Influence of Daily Photosynthetic Photon Flux Density on Foliar Carbon Metabolite Levels in Nitrogen-Limited Soybean Plants

Abstract

A study was undertaken to examine how the daily photosynthetic photon flux density (PPFD), during prolonged inorganic nitrogen (N) limitation of vegetative soybean plants, influenced the magnitude of leaf photosynthate partitioning into starch and sucrose as well as into anaplerotic products, e.g., phosphoenolpyruvate and malate. Glycine max cv. Williams plants were grown from emergence to preflowering (≈27-32 d) under either N-Sufficient (N-Suff) conditions (14.5 mM NO-3 plus NH+4) or N-limited (N-Lim) conditions (3.0 mM NO-3 plus NH+4). N-Suff and N-Lim plants were grown from emergence in the growth chamber with daily PPFD of 500 or 750 μmol photons m-2 s-1. Measurement of foliar gas exchange revealed that regardless of the PPFD level, net photosynthesis (Pn) rate in mature leaflets of N-Suff plants was 1.2 times greater than those of N-Lim plants. Additionally, Pn rate in N-Suff leaflets, as well as N-Lim leaflets, was 1.4 times greater for plants grown at 750 compared with those grown at 500 μmol photons m-2 s-1. Regardless of the daily PPFD level, foliar stomatal conductance was higher in N-Suff compared with N-Lim leaves, but this did not appear to be a factor influenced by higher light in N-Lim plants. The shoots of N-Suff and N-Lim plants grown in the higher light accumulated ≈1.7 and 1.6 times more dry matter, respectively, than their counterparts grown at lower light energy. Further, both N-Suff and N-Lim plants grown at the higher light energy displayed at least two times higher root mass. In both N-Suff and N-Lim plants, higher daily PPFD, compared with lower PPFD level, resulted not only in higher starch and sucrose accumulation but also in a more rapid daily rise in malate levels and higher malate net accumulation rates. During prolonged N limitation of soybean plants in elevated PPFD, foliar starch and sucrose accumulation was predominant over anaplerotic metabolites. It was concluded that in spite of N limitation, soybean plants growing in elevated PPFD displayed a more active foliar starch and sucrose accumulation as well as a more active anaplerotic anabolism than the lower-PPFD-adapted plants, in part because the higher rate of photosynthesis supplied higher levels of carbon metabolites.