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

Stable carbon isotope discrimination (Delta C-13), photosynthetic performance (A), dry matter accumulation (DW), and sucrose yield (Y-s) 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 Y-s, 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 Y-s 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 Y-s 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 V-cmax and J(max) and then compared with Delta C-13. Mesophyll conductance (g(m)) was estimated by comparing Delta C-13 measured on soluble sugars and gas-exchange data. This approach confirmed the expectation that g(m) 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 gas-exchange measurements, and V-cmax was consistently underestimated when g(m) was not taken into account, especially under severe drought. Root Delta C-13 was found to be strictly related to sucrose content (brix%), Y-s and root dry weight, and this was especially clear when Delta C-13 was measured on bulk dry matter. By contrast, leaf Delta C-13 measured in soluble sugars (Delta(s)) and bulk dry matter (Delta(dm)) were found to correlate weakly to brix% and yield, and this was not surprising as the integration time-scale of leaf Delta(s) and Delta(dm) were found to be shorter than that of root Delta C-13 in bulk dry matter. The effect of water stress on diffusive and biochemical limitations with different integration times ranged from 1 d (leaf Delta(s)) to more than 1 month (root Delta(dm)).