Steady state nutrition by transpiration controlled nutrient supply

Programmed nutrient addition with a constant relative addition rate has been advocated as a suitable research technique for inducing steady state nutrition in exponentially growing plants. Transpiration controlled nutrient supply is proposed as an alternative technique for plants with a short or no exponential growth phase. A two-weeks experiment with transpiration controlled nitrogen supply to Pennisetum americanum was carried out to evaluate this method. After an adaptation phase a constant plant N-concentration was maintained, while the relative growth rate decreased rapidly. The transpiration coefficient was almost constant in time and insensitive to moderate N-stress, but increased sharply when plant N-concentration dropped below 1760 mmol/kg DW. Relative growth rate and nitrogen productivity showed a steep decline at the lowest N concentrations (about 1000 mmol/kg DW). Nitrogen productivity was optimal at about 1760 mmol/kg DW. The results show that transpiration controlled nutrient supply is applicable in research and gives accurate results in growth analysis. When the transpiration coefficient is known, the nutrient solution can be adjusted to give any desired plant N-concentration, except for the lowest concentrations.