Effect of mycorrhizal-enhanced leaf phosphate status on carbon partitioning, translocation and photosynthesis in cucumber

An assessment of the effects of arbuscular mycorrhizal (AM) infection on ph otosynthesis, carbon (C) allocation, translocation and biomass production o f cucumber, grown in sand culture, was made using a previously determined p hosphorus (P) supply (0.13 mol m(-3) P) which had a significant impart on A M infection, Separation of a direct effect of AM infection from an indirect one due to an enhanced leaf P status was achieved using a comparable non-m ycorrhizal treatment (NAM + P) supplemented with extra P (0.19 mol m(-3) P) . Total leaf P concentration, specific leaf mass, photosynthetic capacity, and incorporation of C-14 into non-structural carbohydrate pools were depen dent on leaf age, Both maximum and ambient photosynthetic rates were signif icantly higher in the youngest fully expanded leaves from AM and NAM + P pl ants which also had the higher leaf P concentrations. There were no differe nces in the total concentrations of starch, sucrose, raffinose or stachyose in young or old leaves among AM, non-mycorrhizal (NAM) and NAM + P treatme nts, However, younger leaves of NAM plants showed a shift in C-14-partition ing from stachyose and raffinose synthesis to starch accumulation. Determin ation of ADP-glucose pyrophosphorylase (AGPase), sucrose synthase and sucro se phosphate synthase enzyme activities revealed that only AGPase activity was correlated with the increased incorporation rate of C-14 into starch in young leaves of NAM plants, Although there were significant AM-specific ef fects on C translocation to the root system, AM plants had similar rate of photosynthesis to NAM + P plants. These results suggest that the increase i n photosynthetic rate in leaves of AM-infected cucumber was due to an incre ased P status, rather than a consequence of a mycorrhizal 'sink' for assimi lates.