|Authors: ||E. Lötze, M. Frazenburg, L. Dreyer|
|Keywords: ||'Golden Delicious', leaf primordia, primary xylem, spur, WD SEM|
At the beginning of the season, large amounts of nutrients are remobilized from reserve tissues towards developing shoots and flowers in deciduous crops.
Calcium (Ca) is mobilized before root activity is initiated in the Western Cape of South Africa.
In addition, a decline in Ca concentration Ca in the bark of one-year-old shoots was observed at bud break.
This early remobilization can supply 25% of the Ca contained in the new growth.
About 40% of the Ca in the shoot is located in the bark.
Most of the Ca in new growth will therefore be initially derived from remobilized Ca from reserve Ca.
Ca located in the spur, leaf primordia and primary xylem of dormant reproductive apple buds of four commercial apple cultivars was quantified during dormancy, for two seasons in: ‘Royal Gala’ and ‘Cripps Pink’ (less susceptible to Ca related deficiencies) and ‘Golden Delicious’ and ‘Braeburn’ (more susceptible). Commercial management practices were followed and no nutrient treatments were applied.
Ca concentrations were quantified with a wavelength-dispersive X-ray spectrometry detector (WD SEM). Results showed a difference in Ca distribution between the different cultivars, as well as Ca concentration in the tissues, in specific tissue areas.
Early primary xylem formation in ‘Cripps Pink’ and ‘Royal Gala’ was associated with a higher Ca concentration in the xylem in June (winter), followed by an increase towards September (spring). The reduction in Ca concentration in the spur during the same period indicated allocation towards the xylem.
Trends were not as clear in ‘Golden Delicious’ and ‘Braeburn’, but differed substantially from those of ‘Cripps Pink’ and ‘Royal Gala’. The later formation or presence of primary xylem may partly explain the lower Ca concentrations and difference in Ca distribution between tissues.
In addition, the inherent Ca concentration in the xylem differed between cultivars and may partly explain why these cultivars differ in their susceptibility to Ca deficiencies.
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