Effects of growth light and nitrogen nutrition on the organization of the photosynthetic apparatus in leaves of a C4 plant, Amaranthus cruentus

Properties of C-4 photosynthesis were examined in Amaranthus cruentus L. (NAD-malic enzyme (ME) subtype, dicot) grown under different light and nitrogen (N) conditions, from the viewpoint of N investment into their photosynthetic components. In low-light (LL) leaves, chlorophyll content per leaf area was greater and chlorophyll a/b ratio was lower than in high-light (HL) leaves. These indicate that LL leaves invest more N into their light-harvesting systems. However, this N investment did not contribute to the increase in the quantum yield of photosynthesis on the incident photon flux density (PFD) basis (Q(i)) in LL leaves. N allocation to ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) was significantly higher in HL-high N (HN) leaves than in other leaves. On the other hand, N allocation to C-4 enzymes [phosphoenolpyruvate carboxylase (PEPC) and pyruvate P-i dikinase (PPDK)] was unaffected by the growth conditions. Maximum photosynthetic rates (P-max) per Rubisco content were similar irrespective of the growth light treatments. Carbon isotope ratios (delta C-13) in the leaf dry matter were more negative in LL leaves than in HL leaves (LL = -19.3 parts per thousand, HL = -16.0 parts per thousand) and independent of leaf N. Vein density was highest in HL-HN leaves, and leaf thickness was unaffected by the growth light treatments. From these results, we conclude that A. cruentus leaves would not acclimate efficiently to low growth light.