TREES
9. FACTORS INFLUENCING CALCIUM NUTRITION 1. Soil factors
The absolute concentration of Ca in soil solution is less important in controlling Ca uptake than the relationship of Ca to the total salt concentration and its pro-portionate concentration to that of other ions in solution (Shear, 1975). Along with the ratio of Ca to total salt concentration, specific ions in soil solution may inhibit its uptake. It is well known that absorption of Ca may be depressed by NH4, K and Mg ions. This negative effect on rate of Ca uptake is more pronounced when concentrations of above-mentioned cations in soil solution are high and Ca level is relatively low. Thus, high rates of N-NH4-, K- and Mg-fertilizers applied on acid soils will reduce the rate of Ca absorption by plants. According to Kotze (1979) the greatest effect on reduction of Ca uptake have NH4and Al ions. Therefore, on acid sand with high Al concentrations in soil solution, NH4-N fertilizers should not be applied in apple orchards. Calcium uptake may also be stimulated by the syn-ergistic effect of other ions in soil solution such as NO3 and HPO4 or H2PO4 (Jakobsen, 1979). Because NO3 concentration in soil solution is usually higher compared to HPO4 or H2PO4, it seems that NO3 ions have a greatest effect on stimulation of Ca absorption. However, it is worth noting that N-NO3stimulates not only uptake of Ca but also other cations (Kirby and Mengel, 1967).
Under field conditions, soil water deficiency is critical factor reducing absorp-tion of nutrients by plants. Decrease in Ca uptake as a result of low soil moisture is related to the fact that Ca moves in soil solution mainly by mass flow. Moreover, low water content in soil results in increase of total salt concentration in soil solution, which additionally decreases rate of Ca uptake. Slowik (1979) showed that
‘McIntosh’ apple trees grown in zones with high rainfalls during the growing seasons had higher Ca levels in leaves compared to ones from zone with poor precipita-tions. Goode and Ingram (1971) in an irrigation experiment with ‘Cox’s Orange Pippin’ apple trees showed lower concentration of fruit Ca with decreased soil moisture. Thus, on sandy soils in seasons with low rainfalls, irrigation may improve Ca nutrition of apple trees.
9.2. Biological factors
Young, newly formed roots tend to take up the most Ca. Thus, absorption of Ca will be the highest under conditions of active root growth. With increasing distance from the tips, Ca uptake by apple tree roots declines rapidly (Clarkson and Sanderson, 1971).
Atkinson and Wilson (1980) have showed that mature ‘Golden Delicious’
apple/M.9 and Worcester apple/MM.104 had two peaks of root growth during the growing season; one peak occurred in late spring and a second one in midsummer.
These authors proved also that newly planted apple trees had different pattern of seasonal root growth compared to mature trees. When trees were young, root growth and shoot growth occurred simultaneously. After three years, the main peak of root growth did not begin until the rate of shoot growth had started to decrease.
According to many authors, uptake of nutrients by apple trees is influenced by rootstocks. Skrzynski (1998) showed that P2 and P22 rootstocks had higher ability to take up Ca than P60 and M.26. In this experiment, P14 and M.9 rootstocks took up the least Ca. Fallahi et al. (1984) reported that leaves of ‘Starkspur Golden Delicious’ on OAR-1 rootstock had significantly lower Ca concentrations compared to those on M.7, MM.106, and M.1. Granger and Looney (1983) showed that one-year-old apple trees on M.26 accumulated more Ca in leaves than those on M.7, MM.106 or MM.111. Recently, Fallahi et al. (2001) showed that ‘BC-2 Fuji’ leaves on B9 rootstock had higher Ca status than those on Ottawa 3 and M.7 EMLA.
Head (1969) reported that fruiting of apple trees reduced a rate of root growth;
this effect was observed even at light fruit load. In this experiment, heavy fruiting might even eliminate the growth peak found in de-blossomed trees in July through September. The negative effect of cropping on root growth and consequently on Ca uptake is particularly pronounced on apple trees grafted on dwarf rootstocks (Avery, 1970). Pruning of apple trees may also affect root growth. Head (1967) showed that severe dormant pruning stimulated shoot growth and reduced root growth during summer. Therefore, in apple orchards with severe Ca deficiency in fruit, pruning should be performed 1–2 weeks before flowering. Calcium uptake may also be influenced by planting density. With increasing planting densities, uptake of water and Ca per root length unit usually decreases. This is caused by overlap-ping of the depletion zones of individual roots and reflects interroot competition
for Ca. The abundance of newly formed roots depends also on physical conditions of soil. Generally, a higher number of small roots usually is found in porous soils.
Thus, on soils with high bulk density having reduced porosity (macropores > 30 µm), absorption of Ca may be limited.
It seems that the status of Ca in fruit is cultivar dependent. Under Polish con-ditions, ‘Jonagold’, ‘Szampion’ and ‘Gala’ apples have usually lower Ca levels (150–350 mg·kg–1 dry weight) compared to ‘Idared’ and ‘Lobo’ fruit (500–700 mg·kg–1 dry weight). Consequently, ‘Idared’ and ‘Lobo’ fruit have high storability even when they are stored in a cold storage. Concentration of Ca in fruit flesh is also associated with fruit size and the number of seeds. Large fruit usually have low Ca concentrations in flesh tissues. Therefore, heavy thinning, severe winter pruning and spring frost decrease fruit Ca as a result of increase in fruit size. The fruit with higher number of seeds are usually rich in Ca which probably is caused increased production of auxins in fruitlets. The number of seeds in the fruit depends mainly on weather conditions during flowering and the presence of bees in this period. If unfavorable conditions to pollination occur during flowering (heavy rain-falls and/or low temperatures), fruit are poor in seeds and consequently in Ca. The number of seeds is also cultivar dependent. Generally, ‘Lobo’, ‘Idared’ and ‘Elstar’
fruit having high number of seeds are rich in Ca. Status of Ca in fruit depends also on fruit position on tree. The fruit from the upper regions of trees have lower Ca concentrations compared to those from the bottom ones. Lower concentrations of Ca in fruit from upper zone of the canopy is caused not only by larger fruit size but also by decreased Ca accumulation into these fruit. Consequently, the fruit from tree top are usually more mature at harvest than those from the bottom of the canopy. To obtain fruit with high storability, harvest should be performed many times in season beginning from fruit from upper tree regions. Status of Ca in fruit is also related to position of fruit on the spur. Central fruit on a spur have higher Ca concentrations than lateral fruit. However, when fruit are thinned to one per spur, central and lateral fruit have similar Ca concentrations. This indicates that compe-tition for Ca rather than position per se is critical factor in Ca accumulation into fruit. An important role in accumulation of Ca into fruit plays spur leaves (Wojcik and Mika, 1998a; Volz et al., 1996). As transpiration rate of spur leaves increases, movement of Ca into fruit enhances. It is suggested that this phenomenon is related to diffusion of Ca from spur leaves to the fruit. The effect of spur and bourse leaves on rate of Ca accumulation into the fruit is particularly pronounced in the early stages of fruit development. Another factor influencing fruit Ca level is crop load. It is well known that fruit from lightly-cropping trees have low Ca levels.
This phenomenon is observed regardless of fruit size. Low Ca concentrations in fruit from lightly-cropping trees results not only from increased fruit size but also from strong competition for Ca between leaves and fruit; leaf tissues have higher capacity to accumulate Ca than fruit tissues. Therefore, on young trees or in seasons with low tree cropping, fruit are particularly sensitivity to Ca-related physiological disorders.
10. TREATMENTS INCREASING FRUIT CALCIUM