Carbohydrate regulation of leaf development: Investigating the role of source strength, carbon partitioning and hexokinase signaling in regulating leaf senescence

For many plants, growth in elevated CO2 leads to reduced rates of photosynthesis. To examine the role leaf ontogeny plays in acclimation; we monitored photosynthesis and some related parameters throughout the development of tobacco leaves under ambient and high CO2 conditions. The pattern of photosynthetic rate over time was consistent with the expected pattern for a typical dicot leaf for both. However, the photosynthesis pattern in high-CO2-grown tobacco was shifted temporally to an earlier maximum and subsequent senescent decline. Rubisco activity appeared to be the main factor regulating photosynthetic rates in both treatments. Therefore, we propose a new model for interpreting acclimation. Lowered photosynthetic rates appear to be the result of a shift in timing of the normal photosynthetic stages of leaf ontogeny to an earlier onset of the natural decline in photosynthetic rates associated with senescence. To evaluate the converse situation of decreased source strength we examined leaf development in rbcS antisense mutants of tobacco. We found that antisense leaves are longer-lived than wild type and that this appeared to be due to a prolongation of senescence, as monitored by photosynthetic rates, chlorophyll content, and the abundance and activity of Rubisco. These declines were accompanied by coordinate reductions in the levels of rbcS and rbcL mRNA, chloroplast rRNA, chloroplast DNA and total protein. We suggest that the prolongation of senescence in the antisense leaves is due to an impact of source strength on leaf developmental programming that occurs at the level of transcript abunDance; To better understand the relationship between carbohydrates and development, we characterized carbon partitioning over ontogeny. Partitioning into ionic fractions, such as amino acids, was maintained at a steady state longer than partitioning into carbohydrates, which followed photosynthetic rate patterns. The ratio of hexoses to sucrose increased up to full leaf expansion, then declined again; however changes in invertase and hexokinase activities did not correlate with this pattern, although they paralleled each other