Study on the optimum contents of mineral elements in the leaves of passion fruit (Passiflora edulis)

Abstract: 【Objective】The present experiment was undertaken to analyze and formulate the suitable value standard of mineral elements in the leaves of annual passion fruit in growing season, so as to establish the BDRIS diagnostic system of main mineral elements.【Methods】In 2021, 81 representative orchards of purple passion fruit (P. edulis) and yellow passion fruit (P. edulis f. flavicarpa) were selected in the passion fruit production area of Fujian province. The number of fruits per m² and the yield grade were investigated, and the high yield was statistically analyzed, which was based on 810 records. According to the standard, 63 high-yield orchards were selected, and 5 high-yield plants were selected according to the quincunx sampling method, numbered and marked. In the mid-June and late September, 10 leaves at the 10th leaf position were collected from each plant, and 50 leaves were mixed into one leaf sample, which were cleaned for standby; at the fruit maturity stage, according to the numbermarked above, 4 mature fruits were randomly collected from each plant, 20 fruits were taken as one fruit sample, and 10 normal and non-destructive fruits were randomly taken from each fruit sample for testing. The single fruit weight, fruit longitudinal diameter, fruit transverse diameter, total soluble solid and titratable acid contents of the fruit were tested. Finally, according to the data of leaves and fruits, the suitable value of mineral element content in passion fruit leaves was studied.【Results】①According to the best regression simulation between the yield grade and the number of fruit bearing per Square meter, and taking into account the actual production and the operability of the evaluation, it is suggested that the average number of fruit bearing varieties should be taken as the defining standard for high-yield orchards: yellow passion fruit ≥ 16 fruit·m^-2 , and purple passion fruit ≥ 20 fruit·m^-2 , which is basically consistent with the production practice of the two varieties under the same cultivation and management conditions. ②The results of covariance analysis on fruit quality showed that the quality of summer fruits was significantly better than that of autumn fruits, which were characterized by large fruit, high total soluble solid content, low titratable acid content and high solid/acid ratio; The variance of fruit quality among varieties was greater than that of seasonal variation, that is, the quality difference caused by varieties was more significant than seasonal difference. ③The covariance analysis of 126 leaf samples in different collection seasons (variety and season are mutually covariates) showed that there were significant differences in N, K, Fe, Cl contents among varieties (yellow passion fruit ＞ purple passion fruit); there were extremely significant seasonal differences in N, Mg and Fe contents between seasons; except Fe, the content in summer was greater than that in autumn; The content of movable elements like N and Mg in summer was higher than that in autumn, which may be related to the yield of autumn fruits being higher than that of summer fruits (the yield of summer fruits∶the yield of autumn fruits≈ 4.5∶5.5). ④The critical value diagnostic criteria for mineral elements of standard leaves with petioles at the 10th leaf position in the summer green fruit stage were recommended as“elements (＜deficiency; optimal lower limit-optimal upper limit; ＞excess)”: N (yellow passion fruit: 3.90%-5.40%; purple passion fruit: 3.35%-4.90%), P (0.19%-0.31%), K (yellow passion fruit: 2.10%-4.25%; purple passion fruit: 2.10%-3.50%), Ca (＜1.50%; 1.90%-3.41%; ＞3.81%), Mg (＜0.15%; 0.20%-0.40%; ＞0.45%), Cu (3.0-20.0 mg · kg^-1 ), Zn (24-80 mg · kg^-1 ), Fe (＜70 mg · kg^-1 ; 90-160 mg · kg^-1 ; ＞240 mg · kg^-1 ), Mn (＜40 mg·kg^-1 ; 80-550 mg·kg^-1 ), B (＜16 mg·kg^-1 ; 25-70 mg·kg^-1 ; ＞85 mg·kg^-1 ), Mo (0.15-1.5 mg·kg^-1 ), Cl (＜2.0%), S (0.32%-0.48%; ＞0.52%). ⑤The data distribution of the main mineral nutrients including N, P, K, Ca and Mg in the study sample leaves meets the normal assumption that p (normal) ≥ 0.15, which conforms to the BDRIS modeling conditions, and thus a diagnostic system is established.【Conclusion】According to the relative physiological stability of mineral nutrition in leaves of fruit trees in different growth regions or under different environmental conditions, the critical value standard and BDRIS recommended in this study for mineral nutrition in leaves of annual passion fruit in growing season are also applicable to the diagnosis of mineral nutrition in growing season of perennial passion fruit in the south subtropical and tropical regions, which broadens the regional limitations of the diagnostic standard in non growing season in winter before.