EFFECTS OF AIR-FILLED SOIL POROSITY AND AERATION ON THE INITIATION AND GROWTH OF SECONDARY ROOTS OF MAIZE (ZEA-MAYS)

We studied the possibility whether the initiation of secondary roots i s regulated by the air-filled porosity in soil, i.e. the availability of oxygen in the soil. Maize plants were grown in long PVC tubes (1 m long and 12 cm diameter) and were unwatered for different numbers of d ays so that variations of soil water content with depth were achieved on the same date with plants at the same age. The plants were harveste d when their root systems were established in the whole soil column an d watering had been withheld for 0, 15, 20, 25 days. A decrease of soi l water content was significantly correlated with an increase of air-f illed porosity in soil. The number of secondary lateral roots from seg ments of primary adventitious roots increased dramatically when soil w ater content decreased from field capacity to about 0.05 g water g(-1) dried soil. The total dried mass of roots at different soil depths wa s also positively correlated with soil air-filled porosity. It was obs erved that the elongation of the initiated secondary roots responded d ifferently to the variations of soil air-filled porosity. The length o f secondary roots increased initially when the soil was dried from fie ld capacity to 0.18 g g(-1) dried soil (water potential at about -0.2 MPa, air-filled porosity 0.26 cm(3) cm(-3)), but was drastically reduc ed when the soil was dried further. Obviously elongation of secondary roots was inhibited when soil water potential began to deviate substan tially from an optimum value. The present results suggested that the i nitiation of secondary roots was greatly promoted by the increase of a ir-filled soil porosity, i.e. availability of oxygen. This conclusion was further verified in a separate experiment where solution-cultured maize seedlings were subjected to different aeration treatments. An ob vious increase in secondary root initiation was found in plants which were aerated with normal air (21% O-2) than in plants which were eithe r not aerated or aerated with 5% O-2 air.