Yield response factor to water (Ky) and water use efficiency of Carthamus tinctorius L. and Solanum melongena L.

Results of a study carried out on safflower and eggplant, in order to calculate the yield response factor to water (Ky) and the water use efficiency (WUE) changes in relation to soil water availability are reported. The final aim of this study is to verify the possibility to use the Ky coefficient to select the better irrigation scheduling criteria for these species. In particular, we considered the WUE at different scale levels, through an up-scaling process of this parameter from the leaf level to the total and marketable yield, and we evaluated the validity of the relation proposed by Kirda [Kirda, C., 2002. Deficit irrigation scheduling based on plant growth stages showing water stress tolerance. Deficit Irrigation Practice. Water Rep. 22. FAO, Rome, pp. 3–10] that relates the WUE to the Ky parameter. During the whole growing cycle, the two crops underwent five irrigation regimes, with a restoration of 100, 75, 50, 25 and 0% of the maximum crop evapotranspiration. The WUE was measured at leaf level (WUEl, mmolCO2/mmolH2O) and at crop level as the ratio between total epigeous dry biomass produced and total water use (WUE = SS/ET), as the ratio between total yield and total water use (TYWUE = SS TY/ET) and as the ratio between dry marketable yield and total water use (CYWUE = CY/ET). The Ky, was obtained by the angular coefficient of the regression line passing through the origin between the relative evapotranspiration decrement and the relative production decrements observed in the different irrigation treatments. The obtained results confirm the validity of yield response coefficient to water (Ky) as a synthesis parameter to quantify the crop tolerance to water stress and to better direct irrigation scheduling. In fact, safflower, with a value lower than 1 of Ky, shows a good tolerance to water deficit regimes with little production decrements and a substantial stability in water use efficiency. Thus, this specie adapts well to deficit irrigation scheduling criteria, in consideration also of economic sustainability that this crop can maintain in deficit irrigation regimes. On the contrary, eggplant, with a greater than 1 Ky, pointed out some sensitivity to water stress, with high marketable yield decrements and a drop in water use efficiency with respect to fresh marketable yield when deficit water regimes are adopted. So, for this last specie both for a more efficient water use and to preserve the crop economic sustainability, an irrigation scheduling criterion which foresees a complete restoration of water consumption during the whole cycle is advisable. Moreover, the obtained results confirm the validity of the relation between Ky and YWUE, proposed by Kirda [Kirda, C., 2002. Deficit irrigation scheduling based on plant growth stages showing water stress tolerance. Deficit Irrigation Practice. Water Rep. 22. FAO, Rome, pp. 3–10]; so, Ky, although it is an empirical parameter which needs further tests, could promote useful indications for irrigation scheduling in the future.