Effect of soil oxygen deficit on iron nutrition of orange seedlings

Iron deficiency chlorosis in plants is often associated with soil conditions that appear to restrict exchange of gases with the atmosphere, such as fine texture, lack of aggregation, excessive moisture, or discontinuity in the profile in the form of plowsole, hardpan, claypan, or stratified deposits. These conditions can cause CO2 to accumulate in the soil and, particularly in calcareous soils, produce substantial concentrations of HCO3 ions. Solution culture studies (1, 14) have shown that NaHCO3 can cause a marked reduction in iron absorption by some kinds of plants. Thus has arisen the so-called bicarbonate hypothesis to explain the variable chlorosis of plants growing in calcareous soils. The bicarbonate hypothesis represents a sound approach to the problem. The difficulty is that a causal relationship between bicarbonate ion and chlorosis in soil-grown plants has not been established. On the contrary, Reuther and Crawford (9, 10) increased concentrations of CO2 in the root zone of young citrus trees growing in calcareous soils to levels of 20 to 25 % without inducing iron chlorosis. The present authors grew sweet orange seedlings in calcareous soil in the greenhouse and added 1 % of CO2 to the soil atmosphere over a period of several months without causing chlorosis or reducing the iron concentration in the leaves (data unpublished). Impeded exchange of gases between the soil and the atmosphere can result, in addition to the accumulation of CO2, in reduction of oxygen concentration. The present study was undertaken to determine if oxygen deficit in the soil may bear a causal relation to iron chlorosis in plants.