CO2 ASSIMILATION AND PARTITIONING OF CARBON IN MAIZE PLANTS DEPRIVED OF ORTHOPHOSPHATE

In order to study the effects of inorganic phosphate (Pi) starvation on C4 plants, 3-week-old maize plants (Zea mays L. cv. Brulouis) were grown in a growth chamber on a nutrient solution without Pi over 22 d. During the first 2 weeks, Pi-starved plants grew as well as control plants. The Pi concentration in the plant tissue decreased rapidly with time, which suggests that normal biomass production can be maintained at the expense of internal Pi. In addition, photosynthetic CO2 assimilation measured 4-6 h after dawn was not affected, but the concentration of glucose, sucrose, and starch in leaves was much higher than in the controls. (CO2)-C-14 pulse-chase experiments carried out on the ninth day of treatment showed that (CO2)-C-14 assimilation was perturbed during this initial period, resulting in a larger flow of carbon to both starch and sucrose. At the beginning of the third week of Pi starvation (15 d after treatment) C-14 incorporation into sucrose high relative to controls but this was not the case for starch. At the end of the third week of Pi-deficiency, shoot growth was considerably reduced and fresh weight was only one-third of that of the control plants. The Pi concentration of both the leaf and root tissues was less than 1.0 mu-mol g-1 FW compared to 20-25 mu-mol g-1 FW in the controls. Photosynthetic CO2 assimilation was reduced and the leaf concentration of sucrose and starch, which had begun to decrease after the end of the second week of Pi limitation, became lower than in the controls. These results obtained on maize plants show that photosynthesis and carbon partitioning between sucrose and starch were strongly affected by Pi deficiency, similar to C3 species.