Spraying calcium is not an effective way to increase structural calcium in litchi pericarp

Calcium in different forms was compared in the pericarp of cracked and healthy fruit in the cracking-susceptible litchi (Litchi chinensis Sonn.) cultivar 'Nuomici'. Incorporation of 45Ca into the cell walls of the pericarp and effects of sprays of calcium chloride (0.2% or 0.5%) on endogenous calcium and cracking incidence were examined. The results showed that calcium concentration in cracked fruit was significantly lower than in healthy fruit. Structural calcium as part of cell wall components contributed to the majority of the difference in calcium concentration between cracked fruit and healthy fruit. Surface spray of calcium chloride, either once or three times at different stages of fruit development exhibited no significant effect on fruit cracking and hardly increased calcium, especially structural calcium in the pericarp. X-ray microanalysis revealed that the applied calcium remained on pericarp surface. Radioisotope test showed that less than 0.1% of 45Ca applied on fruit surface was incorporated into the cell walls. It is suggested that spraying calcium on fruit surface is not an effective way to prevent litchi fruit cracking due to the difficulty in uptake of the calcium sprayed on fruit surface.