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