Water Potential, Osmotic Potential, and Cell Turgor in Developing European Plums

Neck shrivel is a fruit disorder of european plum (Prunus domestica L.). We investigate whether an asymmetrical distribution of osmolytes might explain the observations of a turgid stylar end and a flaccid stem end, in a selection of 17 plum cultivars sourced from two sites. The osmotic potential (ΨΠ) of the juices expressed from stem or stylar end fruit samples decreased (became more negative) during development. The cell turgor (ΨP) slightly increased during development up to 352 ± 42 kPa at 78 days after full bloom (DAFB) in the stem end and up to 331 ± 51 kPa at 92 DAFB in the stylar end, and then decreased. At maturity, ΨP averaged 22 ± 3 kPa in the stem end and 22 ± 4 kPa in the stylar end. These values are negligibly low compared with the very negative values of ΨΠ in the stylar end (−3188 ± 73 kPa) and stem end (−3060 ± 74 kPa). There was a transient gradient in ΨΠ between stylar end and stem end that almost disappeared by maturity. Marked differences in ΨΠ and its distribution were measured across 17 cultivars. In the majority (14), ΨΠ was more negative at the stylar end than at the stem end. A more negative ΨΠ in the stem was only detected in ‘Aprimira’, ‘Topfive’, and ‘Tophit’. Our results demonstrate that cell ΨP is very low and is essentially independent of ΨΠ in developing european plums. In most cultivars, ΨΠ in the stylar end is more negative than in the stem end. The absence of an axial gradient in ΨP and the small differences in ΨΠ between the stem and stylar end make both factors unlikely candidates for explaining neck shrivel.