Photosynthetic Induction in a C4 Dicot, Flaveria trinervia

Labeling patterns from (14)CO(2) pulses to leaves and whole leaf metabolite contents were examined during photosynthetic induction in Flaveria trinervia, a C(4) dicot of the NADP-malic enzyme subgroup. During the first one to two minutes of illumination, malate was the primary initial product of (14)CO(2) assimiltion (about 77% of total (14)C incorporated). After about 5 minutes of illumination, the proportion of initial label to aspartate increased from 16 to 66%, and then gradually declined during the following 7 to 10 minutes of illumination. Nutrition experiments showed that the increase in (14)CO(2) partitioning to aspartate was delayed about 2.5 minutes in plants grown with limiting N, and was highly dampened in plants previously treated 10 to 12 days with ammonia as the sole N source. Measurements of C(4) leaf metabolites revealed several transients in metabolite pools during the first few minutes of illumination, and subsequently, more gradual adjustments in pool sizes. These include a large initial decrease in malate (about 1.6 micromoles per milligram chlorophyll) and a small initial decrease in pyruvate. There was a transient increase in alanine levels after 1 minute of illumination, which was followed by a gradual, prolonged decrease during the remainder of the induction period. Total leaf aspartate decreased initially, but temporarily doubled in amount between 5 and 10 minutes of illumination (after its surge as a primary product). These results are discussed in terms of a plausible sequence of metabolic events which lead to the formation of the intercellular metabolite gradients required in C(4) photosynthesis.