CO2 RESPONSIVENESS OF PLANTS - A POSSIBLE LINK TO PHLOEM LOADING

Of the many responses of plants to elevated CO2, accumulation of total non-structural carbohydrates (TNC in % dry weight) in leaves is one o f the most consistent, Insufficient sink activity or transport capacit y may explain this obvious disparity between CO2 assimilation and carb ohydrate dissipation and structural investment, If transport capacity contributes to the problem, phloem loading may be the crucial step, It has been hypothesized that symplastic phloem loading is less efficien t than apoplastic: phloem loading, and hence plant species using the s ymplastic pathway and growing under high light and good water supply s hould accumulate more TNC at any given CO2 level, but particularly und er elevated CO2. We tested this hypothesis by carrying out CO2 enrichm ent experiments with 28 plant species known to belong to groups of con trasting phloem loading type. Under current ambient CO2 symplastic loa ders were found to accumulate 36% TNC compared with only 19% in apopla stic loaders (P = 0.0016), CO2 enrichment to 600 mu mol mol(-1) increa sed TNC in both groups by the same absolute amount, bringing the mean TNC level to 41% in symplastic loaders (compared to 25% in apoplastic loaders), which may be close to TNC saturation (coupled with chloropla st malfunction), Eight tree species, ranked as symplastic loaders by t heir minor vein companion cell configuration, showed TNC responses mor e similar to those of apoplastic herbaceous loaders, Similar results a re obtained when TNC is expressed on a unit leaf area basis, since mea n specific leaf areas of groups were not significantly different, We c onclude that phloem loading has a surprisingly strong effect on leaf t issue composition, and thus may translate into alterations of food web s and ecosystem functioning, particularly under high CO2.