Silicon Improves Flowering Characteristics of Sunflower and Petunia at Low Phosphorus Rates

Silicon (Si) is recognized as a beneficial element for plant growth.  In addition, it has been shown to reduce the severity of biotic and abiotic stress.  The objective of this study was to determine the effects of Si on plant growth and development at low to moderate phosphorus (P) concentrations.  Sunflower (Helianthus annus ‘Pacino Gold’) and petunia (Petunia ×hybrida ‘Bravo Pink’) seedlings were transplanted into 12-cm diameter pots, one plant per pot, filled with an 85 peat : 15 perlite substrate amended with 1.55 kg∙m^-3 CaCO₃ and 0.52 kg∙m^-3 MgCO₃ (pH adjustment) and a wetting agent.  Plants were irrigated as needed with a complete nutrient solution providing 0.08, 0.16, 0.33, 0.5 or 0.66 mM P, with or without 2 mM Si (potassium silicate).  Plants were harvested nine weeks after transplant.  Sunflowers grown with Si supplementation flowered 2 days earlier than non-amended plants.  Phosphorus application rate affected relative chlorophyll content, flower diameter, stem diameter, and shoot dry weight of sunflower.  Relative chlorophyll content at harvest increased as P increased from 0.08 to 0.33 mM P, then decreased at higher P rates.  Sunflower flower diameter, stem diameter, and shoot dry weight increased as P increased from 0.08 to 0.33 mM, with no further increase above 0.33 mM P.  Net photosynthetic rate of sunflower increased as P increased, from 12.0 µmol∙m^-2∙s^-1 CO₂ at 0.08 mM P to 14.2 µmol∙m^-2∙s^-1 CO₂ at 0.5 mM P.  Petunia shoot dry weight increased and days to flower decreased as P increased, but were unaffected by Si.  Petunia flower number increased as P increased.  Flower number in the 0 and 2 mM Si treatments was similar at 0.08 mM P (7 and 6.4, respectively), but Si-supplemented petunias had increasingly more flowers than non Si-supplemented plants as P increased (17.3 and 22.3 with 0 and 2 mM Si, respectively, at 0.66 mM P).  Overall, Si supplementation did not strongly affect vegetative growth of sunflower or petunia across the range of P concentrations trialed, but did enhance flowering characteristics.