal cells than 'Jewell' and 'Sweetcrisp'.'Jewell' presented smaller cells in the outer mesocarp when compared to 'Flicker' and 'Sweetcrisp'.'Jewel' had no sclereids in the hypodermal layers, while 'Flicker' and 'Sweetcrisp' exhibited high degrees of sclerification.The higher degree of sclerification observed below the epidermal tissue was directly associated with increased firmness.Firm fruit cultivars possess anatomic differences compared to soft fruit cultivars, suggesting an intimate relation between anatomical traits and fruit firmness.