A study of broken stones in Japanese plums

The effect of growing area and season on the incidence of broken stones, why cultivars differ in their susceptibility to broken stones, and if calcium and silicate treatments can reduce stone breakage in Japanese plums were studied.

• ‘Best’ subset regressions were performed to determine if the incidence of broken stones at harvest could be predicted in ‘Laetitia’.
• CT scans were performed on ‘Laetitia’ and ‘Songold’ fruit from 21 days after full bloom (dafb) until stone hardening was completed.
• A randomised complete block design with eight blocks per treatment were used to determine the effect of Ca(NO₃)₂ and K₂O₃Si on broken stones compared to a control.

Growing area did not affect broken stones, but seasonal climatic factors had an important influence on broken stone incidence. Generally lower temperatures during early spring cause less competition between the growing plant organs and aid in the development of larger fruit, which are more vulnerable to stone breakage. Rapid lengthwise fruit growth during the early parts of the stone hardening period pulls the stone apart at the interface between the parts of the stone that have started to lignify and the parts that are still ‘soft’. Also, stones with higher moisture content (due to wet conditions early in the season) tend to be more susceptible to breaks, especially if this is coupled with increased growth of the mesocarp, which was observed in terms of lengthwise fruit growth. However, data from more seasons are needed to accurately predict the incidence of broken stones at harvest.

CT scans indicated that the tip of the stone at the stylar end started to lignify first for all the tested cultivars. It seems that ‘Laetitia’ and ‘Sapphire’ (susceptible to stone breakage) form more endocarp cells in warmer springs which causes the stones to be more rigid and more susceptible to stone breakage if coupled with rapid radial fruit growth before the stone has hardened completely. ‘Songold’ (less susceptible to broken stones) forms a complete endocarp irrespective of spring temperatures. However, the enzymes responsible for lignification are influenced by spring temperatures. Hence, the cultivar has a more flexible stone, resisting stone breakage in a warmer spring when lignification is less pronounced and forms a much more rigid stone in a cooler spring when lignification is upregulated. Hence, susceptible cultivars will have more broken stones after warm springs, but less after cool springs while non- susceptible cultivars will have broken stone incidence after a cool spring with negligible levels after a warm spring.

Neither foliar Ca(NO₃)₂ and K₂O₃Si, post-harvest Ca(NO₃)₂, nor root K₂O₃Si applications had an effect on the incidence of broken stones.

Data from more seasons and/or farms are needed to accurately predict the incidence of broken stones at harvest. Higher temperatures spring temperatures cause higher endocarp density in susceptible cultivars, and, if coupled with rapid radial fruit growth, will lead to a higher incidence of broken stones. Neither calcium nor silicate treatments can currently be recommended to reduce broken stones in plums.