SPATIAL DISTRIBUTIONS OF TISSUE EXPANSION AND CELL-DIVISION RATES ARERELATED TO IRRADIANCE AND TO SUGAR CONTENT IN THE GROWING ZONE OF MAIZE ROOTS

We have investigated the way in which the radiation absorbed by leaves affects the rate of elongation of maize (Zea mays L.) roots. In five repeated growth chamber experiments, plants previously grown at a phot on irradiance of 23 mol m(-2) d(-1) received either 7 or 34 mol m(-2) d(-1) from day 10 to day 20 after germination, The elongation rate of primary roots steadily decreased for 4 d after reduction in irradiance and then stabilized at 60% of that in plants at high irradiance. The elongating zone was slightly shorter after 2 d at low irradiance, and was further reduced after 8 d, The concentrations of sucrose and gluco se in the elongating zone were greatly decreased after 2 d at low irra diance and the gradient of both sugars was suppressed. The longer peri od at low irradiance affected neither sugar content nor gradient. In t he same way, cell production rate was reduced after 2 d at low irradia nce and was not appreciably decreased afterwards, The root zone with c ell division was shorter in plants at low irradiance, but cell divisio n rate remained nearly constant temporally and spatially; and was unaf fected by the irradiance treatment. Our results suggest that primary e vents after a reduction in irradiance were a change in cell flux and s ugar content in the elongating zone, Change in elongation rate was slo wer and probably the result of a time-related developmental effect, wh ich may be related to the change in cell production.