The growth and tissue mineral concentrations of date palm (Phoenix dactylifera L.) cultivars in response to the ultraviolet-B radiation

The ultraviolet-B (UVB) radiation is an integral part of the sun light and adversely affects the growth and development of date palm (Phoenix dactylifera). Although, date palm is an economically important fruit tree and the majority the world's production is centered in the Arabian Peninsula, the studies evaluating date palm genotypic diversity and response to the UVB radiation are limited. To investigate this, five commercially grown date palm cultivars were exposed to the control and two elevated levels (four and eight hours per day for one year) of UVB radiations. The results showed that UVB radiation decreased plant height and leaf number by 8-14%, shoot root fresh weight by 66-87% and dry weight by 20-23%, total chlorophyll and carotenoids concentrations by 22-28%, while increased the proline and UVB absorbing compounds (phenolics) by 142% and 17.5%, respectively, across cultivars. These changes were more pronounced under the eight-hour UVB exposure. In general, roots tended to have greater concentrations of S and N by ≈1.4 folds, Co, Na, and P by 3.3-7 folds than shoots, compared to average across cultivars and treatments. However, under UVB, several mineral concentrations were either increased (e.g, Ca, Co, Fe, Mn, P) or decreased (e.g, K, Mo, S, N) consistently in shoots and roots. One exception to this rule was the Na concentration that increased in the shoot (9-45%) but decreased in the roots (8-10%) under UVB. Thus, the response of the date palm cultivars to the UVB was dependent on the measured traits, exhibiting decline in the growth parameters despite the increase in the UVB absorbing compounds and the tissue concentrations of several minerals. The principal component analysis (PCA) showed that the cultivars differed similarly in response to a given level of UVB treatments. Based on the PCA, the cultivars were grouped into two groups as BARHI and FRDWT in one group, while KHD, NBSTF, and RFDRD in other group.