effect of different levels of continuous water stress on the yield and quality of three sugar beet lines

a field experiment was carried out during 2001 in the experimental fields of sugar beet seed institute in karaj. the objective of this study was to determine the response of some quantitative and qualitative traits of three sugar beet lines subjected to continuous water stress and normal conditions. three sugar beet lines 7219-p.69, bp-karaj and 7112 as tolerant, semi tolerant and sensitive to water stress , respectively, were subjected to continuous water stress gradient using a line-source irrigation system. three irrigation treatments (full irrigation, slight and severe stress) were applied after complete plant establishment. that these treatments received 1274, 1133 and 740 mm of water and produced root yield of 58.6, 45.8 and 34.7 t/ha, respectively.  in severe water stress, the concentration of k+ and α-amino-n increased, but the concentration of na+ decreased.  thus, in the stress conditions white sugar yield (wsy) was reduced. the comparison between sugar beet lines showed that the line 7112 had greater wsy under both stress and non-stress conditions.  however, the wsy loss of the sugar beet lines due to water stress was 36, 32 and 24% for bp-karaj, 7112 and 7219-p.69, respectively.  in conclusion, the line 7219-p.69 demonstrated the less wsy loss when subjected to continuous water stress conditions. this phenomenon is an indication of different responses of sugar beet lines to type and  severity of induced drought stress and possibly it is due to complexity of plant tolerance to drought stress.