Decrease in phosphoribulokinase activity by antisense RNA in transgenic tobacco. Relationship between photosynthesis, growth, and allocation at different nitrogen levels

To study the direct effects of photosynthesis on allocation of biomass by a ltering photosynthesis without altering leaf N or nitrate content, phosphor ibulokinase (PRK) activity was decreased in transgenic tobacco (Nicotiana t abacum L.) with an inverted tobacco PRK cDNA and plants were grown at diffe rent N levels (0.4 and 5 mM NH,NO,). The activation state of PRK increased as the amount of enzyme was decreased genetically at both levels of N. At h igh N a 94% decrease in PRK activity had only a small effect (20%) on photo synthesis and growth. At low N a 94% decrease in PRK activity had a greater effect on leaf photosynthesis (decreased by up to 50%) and whole-plant pho tosynthesis (decreased by up to 35%) than at high N. These plants were up t o 35% smaller than plants with higher PRK activities because they had less structural dry matter and less starch, which was decreased by 3- to 4-fold, but still accumulated to 24% to 31% of dry weight; young leaves contained more starch than older leaves in older plants. Leaves had a higher ion and water content, and specific leaf area was higher, but allocation between sh oot and root was unaltered. In conclusion, low N in addition to a 94% decre ase in PRK by antisense reduces the activity of PRK sufficient to diminish photosynthesis, which limits biomass production under conditions normally c onsidered sink limited.