3 ). The activation state of PRK increased as the amount of enzyme was decreased genetically at both levels of N. At high N a 94% decrease in PRK activity had only a small effect (20%) on photosynthesis 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 photosynthesis (decreased by up to 35%) than at high N. These plants were up to 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 shoot and root was unaltered. In conclusion, low N in addition to a 94% decrease in PRK by antisense reduces the activity of PRK sufficient to diminish photosynthesis, which limits biomass production under conditions normally considered sink limited.The relationship between photosynthesis and growth is a complex one, with growth rate not being well correlated with the rate of photosynthesis on a leaf-area basis . This is because growth also depends on the investment of biomass in growing sinks and investment in leaf area (Chapin et al., 1990;). There have been many thorough investigations of the relationship between photosynthesis, growth, environment, and genotype, but they have not provided definitive information on the relationship between photosynthesis and growth. This is because of the complex interaction between factors, so it has not been easy to decide whether changes in sink growth result from changes in photosynthesis or vice versa. The elucidation of the relationship between photosynthesis and growth requires specific changes in photosynthesis or sink growth independent of other changes. Genetic manipulation of enzymes involved in photosynthesis and/or sink processes provides a means to achieve this. The use of transgenic plants with altered amounts of Rubisco revolutionized the analysis of photosynthesis and its interaction with the whole plant (Quick et al., 1991b;Stitt and Schulze, 1994). However, because Rubisco constitutes such a large proportion of the protein in a leaf (up to 40%;Woodrow and Berry, 1988), a decrease in amounts of Rubisco substantially disrupts the N balance of the plant, making it difficult to establish direct links between photosynthesis and growth and allocation than if there were a more specific alteration in the rate of photosynthesis. What is required is genetic modification of a photosynthetic enzyme that accounts for a fraction of the N content in the leaf.PRK is one such enzyme, accounting for less than 1% of the protein in a leaf (Evans, 1989). Tobacco (Nicotiana tabacum L.) plants were transformed with an inverted cDNA encoding tobacco PRK (Knight and Gray, 1994). First, it was determined that the effect of this modification on photosynthesis under standard growth-chamber conditions was minim...

Transgenic tobacco with RuBP-limited photosynthetic assimilation due to a 95% reduction in phosphoribulokinase activity, had higher specific activities of Rubisco in fresh extracts and after full activation, than in the wild type. Differences in the amounts of a daytime tight-binding inhibitor were sufficient to contribute significantly to these activity differences.

N. tabacum lines that lacked functional Rubisco were transformed with plasmids encoding a chloroplast transit peptide in frame with C. vinosum rbcL and stable transformants generated. However, the transgene was transcribed at a low level and no Rubisco activity or C. vinosum large subunits were detectable in any line.

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