Fig 1.
Frequency distribution of cracking susceptibility of sweet cherry genotypes.
(A) Segregating population of full siblings of a ‘Regina’ x ‘Garnet’ cross comprising 117 individuals. The percentage of fruit cracking is based on 8 years of cracking assessment in the field following exposure to natural rain. (B) Partitioning of genotypes from A that had sufficient fruit set in the 2022 growing season into two categories of contrasting cracking susceptibility. Vertical dashed line represents the arbitrary threshold of 25% cracking that was chosen to separate the population into cracking tolerant and cracking susceptible genotypes. (C) Relationship between frequencies of stylar end cracking and overall fruit cracking irrespective of crack position as observed in eight growing seasons following exposure to natural rain. Open symbols in C represent the ‘Regina’ and ‘Garnet’ parent.
Table 1.
Compilation of crack characteristics of sweet cherry fruit from a segregating population of a cross of ‘Regina’ x ‘Garnet’.
Cracking was assessed in a laboratory based immersion assay (n = 3) using the cracking index [38, 39], the time to half maximum cracking (T50) or the percentage of fruit cracked at infinity (asymptote) using fruit from grafted trees. Cracking in the field was recorded in eight growing seasons following exposure to natural rain using the same genotypes as above but on their own roots. These data are taken from Quero-Garcia, Letourmy [33]. The number of replicates was 16 susceptible and 21 tolerant genotypes including the parent cultivars Regina and Garnet.
Fig 2.
(A) Time course of cracking of susceptible and tolerant genotypes when using an immersion assay in deionized water. (B) Normalized fruit mass in the course of an immersion assay. Dashed horizontal line indicates the mean fruit mass of all fruit in the experiment. Asterisk indicate significant differences, Tukey’s Studentized range test, P = 0.05. (C) Relationship between the time to half maximum cracking (T50) and the cracking index as described by [38, 39]. Inset: Relationship between the percentage of fruit cracking at 4 h and the cracking index. Open symbols in C represent the ‘Regina’ and ‘Garnet’ parent.
Table 2.
Coefficients of correlation between cracking as quantified in the lab using an immersion assay and fruit from grafted trees and as observed in the field using trees on their own roots that were exposed to natural rain.
Fruit cracking in the lab was quantified using the cracking index (‘CI’), the time to half maximum cracking (‘T50’) and the percentage of fruit that cracked at infinity (‘asymptote’). Cracking in the field was assessed as the percentage of fruit that cracked in the proximal stem cavity, the equatorial region of cheek and suture and the distal stylar end cracking, the mean cracking, the maximum and the predicted cracking percentage. For details see text.
Fig 3.
Relationship between cuticle properties of mature sweet cherry fruit of siblings of a cross of ‘Regina’ x ‘Garnet’ and their cracking susceptibility as assessed in a laboratory based incubation assay as indexed by the cracking index (A,B,C,D) or as observed in eight growing seasons following exposure to natural rain (E,F,G,H). A,E. Mass per unit fruit surface area of the cuticular membrane (CM). B,F. Mass per unit area of the dewaxed CM (DCM). C,G. Mass per unit area of cuticular wax. D,H. Release of strain of the CM following excision and isolation. Open symbols represent the ‘Regina’ and ‘Garnet’ parent.
Fig 4.
(A). Strain release of the cuticular membrane (CM) as affected by the mass of the CM per unit surface area of fruit of mature sweet cherry siblings of a cross of ‘Regina’ x ‘Garnet’. (B). Relationship between the extent of microcracking of the cuticle and the amount of strain relaxation following excision and isolation of the cuticular membrane. Microcracking was indexed in the stylar end region by the increase in area infiltrated with the fluorescent tracer acridine orange during a 2 h incubation period in deionized water. (C). Relationship between the extent of microcracking of the cuticle and Ca/dry mass ratio in the stylar end region of sweet cherry fruit. Open symbols represent the ‘Regina’ and ‘Garnet’ parent.
Fig 5.
Representative micrographs of the stylar end region of fruit of a cracking susceptible (RxG113) and cracking tolerant genotype (RxG80) before and after a 2 h incubation period in deionized water for induction of microscopic cracks (‘microcracks’) in the cuticle.
Fruit were viewed in incident white light or fluorescent light. Microcracks were stained for 10 min using the fluorescent tracer acridine orange. Penetration of acridine orange is limited to microcracks in the cuticle and does not occur through an intact fruit cuticle. Scale bar for all micrographs = 5 mm.
Fig 6.
Relationship between microcracking of the cuticle in the stylar end region of mature fruit of siblings of a cross of ‘Regina’ x ‘Garnet’ sweet cherry and their cracking susceptibility as assessed in a laboratory based incubation assay as indexed by the cracking index (A,B,C) or as observed in eight growing seasons following exposure to natural rain (D,E,F). Microcracking was indexed by the area infiltrated with the fluorescent tracer acridine orange. Fruit were inspected before and after a 2 h incubation period in deionized water for microcrack induction and the area infiltrated by acridine orange in the stylar end region of the fruit quantified using fluorescence microscopy and image analysis. A,D. Infiltrated area before induction of microcracks. B,E. Infiltrated area after induction of microcracks. C,F. Increase infiltrated area after induction of microcracks. The increase in infiltrated area (Δ Infiltrated) was calculated as the difference between the infiltrated area after minus that before incubation. Open symbols represent the ‘Regina’ and ‘Garnet’ parent.
Fig 7.
Time course of change in Ca/dry mass ratio of fruit of cracking tolerant and cracking susceptible sweet cherry genotypes that cracked in the course of a laboratory-based incubation assay for quantifying cracking susceptibility.
Table 3.
Ca/dry mass ratio in the proximal stem end, the equatorial and the distal stylar end region of fruit of cracking susceptible and tolerant genotypes.
Fruit were taken from a segregating population of grafted trees of siblings of a cross of ‘Regina’ × ‘Garnet’.
Fig 8.
Relationship between the Ca/dry mass ratio of mature fruit of siblings of a cross of Regina x Garnet and their cracking susceptibility as assessed in a laboratory based incubation assay as indexed by the cracking index (A,B,C,D) or as observed in eight growing seasons following exposure to natural rain (E,F,G,H). The Ca/dry mass ratio was determined in the stem end region (A,E), the equatorial region (B,F) the stylar end region (C,G) and as a the average Ca/dry mass ratio per fruit without pit (D,H). Open symbols represent the ‘Regina’ and ‘Garnet’ parent.
Table 4.
Parameters of multiple linear regression equations explaining sweet cherry fruit cracking in a laboratory based immersion assay or as observed in eight years in the field following exposure to natural rain.
Cracking in the immersion assay was indexed by the cracking index or the time to half maximum cracking (T50). In the field the percentage of fruit that cracked or fruit that cracked in the stylar end region was recorded [33]. The same genotypes were used for laboratory and field assessments. Fruit for the laboratory assay was from grafted trees, the field observations were done on trees of the same genotpyes on their own roots.