A review of fruit development in strawberry: high temperatures accelerate flower development and decrease the size of the flowers and fruit

Temperature affects the growth and development of strawberry (Fragaria xananassa Duch.). This review examines the effect of temperature on fruit growth from the published literature. Higher temperatures under global warming will result in lower yields and smaller, less sweet berries in the future. Warm weather has a negative impact on the size of the flowers, germination of the pollen and on carbon assimilation, which all contribute to the production of smaller fruit. The relationship between fruit weight and temperature was assessed across studies. The slope from the linear regression ranged from - 0.11 to - 5.00, with a median of - 0.96, and a mean (+/- s.d. or standard deviation) of - 1.32 +/- 1.22 (N = 23 studies). Differences in the response reflect variations in cultivars, yields, the structure of the inflorescence and the period used to assess growth. There were strong linear relationships between the number of stamens per flower (P < 0.001, R-2 = 0.62) and the number of carpels per flower (P < 0.001, R-2 = 0.77), and the size of the flowers. There were strong relationships between the size of the fruit at harvest and the size of the flowers at anthesis (P < 0.001, R-2 = 0.76; P < 0.001, R-2 = 0.86). High temperatures during flower development decreased the size of the flowers (8.5 mm x 4.8 mm at 15 degrees C and 6.0 mm x 3.7 mm at 25 degrees C) and the number of carpels (achenes) per flower (P = 0.023, R-2 = 0.29; and P = 0.003, R-2 = 0.59). High temperatures decreased the period of floral differentiation (P < 0.001, R-2 = 0.90) and the size of the fruit at harvest (P = 0.024, R-2 = 0.99; P = 0.032, R-2 = 0.99). The percentage of pollen grains which germinated was higher from 20 degrees to 30 degrees C, with lower germination at lower or higher temperatures. The slope from the linear regression between the fruit development period (FDP) and temperature ranged from - 0.77 to - 2.93, with a median of - 1.70, and a mean of - 1.71 +/- 0.72 (N = 15 studies). Net CO2 assimilation was similar from 20 degrees to 30 degrees C, and only lower under extreme conditions (N = 15 studies). Respiration in the leaves increased with warm days and nights (P = 0.003 or 0.004, R-2 = 0.95 or 0.94), while respiration in the fruit increased exponentially from 10 degrees to 36 degrees C (P < 0.001, R-2 = 0.99). The effect of temperature on fruit growth in the field is confounded by changes in the structure of the inflorescences, with smaller fruit in the later inflorescences than in the first inflorescence (P < 0.001, R-2 = 0.85, 0.82 or 0.89). The production of small fruit during warm weather is associated with smaller flowers and fewer stamens, germinated pollen grains, carpels and achenes in each flower or fruit. Warm conditions increase respiration in the leaves and the fruit. Fruit weight is moderately heritable, with a mean broad-sense heritability (H-2) of 0.44 +/- 0.23 (N = 11 studies) and a mean narrow-sense heritability (h(2)) of 0.45 +/- 0.22 (N = 28 studies). High temperatures accelerate the development of the flowers and decrease the size of flowers and fruit. Efforts should be made to develop cultivars with acceptable fruit size under hot conditions. Genome-wide association (GWA) and genomic prediction (GP) will accelerate the identification of large-fruited populations and individuals under global warming. These studies should examine the genes associated with cell division and expansion during the development of the flowers.