Selection of drought-tolerant half-sib families in sugar beet

One of the most effective ways to deal with drought stress and increase water use efficiency is the development of drought-tolerant varieties which display less reduction in yield per unit water reduction. Therefore, the breeding of diploid pollinators was considered as an essential measure for producing drought-tolerant hybrids. In this study, from 4 diploid pollinator populations, 45 half-sib families were developed and evaluated under drought stress condition and subsequently superior families with high root yield were selected. New families were produced and after final evaluation, 23 families with high performance were selected and crossed with a male sterile monogerm single cross as female parent and eventually 23 test-cross hybrids were produced. These hybrids together with Gadook cultivar and a drought-tolerant hybrid (IR7), as controls, were evaluated in a randomized complete block design with four replications in two experiments including normal irrigation and drought-stress condition at Torogh Research Station in Mashhad, Iran for two years, 2007-08. Normal irrigation was performed after 90 mm evaporation from class A evaporation pan but under stress condition, irrigation was performed based on soil moisture measurement at different soil depths (after soil moisture depletion reached to wilting point). Results showed significant difference among the families in terms of sugar yield in both experiments. Sugar yield was decreased from 15.20 t ha-1 in normal irrigation to 7.10 t ha-1 under drought stress condition. Under drought stress, the highest root yield (55.78 t ha-1) was obtained in test-cross hybrid 12 [(436*231)*SBSI-DR I-HSF-14-P.35)] which was placed in superior group with hybrids 7, 6, 14, 2, 5, 10 and 3. Furthermore, the highest sugar yield (7.93 t ha-1) was obtained in hybrid 7 [(436*231)*SBSI-DR I-HSF-14-P.7]. In order to select drought-tolerant hybrids, Stress Tolerance Index (STI), Stress Susceptibility Index (SSI) and Tolerance (TOL) indices were calculated based on sugar yield. According to the results, test-cross hybrids 7, 9, 12, 18 and 22 were identified as drought-tolerant hybrids but based on sugar yield results, hybrids 7 and 12 were selected as promising hybrids with 7.93 and 7.51 t ha-1 sugar yield, respectively.