Growth of Roots Emerged from Excised Phytomers of Three Gramineous Species under a Low Osmotic Potential

The growth of crown and lateral roots emerged from the excised phytomers of pearl millet (Pennisetum typhoideum Rich.), barnyard millet (Echinochloa frumentacea Link.) and maize (Zea mays L.) was studied under normal and low osmotic potential conditions. The plants were grown in two solutions with osmotic potentials of –0.02 and –0.54 MPa for 6 days. The relative growth rate of the roots in total length (RGRL) , was not affected by osmotic stress in pearl and barnyard millets, but that in stressed maize was reduced to 64% of the control. Similarly, the relative growth rate of the roots in dry weight (RGRW), was not affected by osmotic stress in pearl and barnyard millets, but significantly reduced in maize. Osmotic stress increased the specific root length of the lateral roots in pearl and barnyard millets, but did not affect that of maize. The photosynthetic rate (Pr) in the two millets was decreased to ca. 60% of the control by the stress, and that in maize was reduced to 21%. Under the stress, Pr in the three species was limited mainly by low stomatal conductance, but no clear relationship was found between Pr and osmotic adjustment of the leaf. The relative water content of the leaf was lower in maize than in the two millets. The resistance to water flow through the phytomer (R) was significantly increased by osmotic stress in maize, but not significantly in the two millets. The mean root length (RL) was decreased by the stress in maize, but not in the two millets. The maintenance of RGRL in the two millets was associated with sustained R, RL and also with the maintenance of Pr and allocation of assimilates to roots.