Variation in carbon isotope discrimination during growth and at different organs in sugar beet (Beta vulgaris L.)

It is widely documented that higher carbon isotope discrimination (Δ) is generally associated with lower water use efficiency. As a consequence, Δ has been evaluated and used as a possible selection criterion in several crops. Its variation in time and between plant organs has not however been very much studied in sugar beet, despite the likelihood of improving the effectiveness of selection strategies. The research reported here was carried out over 3 years, 2000–2002, with the objective of investigating the variation of Δ in plant organs of sugar beet, as affected by growth phase and water regime. Results showed that Δ can change significantly over time and among organs. Leaf had the highest values of Δ (on average 21.2‰) and root, the lowest (18.8‰). Furthermore, root Δ showed a relatively low variance (i.e. 0.31), being weakly affected by water regime and time. In contrast, leaf Δ changed drastically between young and mature plants. There was a significant ‘time × water regime’ interaction in leaf A in 2001: in irrigated plants, leaf Δ increased over time, while in rainfed plants it decreased, which may be due to the persistence of photosynthetic active leaves. Root Δ was related positively to sucrose content (Brix%) and negatively to root biomass accumulation (r = 0.46** and −0.48**, respectively). These results suggest that the variation of Δ among organs or with plant age, if ignored, can lead to an unwanted increase of source of variance. Selecting on root Δ may be the most effective criterion for the lowest unwanted source of variance.