Phenotypic Plasticity of Cotton as Affected by Different Plant Densities.

The objective of our study was to determine the impact of plant density on the plasticity of cotton (Gossypium hirsutum L.) in terms of 1) mainstem leaf plastochron; 2) growth and biomass of internodes, petioles and leaves of mainstem; 3) branch morphology, development, expansion; 4) fruiting patterns; and 5) root growth and biomass.  A field experiment was conducted with four plant densities (3, 6, 12 and 24 plants m^-2). Overall, increased plant density resulted in taller plants with fewer nodes. Plant density did not have a significant effect on time of leaf initiation and leaf plastochron for earlier nodes. The longest internode lengths were observed at nodes 12 and 13 for 3 and 6 plants m^-2 and at nodes 9 and 10 for 12 and 24 plants m^-2. Petioles were also longer at lower densities. Total number of branches was significantly reduced with increasing plant density, with 16.2, 15.5, 13.9, and 10.3 branches plant^-1 for 3, 6, 12 and 24 plants m^-2. Total branch leaf area per plant were 42, 59 and 82% less at 6, 12 and 24 plants m^-2 compared to that at 3 plants m^-2. Total fruiting sites (TFS) also decreased with increasing densities. There were 110, 60, 45 and 25 TFS plant^-1 at 3, 6, 12 and 24 plants m^-2, respectively. Similar trends were observed for both boll and square numbers. The total number of retained bolls was 25.6, 13.5, 8.4 and 4.1 at 3, 6, 12 and 24 plants m^-2, respectively. Increasing plant density significantly reduced per organ biomass of mainstem leaves, stems, petioles, branch stems, branch leaves, and reproductive biomass. The results demonstrate significant density-mediated vegetative and reproductive plasticity of cotton probably as a result of competition for above and below ground resources between plants.