Developmental characteristics of citrus peel and the effect of gibberellic acid on fruit cracking

Abstract: 【Objective】Fruit cracking is an important physiological disease in citrus. Due to the com- plexity of the fruit cracking mechanism and the variability of the climate, there are still no effective measures to solve fruit cracking problem in practice. Kanpei is a hybrid citrus variety recently intro- duced to China. Its fruit cracking phenomenon is serious regardless of changes in the external environment, which provides a good case for fruit cracking research. This paper comprehensively evaluated the development characteristics of peel and its correlation with fruit cracking in Kanpei fruit. Effects of gib- berellic acid (GA3) on fruit cracking rate and fruit quality were also investigated.【Methods】During the young fruit stage and the fruit expanding stage, 1.5 cm ́2.0 cm peel pieces were cut and subjected to FAA fixation, washing, dehydration, transparency, paraffin impregnation, embedding, sectioning and gluing, dewaxing, staining, dehydration, hyalinization, sealing and other steps to obtain paraffin sec- tions of the peel. The paraffin sections were used for the observation of the structural characteristics of the peel tissues. Fruit shape index and peel thickness were measured with a vernier caliper. The firm- ness of fruit was measured using a digital force gauge, and the equatorial position is taken as the mea- suring point. There were four treatments in the experiment, including three concentration gradients of gibberellic acid, 5 (G5), 10 (G10), and 20 mg · kg- 1 (G20), which was sprayed on the surface of the fruit every 20 days from the young fruit stage (June 15). The fourth treatment involved spraying with 10 mg · kg- 1 GA3 (DG10) every 15 days from the fruit expansion period (July 5). In all the treatments, the GA3 were sprayed three times, each containing 0.05% Tween-20 as the surfactant. Spraying water served as the control group (CK). The initial fruit load (M) of each tree after fruit set (July 20) and the final fruit load (N) of each tree at the end of fruit cracking (mid-late October) were recorded. The fruitcracking rate = (M−N)/M ×100%. The soluble solid content (SSC) of the fruit and color parameters of the fruit surface were measured with a Brix meter and a color difference meter, respectively. The juice filtered through two layers of gauze and diluted by 50 times was used for the citric acid content determi- nation.【Results】After flowering, the vertical and horizontal diameters of the fruit increased rapidly, and the growth rate of the horizontal diameter was faster than that of the vertical diameter, which led to a decrease in fruit shape index from 0.92 at 30 days after full bloom to 0.76 before color break. The thickness of the peel increased rapidly with the development of the fruit, reaching a maximum value of 0.50 cm around 45 days after the full bloom, and then gradually decreased to about 0.19 cm at color break stage. The results of the paraffin section showed the microstructure of peel, where the albedo is the sensitive layer for cracking because cracks appeared between the albedo cells from 45th days after full bloom and expanded with fruit development. There was no significant correlation between fruit shape index and cracking incidence, but thicker peel resulted in higher peel hardness, which was benefi- cial to buffer the stress on the peel caused by the increased turgor pressure of the fruit. The gibberellic acid treatments effectively reduced fruit cracking rate. The fruit cracking rate in G10(10 mg·kg-1) treat- ment decreased by 54.53% and 34.76% compared with the control group and DG10, respectively. How- ever, higher concentration GA3 (20 mg · kg- 1) increased the sunburn injury caused by high- temperature days in summer, while lower concentration GA3 (5 mg · kg- 1) had a limited effect in preventing peel cracking. Spraying GA3 (G10) enhanced the connectivity of albedo cells and improved the arrangement of cuticle layers, which might play an important role in reducing the rate of fruit cracking. All the GA3treatment had no significant effect on fruit internal quality, but 20 mg·kg-1 GA3 increased peel firmness and thickness, by 8.1% and 15.0%, respectively, compared with the control group. Delayed and fre- quent application of GA3 (DG10) postponed the accumulation of red pigment in the peel. It is worth not- ing that the fruit shape index in each GA3 treatment was 1.5%-4.5% higher than in the control.【Conclu- sion】The cracking of citrus fruit is a gradual process, which is the result of both external and internal factors. The fruit cracking is related to the continuous thinning of peel thickness and cell cracks in the albedo tissue. Exogenous GA3 suppresses the development of cracks, and thus effectively control the oc- currence of fruit cracking. However, some attention should be paid upon the concentration and time of GA3 application, otherwise, some adverse effects may occur.