Long term assessment of salinity impact on fruit yield in eighteen date palm varieties
Date palm (Phoenix dactylifera L.) growth and production in the arid regions is adversely affected by increasing soil and water salinities. Although many aspects of date palm nutrition, physiology, genetics, and many more are extensively researched, the evaluation of salinity impact on date palm growth and productivity are limited to short duration studies and/or focused mostly on the seedling stage. A large gap in understanding salinity impact on mature date palms is therefore very evident. A long-term experiment (2001–2016) was established in 2001 using eighteen local and regional date palm varieties from the Arabian Peninsula on 2.5 ha to evaluate the impact of three levels of irrigation water salinity (5, 10 and 15 dS m−1 ECW) on date palm productivity. Eight years of yield data showed that fruit yield at Rutab and maturity stages was severely reduced at high salinity. Rutab fruit yield ranged from 67 to 146 kg tree-1 at low salinity (5 dS/m) while at the high salinity (15 dS m−1 ECw) yields ranged from 21 to 98 kg tree-1. At maturity, fruit yield reduction at the high salinity relative to the low salinity reached 67%. The 18 date palm varieties were widely different with regard to the impact of salinity on yield stability. Regression fitting and Principal Component Analysis (PCA) led to the identification of four different groups of date palm varieties in relation to salinity tolerance and thresholds. Group 1, including varieties Lulu and Barhi, are high-yielding varieties with high salinity tolerance; average salinity level at 50% yield reduction was 12 dS m−1 ECw. The second group includes the varieties Khisab, Sukkari, Jabri, Shahla, which are characterized as high-yielding varieties with sensitivity to increase in salinity, yield decline significantly at elevated salinity. Average salinity level at 50% yield reduction is 10 dS m−1 ECw. The third group includes varieties Fardh, Um Al Hamam, Naghal, Abu-Maan, Rhothan, which are characterized by medium to high yield potential under low salinity with moderate salinity tolerance. Average salinity level at 50% yield reduction is 9 dS m−1 ECw. The fourth group includes varieties Shagri, Khnizi, Nabtat Saif, Ajwat Al Madinah, Khalas, Maktoumi which demonstrate low yield potential with low salinity tolerance. Average salinity level at 50% yield reduction is 8 dS m−1 ECw. The present findings will provide a better understanding of the impact of salinity on date palm yield and guide future work on genetic enhancement and the development of integrated crop management approaches for date palm under marginal conditions.