The alternate bearing effect is when plants produce abundant crops of fruit in some years, but sparse yields in others. This review summarises recent studies that suggest a connection between the alternate bearing effect in the date palm, Phoenix dactylifera, and the presence of certain chemical elements (mostly metals) in the tree tissues. So far, there is evidence that levels of Ag, B, Ba, Ca, Cd, Cu, Fe, K, La, Mg, Mn, N, Na, Se, Tl, and Zn in the dates and leaflets are affected by the alternate bearing phenomenon. The significance of this work, therefore, emphasises the prospect of using our experimental data to overcome sparse yields, due to the alternate bearing effect, and producing more prolific growth of fruits. The potential agricultural, economic, environmental and health implications of these findings are considered.
Alternate bearing is when plants produce good crops of fruit in some years (“on” years) followed by poor crops (“off” years) [
Alternate bearing in some date cultivars is of growing concern [
On several occasions, attempts have been made to reduce the effect of alternate bearing on the date palm through the judicious use of horticultural techniques. Many groups have reported attempts to balance vegetative and reproductive growth [
Alternate bearing seems more difficult to control and understand in monocotyledonous plants such as the date palm than in dicotyledonous ones. In the latter, some branches could be “on”, but others could be “off” [
In the Fard cultivar of the date palm, alternate bearing is not in regular annual cycles. The gap between “off” and “on” states is in the range of 1 - 3 years [
There are three fruit growth stages in the Fard cultivar and these occur from May to September. The first stage is known as the Kimri stage. Here, the dates are immature and green in colour. The second stage is called the Bisir stage. The dates are more advanced now and their colouration is yellowish-red. Finally, there is the Rutab stage. By this time, the dates are mature, soft and honey coloured. They are eaten by humans at this stage of their development.
This review will show that the concentrations of certain elements vary in “on” and “off” date palms, suggesting a possible link between the alternate bearing effect and the levels of these chemicals. The elements could be manipulated by the phenomenon.
What follows is an element-by-element summary of published findings of the effect of the alternate bearing phenomenon on the levels of selected chemical elements in the date palm. Generally, six “on” and six “off” Fard cultivar date palms of the same age and location were considered, unless stated otherwise. A palm was considered to be “on” when it possessed eight or more bunches of dates, but “off” when it had six or less bunches of fruit. Typically, the date palm samples were pre-treated using standard literature digestion and ashing techniques [
Barium is not an essential element in plants [
Boron is a micronutrient and one of the 16 elements classed as essential in plants [
Element | Development stage | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Kimri | Bisir | Rutab | ||||||||||
Fruit | Leaflets | Fruit | Leaflets | Fruit | Leaflets | |||||||
On | Off | On | Off | On | Off | On | Off | On | Off | On | Off | |
Ag | ND | 1.28 | ND | ND | ND | 2.20 | ND | ND | ND | 0.09 | ND | ND |
Al | 37.1 | 32.7 | 55.1 | 54.0 | 1.55 | 5.68 | 17.5 | 19.4 | 18.2 | 8.86 | 22.6 | 24.7 |
B | 4.6 | 4.5 | 1.3 | 1.5 | 2.5 | 2.2 | 4.4 | 2.7 | 2.8 | 3.3 | 2.3 | 3.0 |
Ba | 0.85 | 0.57 | 0.68 | ND | 2.74 | ND | 6.3 | ND | 18.2 | 7.7 | 5.2 | 6.5 |
Ca | 1041.3 | 969.2 | 2075.4 | 1948.8 | 537.1 | 558.8 | 3509.6 | 3341.3 | 709.2 | 724.6 | 2491.3 | 2575.8 |
Cd | 0.054 | 0.096 | 0.038 | 0.057 | 0.004 | 0.012 | 0.066 | 0.004 | 0.020 | 0.025 | <0.001 | 0.004 |
Cu | 2.8 | 3.1 | 1.5 | 1.7 | 2.0 | 1.3 | 2.0 | 1.2 | 3.4 | 1.9 | 0.4 | 1.8 |
Fe | 6.8 | 5.3 | 74.1 | 67.3 | 6.6 | 302.9 | 7.5 | 1.3 | 8.5 | 176.4 | 27.9 | 37.9 |
K | 6666.7 | 6779.2 | 3446.7 | 3666.7 | 3704.2 | 5025.0 | 2172.1 | 1576.3 | 5325.0 | 5591.8 | 2556.7 | 2424.2 |
La | ND | ND | ND | ND | ND | 2.45 | 0.07 | ND | ND | ND | ND | ND |
Mg | 1515.4 | 1487.5 | 2155.8 | 2088.3 | 765.0 | 844.2 | 1209.2 | 1297.9 | 906.3 | 976.3 | 1433.3 | 1714.6 |
Mn | 5.2 | 4.6 | 9.3 | 8.9 | 2.2 | 8.2 | 9.6 | 7.9 | 3.7 | 3.1 | 7.2 | 7.7 |
Na | 180.5 | 165.3 | 116.1 | 126.6 | 43.4 | 35.5 | 136.4 | 133.9 | 66.4 | 74.0 | 132.2 | 121.9 |
Se | 0.23 | ND | 0.17 | ND | ND | ND | ND | ND | ND | ND | ND | ND |
Tl | 0.47 | ND | ND | ND | 1.04 | ND | ND | ND | ND | ND | ND | ND |
Zn | 1.7 | 2.8 | 1.1 | 1.9 | 6.0 | 4.5 | 7.5 | 1.3 | 1.2 | 6.2 | 0.6 | 2.2 |
concentration in the leaflets of greater than 3.5 mg/kg during Bisir, whereas an “off” palm has less than this (
This heavy metal has no known metabolic function in living organisms [
Calcium is an essential element and helps maintain ionic balance in plants [
Copper is an essential element in plants [
Like Cu, Fe is also classed as an essential element in plants [
The role of lanthanum in plants is relatively under studied. There may, however, be some relationship between La and the alternate bearing effect in the date palm. The Bisir stage could be used to distinguish between “on” and “off” date palms. More specifically, Bisir “off” fruit contained La, but Bisir “on” fruit did not [
Magnesium is an essential element in plants and a macronutrient [
Manganese is an essential element in plants and a micronutrient [
Nitrogen is an essential element in plants and a major constituent of organic substances [
Potassium is a macronutrient and an essential element in plants, where it maintains ionic balance [
Selenium has not yet been identified as essential for plants [
This element is believed to have no known metabolic function in living organisms. There may be a connection, however, between Ag in dates and the alternate bearing effect. It is noticeable that date palm fruits growing on the “off” trees had amounts of Ag in them, but in the “on” trees, this element was not detected [
Sodium is classified as non-essential in plants [
The role of Tl in plants is unclear, but it is known to be a trace element [
Zinc is an essential element and micronutrient in plants [
Several other elements have been considered in the scientific literature regarding a possible connection to the alternate bearing effect. It was found that there was no discernable connection between the presence of detectable levels of Al [
The findings in this review could potentially impact agriculturally, economically, environmentally and on human health.
From an agricultural point of view, knowledge of levels of relevant elements (Ag, B, Ba, Ca, Cd, Cu, Fe, K, La, Mg, Mn, N, Na, Se, Tl, and Zn) in date palm tissues analysed at the very beginning of the fruiting season could be exploited to identify “on” and “off” palms. Consequently, it may be possible to predict whether the date palms will produce abundantly or sparsely in the coming fruit-growing season or even which individual palms will do this. In addition, date producers may attempt to manipulate the date palms with elemental supplements added to the soil to circumvent the “off” seasons and obtain the large quantities of dates synonymous with “on” palms more frequently.
The economic impact could manifest in chemical companies producing fertilisers to try to produce a specialist supplement to overcome alternate bearing in date palms and other plant species of agricultural importance. If successful, this may create new jobs in that area. On the other hand, such supplements would be relatively expensive and may increase the cost of dates to the consumer. This could be countered, however, by a drop in the global date price caused by overproduction of the fruit. This could leave the date growers no better off or even worse off economically.
The environmental impact of an excess use of fertilisers or custom-made mineral supplements could be felt in the surrounding area by, for example, contaminating the land. This pollution could run off into nearby bodies of water following heavy rain and cause eutrophication [
The impact on health is a cause for concern too. Feeding early-season dates to livestock could adversely affect their health or cause hazardous elements such as Ag and Tl to bioaccumulate in their tissues [
The data show clearly that the alternate-bearing effect influences some, but not all, elements in the date palm. Certain trace elemental levels could be used to predict good or bad growth of dates in advance. Our work, thus, makes a marked contribution to environmental studies by highlighting the potential of implementing the experimental data to benefit agriculture and the environment by overcoming sparse yields of fruit. However, not all the elements in the periodic table have been studied yet. As an illustration, the effect of the alternate bearing phenomenon on levels of essential elements such as carbon, hydrogen, oxygen, phosphorous, sulphur and chlorine levels could be studied in the future. This review has also highlighted the potential impact of the alternate bearing effect in the date palm on agriculture, the economy, the environment and human health. Furthermore, alternate bearing showed significant effects on the levels of Ca, K, Mn, Fe and other elements in the dates during some or all of the development stages, but in the leaflets only in Mg and K. Regular application of fertiliser containing these elements could be effective in reducing the effect of alternate bearing on the date palm. The experimental data showed inconsistent variations in the levels of elements in the fruit and leaflets from “on” and “off” date palms. This infers the need for studies over more than one fruit growing season to prove the influence of alternate bearing on the mineral composition of the date palm tissues. One season may influence the next season and be influenced itself by the previous season. To know with greater certainty the effect of the alternate bearing on individual elements or small groups of elements in the date palm, carefully controlled experiments should be undertaken with fewer uncontrolled variables. An interesting extension of this review would be to widen the field of study to alternate bearing in other perennials in arid lands. Are the same general trends found in the date palm observed in other species affected by alternate bearing?
The authors wish to thank the staff and students of Sultan Qaboos University, Sultanate of Oman for their assistance with the original research that formed the basis of this review.
John R.Williams,Avin E.Pillay, (2015) Heavy Metals and the Alternate Bearing Effect in the Date Palm (Phoenix dactylifera). Journal of Environmental Protection,06,995-1002. doi: 10.4236/jep.2015.69088