The effect of transient and continuous drought on yield,photosynthesis and carbon isotope discrimination in sugar beet (Beta vulgaris L.)

Stable carbon isotope discrimination (Δ13C), photosynthetic performance (A), dry matter accumulation (DW), and sucrose yield (Ys) of sugar beet were evaluated in a glasshouse experiment under transient (TS) and permanent (PS) water stress. A was significantly reduced under drought, to an extent depending on stress duration. The reduced A was strictly associated with a low DW and Ys, the later being 42% lower in PS than control plants (C). Restoring water steeply increased A and the associated leaf traits (RWC, leaf water potential etc.), but the increase of Ys was negligible. Therefore, the negative effects of severe water stress in the early growth period, though reversible on gas-exchange and most leaf traits, can drastically reduce Ys of sugar beet. Furthermore, A seems not to be effective in predicting sucrose accumulation, although it was very effective in detecting the occurrence of plant water stress. The A/C i model was used to assess the photosynthetic adjustments to continuous or transient drought by calculating the photosynthetic parameters Vcmax and Jmax and then compared with Δ 13C. Mesophyll conductance (gm) was estimated by comparing Δ13C measured on soluble sugars and gas-exchange data. This approach confirmed the expectation that gm was limiting A and that there was a significant drop in [CO2] from the substomatal cavities and the chloroplast stroma both in favourable and drought conditions. Therefore, the carbon concentration at the carboxylation site was overestimated by 25-35% by conventional gasexchange measurements, and Vcmax was consistently underestimated when gm was not taken into account, especially under severe drought. Root Δ13C was found to be strictly related to sucrose content (brix%), Ys and root dry weight, and this was especially clear when Δ13C was measured on bulk dry matter. By contrast, leaf Δ13C measured in soluble sugars (Δs) and bulk dry matter (Δdm) were found to correlate weakly to brix% and yield, and this was not surprising as the integration time-scale of leaf Δs and Δdm were found to be shorter than that of root Δ13C in bulk dry matter. The effect of water stress on diffusive and biochemical limitations with different integration times ranged from 1 d (leaf Δs) to more than 1 month (root Δdm).