INFLUENCE OF PHOSPHORUS LIMITATION ON PHOTOSYNTHESIS, CARBON ALLOCATION AND PARTITIONING IN SUGAR-BEET AND SOYBEAN GROWN WITH A SHORT PHOTOPERIOD

In previous studies with plants grown with a long photoperiod (16 h light: 8 h dark), we demonstrated that phosphorus (P) limitation caused marked reductions in leaf expansion, shoot growth, photosynthesis and leaf sugar phosphates and increases in levels of foliar starch in both sugar beet (Beta vulgaris [L.]) and soybean (Glycine max [L.] Merr.). In the present study, we investigated the effect of P limitation on plants grown with a short photoperiod (8 h light: 16 h dark). Plant P levels were varied nutritionally to provide P sufficient (high-P) and P-limited (low-P) plants. The major effect of low-P was to decrease the growth of leaves, especially the expansion of the leaf surface (total leaf areas were reduced from 60 to 65% and shoot dry mass by 50 to 60%). In contrast, root dry mass was unaffected (sugar beet) or increased (soybean) in low-P plants. Light+CO2 saturated photosynthesis and conductance were decreased in low-P plants (from 29 to 58%) while photosynthesis at ambient light and CO2 was affected less. In sugar beet, the reduction in photosynthetic rate with low-P was most likely due to decreased RuBP regeneration and not to decreased Rubisco activity which remained high. Comparison with earlier long photoperiod studies show that low-P treatment over the shorter photoperiod affected growth, photosynthesis and carbohydrate levels in substantially similar ways (except that in sugar beet, low P did not result in an accumulation of starch, sucrose and glucose in leaves as with long photoperiod). Short photoperiod decreased plant growth rate (photosynthate production was diminished by halving of the light period) but did not affect the partitioning of dry matter between root and shoot. Compared to long photoperiod, short photoperiod increased substantially the rate of starch accumulation during the day but had little effect on starch pool sizes.