Identification of pollination compatibility and exploration of distance of pollinizer trees for rough skin strains of Majiayou

【Objective】The study identified the S-genotype of the rough-skinned strains of Citrus maxima‘Majiayou’(Cupi Majiayou), clarified pollination compatibility among the main varieties of Majiayou, and explored the comprehensive quality Majiayou under different distances from the pollinizer trees so as to provide references for the quality improvement with better arrangement of pollinizer trees for Majiayou.【Methods】21 pomelo S gene-specific primers used for the amplification and identification of the S-genotype rough-skinned strains of Majiayou. Before cross-pollination, pollen viability and in vitro germination rate of both rough-skinned and smooth-skinned Majiayou were determined to ensure pollen fertility. The growth of pollen tubes in the styles from self-pollination and cross-pollination between the two strains was observed using the aniline blue staining method. Quality indexes including soluble sugar, and contents of various organic acid components of fruit from different trees at different distances from the pollinizer trees were measured.【Results】21 pomelo S gene- specific primers were used for the specific amplification and identification of S-genotypes of the rough-skinned variety of Ma-jiayou and also for identification of the semi-compatible/fully compatible varieties reported previously to verify the accuracy of the method. The amplification results for the smooth-skinned Majiayou were consistent with previous studies, being S10S16. The S-genotype of the Cupi Majiayou variety was found to be S10S12, sharing one identical S gene with the smooth-skinned variety. Based on the S-genotype, it was predicted that cross-pollination between the two varieties might exhibit semi-compatibility. To further verify cross- pollination compatibility between the rough- skinned and smooth- skinned strains of Majiayou, pollen viability and in vitro germination experiments were conducted to confirm the fertility of the pollen. The results showed that the pollen activity of the smooth-skinned and rough-skinned varieties of Majiayou were 98.17% and 97.04%, respectively. The in vitro germination rates of the pollen of the smooth-skinned and rough-skinned varieties were 45.96% and 42.67%, respectively. Therefore the pollens were fertile and suitable for subsequent cross-pollination. The styles of the rough-skinned and smooth-skinned varieties after self-pollination and cross-pollination were used for aniline blue staining to observe the growth status of the pollen tubes. The staining results showed that after self-pollination, the pollen tubes of both the smooth-skinned and the rough-skinned varieties stopped elongating at the upper part of the style, indicating complete incompatibility during self-pollination. However, after crosspollination between the rough-skinned and smooth-skinned varieties, some pollen tubes grew normally to the middle and lower parts of the style, while others stopped growing at the top, indicating semi-compatibility between the rough-skinned and smooth-skinned varieties during cross-pollination. To explore the seed set and fruit quality of Majiayou under natural pollination at different distances under semicompatible pollination father configuration, the seed set of Majiayou under pollinizer arrangements was investigated. The results showed that under the row-column pollination tree pattern, the seed set of each row from P1 to P8 was above 80.0%, while the seed set of rows P9 to P13 decreased, ranging from 66.7% to 73.3%. Under the central radiation pollination tree pattern, the seed set of rows P1 to P2 was above 87.5%, while the seed set of row P3 decreased to 59.7%. With the increase of pollinizer distance, the peel thickness of the main cultivated Majiayou under the row-column pattern showed a trend of decreasing first and then increasing, with the peel thickness of rows P1 and P9 being 23.5 and 27.3 mm, respectively, which were 27.7% and 16% lower than CK1 (32.5 mm), and the peel thickness of row P13 was not significantly different from the control. In addition, the number of seeds showed a continuous decrease with the increase of the distance from the pollinizer. Soluble sugar (TSS) content of the fruit from row P13 was 16.3%, significantly higher than CK1’s 11.8%. The titratable acid (TA) content of the fruit from row P1 (0.97%) was significantly higher than CK1, while that of row P9 (0.3%) was the lowest among all rows. The TSS/TA ratio was the lowest in row P1, at 14.9, which was 32.0% lower than that of CK1, while that in rows P9 and P13 increased by 57.1% and 34.7% compared to CK1, respectively. Vitamin C content also showed a continuous increasing trend with the increase of pollinizer distance. Under the central radiation pollination pattern, the peel thickness of the pollinated pummelo were significantly lower than CK2’s 33.7 mm, with an average reduction of 18.7% to 35.0%. The TA content of the fruit in the upper part of rows P1 and P2 was significantly lower than that of CK2, while there was no significant difference between the fruit from row P3 and CK2. The solid acid ratio of Majiayou; at different fruiting positions showed a clear trend of increasing first and then decreasing with the increase in the distance from pollen donor trees, with the TSS/TA of the fruit in the upper part of rows P1 and P2 increased by 29.2% and 49.5% compared to CK2, respectively; the middle fruit increased by 21.0% and 27.4%, respectively, while there was no significant difference between the fruit in different positions of row P3 and CK2.【Conclusion】The scientific arrangement of pollinizer trees using thesemi-compatible rough-skinned variety of Majiayou is important for the production of high-quality commercial fruit of Majiayou.