Plant development and reproduction

Gene modification to produce plants more suitable for high-density orchards can be performed both on cultivars and rootstocks (Table 3.1). Plants with an extensive root system and/or with reduced water consumption or changes in canopy architecture, dwarf and semi-dwarf canopy, with short and numerous shoots, could increase orchard density and improve plant performance. At present, reduction of plant size is achieved by using mainly dwarfing rootstocks and, in a few cases, by using spur varieties, selected by clonal selection or among seedling population. Both dwarfing rootstocks and spur varieties are available for only a few species and graft compatibility often presents a problem. Biotechnology techniques may contribute to the creation of dwarfing rootstocks and dwarf varieties either by somaclonal variation (better if in combination with gamma irradiation treatments) or by genetic engineering to modify hormone activity or light receptors. in vitro cultures treated with ionising radiation frequently produce shoots modified in their growth which maintain this characteristic also in the field. More interesting, however, is a transformation approach with some already available genes. Both phytohormone, or phytohormone- like and phytochrome genes seem to be good candidates in modifying plant architecture, particularly in response to different light conditions. Phytohormones are recognised as modulators of growth and differentiation in plants, since their levels can influence the growth rate such as branching, apical dominance, flowering, sex determination, regrowth and rest period. Since their synthesis is linked to light quality, modifying genes for hormone synthesis or for light receptors (phytocromes), some interesting modifications should be expected.