YIELD RESPONSE TO WATER AND STRESS INDEXES ON TOMATO

Results of a research carried out in Gaudiano area in 1993 to study the influence of different irrigation regimes on some ecophysiological and yield caracteristics of different cultivars of tomato ('Ponderosa' for long time storage; 'Brigade By' for processing; 'Pomored 1232 Hy' for fresh market;) are reported.

Four irrigation regimes were compared: unirrigated control (U.C.) supplementary irrigation whenever the soil water potential dropped up to - 1.5 MPa (S.I.) and restoration of water volumes equal to 50% and 100% of crop maximum evapotranspiration (ETc).

Soil water potential, leaf water potential and diffusive resistance of lower leaf surface were measured during the crop cycle; total and marketable yield were measured at the harvest. Dry matter accumulation and water use were also measured in order to evaluate the water use efficiency.

Results show that tomato crop water status was strongly influenced by the water regime; diffusive resistance to gaseous exchange increased with water shortage, and stomata were almost fully closed during the most of unirrigated control crop cycle. As consequence of stomatal behaviour, dry matter accumulation was gradually reduced with the increase of water deficit; total biomass dry matter values ranged from 935.7 to 267.5 gm^-2, as average of the three cultivars, passing from the most irrigated treatment to the unirrigated control.

The highest marketable yield was obtained with a restoration of 100% of ETc with a value of 66.4 t ha^-1, as average of the three cultivars, while only 6.21 t ha^-1 were obtained in the unirrigated control. However yield water use efficiency (YWUE) exhibited in average, higher values in the treatment with a restoration of 50% of ETc (1.09 kg m^-3)

Slight differences were observed among the three cultivars in the leaf water potential trend. The 'Ponderosa' diffusive resistance resulted lower than 'Brigade' and 'Pomored' only under water stress conditions, giving a better performance of this cultivar in water limiting situations.