Fruit growth rate influences cortical xylem transport, calcium delivery, and bitter pit incidence of apple

Abstract

Bitter pit of apple is commonly ascribed to calcium dilution because symptoms are positively associated with fruit mass. A dilution effect implies that calcium transport or supply cannot keep pace with fruit growth but does not identify the mechanism. Disparate sink:source ratios or transient photosynthetic inhibition were used to alter ‘Honeycrisp’ fruit growth rate (FGR) to study fruit xylem transport kinetics and peel calcium concentration. FGR and xylem dysfunction increased with decreasing sink:source ratio. Xylem transport declined to 50% of maximum 1.7 weeks earlier under Low than High sink:source ratios and peel calcium decreased concomitantly with xylem transport. Treatment effects on bitter pit incidence were explained by integrating time-dependent xylem transport curves into an empirical ‘cumulative xylem’ function. Regressing this function on FGR conflated treatment differences. When photosynthetic light reactions were inhibited by episodic limb shading, FGR was reduced and xylem transport increased; shade removal caused transient surges in FGR and concomitant reduction in xylem transport. Changes in calcium concentration were less responsive to shade due to stomatal limits to water transport. Combined, the results support that FGR compromises xylem transport, effectively limiting calcium supply and exacerbating bitter pit, the magnitude of which depends on the onset of xylem dysfunction.