Growth-limiting phosphate nutrition suppresses nitrate accumulation in greenhouse lettuce

Lettuce was grown in aerated nutrient solutions containing all necessary plant nutrients except phosphate. Phosphate was supplied at four different exponentially increasing rates of daily dosage. In addition, two levels of light and atmospheric CO2 were imposed. The four phosphate addition treatments resulted in different, fairly constant relative growth rates, whereas little effect was observed from light or CO2 levels. Growth reduction associated with limiting phosphate nutrition resulted in increases in root/shoot ratio, dry matter content and concentrations of sugars and organic acids. By contrast, the concentration of nitrate in the shoots was decreased. The concentrations of nitrate and the sum of sugars and organic acids in the shoot showed a strong negative correlation. Two opposing mechanistic models concerning the effects of environmental conditions on nitrate accumulation in plants are discussed in the light of these results.