Boron and the alternate-bearing phenomenon in the date palm (Phoenix dactylifera)

The distribution of boron (B), in date palms is an area that has not been widely explored and is of considerable interest to environmental science. Boron is known as an essential micronutrient, and of all the metalloids, plays a particularly significant role in plant studies. The thrust of this work, therefore, involved measurement, by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), of boron in dates and corresponding leaf and soil specimens, and evaluation of its toxicity and deficiency during the developmental stages of the fruiting season. Thirty-six date samples and 36 leaf specimens of the Fard cultivar; and 36 corresponding soil samples were collected during the Kimri, Bisir and Rutab stages of the fruiting season and subjected to suitable digestion procedures. Sample masses of typically 1 g (dry weight) were prepared in 25 mL dilute acid solution and investigated for trace levels of boron. Variations in boron concentration were observed in all the analysed samples and studied as a function of the phases of the fruit-bearing season. The boron levels were typically between 3 and 8 mg kg^-1 in soil specimens, and 1-6 mg kg^-1 in fruits and leaves. A connection was sought to the alternate-bearing phenomenon by examining the relevant data in "on" and "off" trees. It was discovered that an "off" tree could possibly be identified by lower levels of boron in date leaves than for an "on" tree in the Bisir stage of development. In this regard, the experimental results revealed that the "on" trees produced boron levels above about 3.5 mg kg^-1, and the "off" trees could be characterized by boron levels below this value. In addition, K/B and Ca/B ratios were evaluated as a possible guide to boron toxicity and deficiency. It was found that for normal growth, the magnitude of the K/B ratios for dates, in most cases, fell between 1000 and 2000, and for corresponding leaf specimens between 400 and 2000; and the Ca/B ratios, in most cases, occurred between 700 and 1500 for leaves, and between 150 and 300 for fruit. An interesting observation was that K/B ratios in excess of 2500 were found only in the "off" trees for the leaves in the Kimri stage. The study, therefore, formed an interesting contribution to environmental science in arid regions.