The absence of photoinhibition during the mid-morning depression of photosynthesis in Vitis vinifera grown in semi-arid and temperate climates

Fernao Fires and Pinot noir grapevines were grown in field-sites in Lisbon and Versailles respectively. The grapevines at the Versailles site were grown in pan in artificial media while the vines in Lisbon were grown in soil in the field. A substantial inhibition of net CO2 assimilation from mid-morning onwards was observed at both sites. Prior to the period of measurement the Lisbon site had received no rain for 45 days but the Versailles vines were watered twice a day. In neither case was there any evidence to suggest that substantial photoinhibition was occurring during the photoperiod. The F-v/F-m ratio was relatively constant from dawn until dusk. The foliar zeaxanthin content showed a marked diurnal variation with maximum values obtained at midday and at minimum values at dawn and dusk. Interestingly, the total xanthophyll poor was greater in the leaves of Versailles Vines than the Lisbon site. The maximum zeaxanthin levels of the former at midday were three times those of the latter. The foliar pools of ascorbate and glutathione either remained constant or increased slightly from dawn to dusk. The pools of glycine and serine in the leaves declined from mid-morning onwards at both sites but the ratio of 3-PGA/triosephosphates remained constant: throughout the photoperiod. This implies that: energy supply and utilisation in the leaves were well-balanced throughout the photoperiod despite the decrease in CO2 assimilation. We conclude that similar strategies for the regulation of the photosynthetic apparatus occurred in both cases. This afforded effective protection against photoinhibition. Even though the vines grown in Lisbon were subjected to a significant period of drought they exhibited lower levels of stress-indicating substances particularly proline and zeaxanthin than the vine leaves grown in Versailles. We are drawn to the conclusion that vines grown in pots with the resultant restriction on root growth suffer greater stress than the field-grown vines. The cooler growth conditions encountered in Versailles may be suboptimal for grapevines and, hence, add additional stress factors.