Development of “stem-topple” disorder in Ranunculus asiaticus is related to localised disturbances in tissue calcium levels

SUMMARY A variety of physiological disorders in plants are known to be induced by calcium deficiency, and display localised symptoms of necrosis, mostly in slowly transpiring organs or rapidly growing tissues. Flower “stem-topple”, identified in some geophytic plants, was suggested to be a calcium-related disorder. In Ranunculus asiaticus L., the zone susceptible to toppling is the growing section of the flower-stem. Since rapidly growing tissues use large amounts of calcium, and growth involves dilution of the cell contents by the deposition of water, a deficient supply of calcium may be induced locally in such tissues, even when the supply of calcium to the plant is sufficient. In the present study, to investigate the potential involvement of localised tissue calcium levels in the induction of “stem-topple” disorder in R. asiaticus, we attempted to correlate the occurrence of the disorder with the spatial distribution of minerals along the flower stem. Since the amount of N applied as fertiliser has been reported to affect the severity of “stem-topple” in R. asiaticus, and since high levels of NH₄⁺ are known to interfere with calcium nutrition and to increase the severity of calcium-related disorders, we evaluated the effects of N levels, as well as the level of NH₄⁺ supplied to plants, on the occurrence of “stem-topple” damage. Three levels of NH₄⁺ fertigation were compared (10, 20 and 30% of the total weight of N supplied, at 100 mg N kg–¹) and two levels of N (50 and 100 mg N kg–¹at 20% NH₄⁺). The least damage and the highest levels of Ca in tissue susceptible to toppling occurred under 50 mg N kg–¹. The total amount of N supplied changed the size-class distribution of toppled flowers. Increasing the proportion of NH₄⁺, or its concentration in the fertiliser did not exacerbate the severity of the disorder. The results demonstrated a negative correlation between the occurrence of “stem-topple” and calcium concentrations in stem tissues susceptible to toppling, suggesting that “stem-topple” is indeed a calcium-related disorder.