Substantial Reduction of Water and Nitrogen Supply Does Not Negatively Affect Quality or Yield of Organically Grown Tomatoes

Tomato (Solanum lycopersicum L. 'Cauralina') plants were grown organically in a plastic tunnel, from 30 March to the beginning of August 2011. From mid-May, four treatments were applied: high water supply/high nitrogen (W+/N+), reduced water supply/high nitrogen (W-/N+), high water supply/low nitrogen (W+/N-) and low water supply/low nitrogen (W-/N-). Irrigation was triggered beginning on 20 May to keep water tension between 10-30 cbars in the W+ treatments, and 30-50 cbars in the W- treatments. In the N+ treatments, 10 kg N ha-1 x 4 was supplied, while no N was supplied during the cropping season in the N-treatments. Water supply was reduced by 23% at the end of the trial in the W-treatments, whereas the nitrate content of plants and maximal photosynthesis were not significantly modified in the N- treatments. By contrast, from mid-June, chlorophyll content was found to be lower in the N-treatments. Midday leaf water potential and stomatal conductance were lower only from the end of June in the W-treatments. Leaf net photosynthesis, respiration, yield, mean fruit mass, fruit taste and concentration in organic acids were not modified by the treatments. In the W+/N-treatment, fruit harvested on 4 August contained more reducing sugars (+ 13%) and vitamin C (+ 35%) than the W+/N+ fruit. There was also a 15% increase in vitamin C in the fruit of the W-/N-treatment when compared with W-/N+ fruit. Our observations suggest that it is possible to reduce irrigation and nitrogen supply in organic tomato crops without any negative effect on yield and quality.