Characterization and optimization of tillering in direct seeded rice in Ebro Delta Area
Tillering in rice crop plays an important role in yield because of its strong influence on panicle density, which is an important yield component. However, little studies on tillering characterization in direct seeded rice in a Mediterranean environment, such as Ebro Delta area, are known.
Tillering in rice plants starts at the fourth leaf stage and the growing and development of tillers continues until reaching the maximum tiller number stage. From that moment on, some tillers start dying. At the end of tiller senescence, the final productive tiller number or, what is the same, the panicle density, is set.  The global balance between tiller production and tiller senescence is evaluated through the tillering efficiency, which is the ratio of maximum tiller number and panicle density. Tiller mortality is regulated by intraspecific competence established among tillers for sources such as light, water and nutrients. In general, late emerging tillers show lower survival rates because of disposing of less time. Such competition could result in a reduction in potential yield in rice crop. 
The objectives of this Thesis were set according to the general performance of rice tillering process and consist of promoting early tillering and reducing the later one by agronomical practices so as to improve rice yield. The study was structured in two lines of research through which different experiments were designed. The first line consists of the characterization of tillering in direct seeded rice and in a Mediterranean environment under different cultural practices: nitrogen fertilization, sowing dose and water management. Influence of genotype is also considered.  For that purpose, two experiments were carried out from 2007 to 2009. In the first one, sowing dose and two varieties were studied; in the second experiment, water management, nitrogen fertilization and sowing dose were considered. The aim of the second line of research was the control of tillering process by water management is pretended in order to improve tillering efficiency and rice yield. In this occasion, an experiment was set from 2008 to 2009 were different water managements for tillering control were compared. 
In reference to the influence of sowing dose on tillering, a wide range of sowing doses (from 60 to 320 Kg. seeds x ha-1) were studied. Rice is a crop with a high tillering ability that confers the crop the property of plasticity. Such plasticity results in a high stability in panicle density and yield. From 80 Kg of seed x ha-1 or 100 plants x m-2 no response in yield is observed. No optimal panicle density is found. 
Shallow and intermittent water management and early nitrogen fertilization at tillering stages promote early tillering contributing to a better vegetative growth. That could cause a higher tiller production and the subsequent increase in rice yield potential because of the positive relationship between maximum tiller number and panicle density. However, that result not always takes place because of the compensatory effects established from the maximum tiller stage. Tiller senescence, which is one of these compensatory effects, is given by competitive relationships among tillers. Finally, no increase in panicle number and, subsequently neither in yield are found. 
Tiller control consists in the cessation of tiller emergence at the beginning of unproductive tillering. Cessation of tiller emergence occurs by increasing water layer in rice fields and, as a consequence, maxim tiller number decreases due to the reduction of competition among tillers. Consequently, tiller mortality decreases leading to a higher panicle density when it is compared with plots with no tillering control. Then, rice yield increases because of the higher panicle density.