Seminars

In the next century the predictions are ominous. The world population will continue to increase. Also climate change predictions appear to be validated each year by real data regarding weather patterns globally. There is every reason to be concerned that with the disappearance of the polar ice caps there will likely be a drastic reduction in the availability of land for traditional agricultural and forestry pursuits especially in low lying coastal areas. The pressure is on mankind to better understand the primary processes that support all life on this planet. To the best of our knowledge the only process supplying reduced C for life as we know it is the process of photosynthesis.There are two primary groups of living organisms that have evolved with the photosynthetic ability, to trap solar energy and convert that energy into a large array of C-skeletons that constitute the backbone of all life on earth. Very broadly considered these are the aquatic organisms, comprised of algae, diatoms and photosynthetic bacteria, and the many vascular “terrestrial”plants of which our major crops such as rice, wheat and corn are common examples. This lecture examines the relationships between canopy photosynthesis and plant development among the vascular plants by focusing on a major role of leaf canopy, the support of the non-photosynthetic plant parts.

Source leaves have two primary functions. First, they serve as organs of CO2 fixation, and second they are the major sources of reduced C, N and S for growth of developing organs and tissues (sinks). Leaves are heterogeneous structures able to export their photo-assimilates both in day and night periods. Although temporary storage of C and subsequent mobilization of assimilates can occur, quantitatively, the leaves of C3, C3/C4 intermediate and C4 species export most of the reduced C while light energy is being trapped. At ambient CO2, leaves of C4 plants fix and export more C than do related C3 types. During C4 metabolism C fixation and sugar synthesis in bundle sheath cells is enhanced by raising the level of CO2. In some families naturally occurring C4 intermediate species may have rates of C fixation approaching those of their C4 cousins, but display lower C-export and growth rates. Interestingly, many C3 plants that export photo-assimilates, other than sucrose, appear to have translocation fluxes that proportional to fixation are comparable to many C4's. When exposed to elevated CO2 as predicted for the 21st century, these C3 plants achieve net C-fixation rates, and, immediate, leaf export rates comparable with those of C4 types. The implication of our observations regarding C-fixation versus export from leaves needs to be understood in terms of over all plant development of vascular plants where source-sink interactions over long periods control survival and productivity.

For example, depending on the stage of canopy development, acclimation to CO2 enrichment often results in a depression in source leaf photosynthesis that in some cases correlates to more rapid turnover of key leaf proteins, such as RUBISCO. Enhancing leaf and canopy photosynthesis through CO2 enrichment results in higher C fixation and export, but, sink demand must be matched to maintain photosynthetic capacity over time. Interestingly, recent studies with selected transgenic lines of a C3 model plant, Arabidopsis, in which dark respiratory processes were targeted rather than source leaf photosynthetic catalytic processes, we noted that canopy photosynthesis, C-partitioning, relative growth rates and seed oil production were enhanced.

In the next century, as CO2 levels continue to increase "naturally", our approach to genetically target activity of the sinks and thus enhance not decrease loss of reduced C, appears counter-intuitive to improving leaf photosynthetic rates per se. However, this approach appears to be a realistic strategy of improving plant photosynthesis and productivity of many economically important vascular plants.