Effect of fertilizer application level on pectin composition of Hakuho peach (Prunus persica Batsch) during maturation

Effects of fertilizer application levels on fruit texture and flesh pectin compositions of a melting peach were investigated. Hakuho trees (Prunus persica Batsch) were supplied with normal (M), high (H; M x 2), and superhigh (SH; M x 4) levels of complete liquid fertilizer twice a week. Flesh firmness of the H and SH treatment fruit was lower than that of M treatment fruit at the hard-mature and firm-mature stages, although no difference was detected at the full ripe stage. Sensory scores for flesh texture at the full ripe stage were highest in the N treatment fruit and lowest in the SH treatment fruit. The content of water-soluble polyuronides (WSP) in flesh was highest in SH fruit and low est in M fruit at the hard-mature stage, although the difference became smaller at the full ripe stage. Molecular mass analysis using a gel filtration column revealed that water-soluble polysaccharides in alcohol-insoluble solids (AIS) of the H and SH fruits had a peak of high molecular mass, ≈200 kDa, at the hard-mature stage, and the molecular mass decreased gradually to ≈23 kDa at the full ripe stage. In the M fruit, however, the molecular mass was rather constant during the ripening period, 112 kDa even at the full ripe stage. The analysis of acidic fractions (pectin) in the polysaccharides using an ion exchange column, as well as juice gellation test by adding Ca and Tris buffer, also indicated that high levels of fertilizer application impairs an early degradation of flesh polyuronides resulting in the accumulation of low-molecular-weight WSP. This may ultimately cause the inferior flesh texture of overfertilized peach fruit.