Plant morphogenesis is subject to environmental influence. The underlying control mechanisms are of great interest for crop improvement. A phytotron study on Nippon Bare rice seedlings was conducted to analyze the effects of P deficiency on plant organogenesis (tiller and leaf appearance, root apex number) and allometric relationships (root/shoot weight ratio, specific leaf area (SLA) and specific root length (SRL), leaf blade/sheath length ratio). The results confirmed that the main effect of P deficiency is a reduction of shoot growth to the benefit of the root system. This phenomenon, apparently caused by a reduction in leaf elongation rate without changing final leaf size, explains other changes in morphogenesis, such as longer phyllochron (because leaves take longer to expand) and lower tillering rate (because tillering is largely under trophic control). Allometric parameters such as SLA, SLR, root apex number per unit length and leaf blade/sheath length ratios remained stable. Further studies on the genetic control of phenotypic plasticity are suggested.