Bases ecofisiológicas de la producción de la cebolla (Allium cepa, L.)aportaciones para la mejora de las técnicas de cultivo en el Pla d'Urgell (Lleida)

Laburpena

The aim of this work was the improvement of agricultural practices on an onion crop (Allium cepa L.) and, at the same time, to study of its eco-physiological production bases. The research was carried out in the Pla d¿Urgell area (Lleida) where it is the most important vegetable crop. The most common cultivar, sown from January onward, is Valenciana de Grano (V. de Grano). It is harvested during the second fortnight of August for fresh consumption. It is usually stored for marketing through the winter. The onions are border irrigated and water is regulated by turns. The area has a semi-arid climate (400 mm year -1) with heavy winter frosts and high summer temperatures. All the experiments were done over six years in the field, except for one done out of soil. Soils were calcareous and free of salinity with textures ranging from sandy to clay. Research was complemented with a field trial at the Horticultural Research International in Wellesbourne (Great Britain). At the beginning of this study, the factors limiting onion production in the area were plant establishment, fertilisation, water management, production orientation and harvest system. All these factors, except the last one, have been analysed in this PhD thesis. The diversification through new cultivars of production for dehydration purposes has been evaluated. Staro and Southport White Globe (S.W.G.), with a similar growth cycle as V. de Grano were evaluated under the different management systems and seem to be promising cultivars. The introduction of a high frequency irrigation system led to a threefold increase in yields over those obtained with the traditional irrigation system given that the latter did not satisfy plant water requirements. Yields of dry bulb matter close to 10 Mg ha-1 were obtained. March sowing is advisable with the new irrigation system. In the old system, sowing in late January or early February was recommended, while earlier sowing was not advisable because of low establishment. The use of priming seeds advances emergence. With a high irrigation frequency, 90 % of root length (RL) is concentrated in the first 40 cm of soil depth, with a root length density of 8 - 9 cm cm-3 in the first 20 cm. A linear relationship between RL and shoot dry matter can be established. According to this relationship and for a fixed shoot dry matter, V. de Grano shows a higher RL than Staro and S.W.G. The maximum RL production is recorded two weeks before the onset of bulbing. In soils with basic or slightly acid pH and low P and K fertility levels, the response to an injection of ammonium phosphate "starter" fertiliser solution in young seedlings sown in a wide range of densities is consistent with the root growth pattern obtained (low root length density and shallow distribution in the early stages). Initial shoot growth was accelerated by conditions of full water availability. Moreover, P and N content was increased and K content was reduced. Advancement in crop maturity is also obtained. This is the why no significant yield increases in early sowings were obtained. In the later sowings, starter fertiliser reduced the percentage of non-maturing "thick-necked" bulbs. When the water needs of the onion plant cannot be satisfied, starter fertilisers can increase total bulb yields. With respect to broadcast fertilisation with flooding irrigation, basal N fertilisation can be reduced before the first irrigation (limiting leaching risks). However, at least one top dressing at the end of May is needed. With water stress the N bulb content is reduced more than the N leaf blade content. High frequency irrigation facilitates P, Fe and Mn absorption. With this irrigation system it is possible to achieve the quoted high yields with a N plant content in the early stages (biomass < 4 Mg ha-1) below the critical levels established in the literature. Furthermore, it is possible to establish a lineal relationship between shoot dry matter K and N content during the onion crop growth period although it has not been possible to establish one between N and P contents. Maximum nutrient uptake rates occurred at the onset of bulbing with 6.3 kg N ha-1 day-1, 0.9 kg P ha-1 day-1, 10.6 kg K ha-1 day-1, 5.4 kg Ca ha-1 day-1 and 0.7 kg Mg ha-1 day-1. With a production system where yields are close to the potential yields in the area, the equilibrium N:P:K:Ca:Mg of bulb extractions shows the ratio 8:1:9:2:0.3. Phosphorous extractions are between 35 and 38 kg P ha-1. A luxury K consumption may exist. Nutrient concentrations in the soil solution necessary to sustain the nutrient fluxes into the root system by diffusive supply have been calculated. They can be useful as complementary criteria in the establishment of new fertility indices for this intensive production system and for fertigation management. Under this intensive management system, the leaf area index (LAI) and the fraction of intercepted photosynthetically active radiation during the leaf growth period, prior to the onset of bulbing, can be estimated from vegetation indices calculated from onion canopy reflection measurements on the 660.9 nm and 813.4 nm bands. Biomass can also be predicted from spectral reflectance as a result of the previous relationships. With a high frequency irrigation system where the sowing date is delayed by even two months with respect to the traditional flood irrigation system, bolting increases very sharply as it also does with density. This phenomenon, observed when vernalisation requirements are satisfied, can be related to LAI. Differences in sheath and blade N content cannot explain density differences in bolting percentage. If high yields are to be maintained, a LAI reduction is not feasible. One alternative could be to reduce the number of visible inflorescences. In this case, it would be advisable to pull up onion plants at the onset of maturity but this solution deserves further study.