Yield and water-production functions of two durum wheat cultivars grown under different irrigation and nitrogen regimes

Wheat (TriticumdurumL.) yields inthe semi-aridregions are limitedby inadequatewater supply
late in the cropping season. Planning suitable irrigation strategy and nitrogen fertilizationwith
the appropriate crop phenology will produce optimumgrain yields. A 3-year experiment was
conducted on deep, fairly drained clay soil, at Tal Amara Research Station in the central Bekaa
Valley of Lebanon to investigate the response of durumwheat to supplemental irrigation (IRR)
and nitrogen rate (NR). Threewater supply levels (rainfed and two treatments irrigated at half
and full soil water deficit) were coupled with three N fertilization rates (100, 150 and
200 kg N ha1) and two cultivars (Waha and Haurani) under the same cropping practices
(sowing date, seeding rate, row space and seeding depth). Averaged across N treatments
and years, rainfed treatment yielded 3.49Mg ha1 and it was 25% and 28% less than half
and full irrigation treatments, respectively, forWaha,while for Haurani the rainfed treatment
yielded 3.21Mg ha1, and it was 18%and 22%less than half and full irrigation, respectively.On
theotherhand,Nfertilizationof150and200 kg N ha1 increasedgrainyieldinWahaby12%and
16%,respectively, incomparisonwithNfertilizationof100 kg N ha1,whileforcultivarHaurani
the increases were 24% and 38%, respectively. Regardless of cultivar, results showed that
supplemental irrigation significantly increased grain number per square meter and grain
weight with respect to the rainfed treatment,while nitrogen fertilizationwas observed to have
significant effects only ongrain number per squaremeter.Moreover, results showedthat grain
yield for cultivar Haurani was less affected by supplemental irrigation and more affected by
nitrogen fertilization than cultivarWaha in all years. However, cultivar effects were of lower
magnitude compared with those of irrigation and nitrogen.We conclude that optimum yield
wasproducedforbothcultivarsat50%ofsoilwaterdeficitas supplemental irrigationandNrate
of 150 kg N ha1. However, Harvest index (HI) and water use efficiency (WUE) in both cultivars
were not significantly affected neither by supplemental irrigation nor by nitrogen rate. Evapotranspiration
(ET) of rainfed wheat ranged from 300 to 400mm, while irrigated wheat had
seasonal ETranging from450 to 650mm.Onthe other hand, irrigationtreatments significantly
affected ET after normalizing for vapor pressure deficit (ET/VPD) during the growing season.
Supplemental irrigation at 50% and 100% of soil water deficit had approximately 26 and
52mmmbar1 more ET/VPD, respectively, than those grown under rainfed conditions.