Activity in Sun and Shade Plants1

The consequences of light adaptation and acclimation of photosynthesis on photosynthetic nitrogen use efficiency (NUE), particularly as it relates to the efficiency of ribulose-1,5-bisphosphate carboxylase (Rubisco) use in photosynthetic CO2 assimilation, was studied in the sun species Glycine max and the shade species Alocasia macrorrhiza. Both G. max and A. macrorrhiza were found to possess the capacity for light acclimation of CO2 assimilation, but over distinctly different ranges of photon flux density (PFD). For each species, light acclimation of photosynthesis had little effect on the rate of photosynthesis per unit Rubisco protein or the light response of Rubisco carbamylation and CAIP metabolism. In contrast, photosynthesis per unit Rubisco protein was significantly higher in G. max than in A. macrorrhiza, due in part to a lower total (fully carbamylated) molar activity (activity per unit enzyme) of A. macrorrhiza Rubisco than that of G. max. Comparison of the light response of Rubisco regulatory mechanisms between G. max and A. macrorrhiza indicated some degree of adaptation, such that carbamylation was higher and CAlP levels lower at lower PFDs in the shade species than the sun species. However, this adjustment was not sufficient for Rubisco in low light grown A. macrorrhiza to be fully active at the growth PFD. Photosynthesis in A. macrorrhiza appeared to become RuBP regeneration-limited at lower PFDs than G. max, and this was probably the determinant of the light saturated rate of photosynthesis in the shade species. The low efficiency of Rubisco use in A. macrorrhiza was a major contributing factor to its fiveto sixfold lower photosynthetic NUE than G. max. Shade species such as A. macrorrhiza appear to make far from maximal use of Rubisco protein N. Plant species are typically genetically predisposed (adapted) for growth over a specific range of PFD.2 These so-called sun or shade plants may also possess the capacity to respond to differences in PFD within the PFD range which they are adapted to grow (acclimation). Both adaptation and acclimation to different PFDs involve numerous changes in the 'This research was supported by the U.S. Department of Agriculture Competitive Grants Office under Grant No. 87-CRCR-1-2470 and by the National Science Foundation under Grant No. DCB-8796314. 2Abbreviations: PFD, photon flux density; CA IP, 2-carboxyarabinitol 1-phosphate; C,, intercellular CO2 partial pressure; N, total nitrogen; NUE, nitrogen use efficiency; RuBP, ribulose 1,5-bisphosphate; Rubisco, RuBP carboxylase (EC 4.1.1.39). morphology, physiology, and biochemistry of the plant, including photosynthesis (for review, see ref. 2). Adaptation/ acclimation of the photosynthetic apparatus involves changes in the levels of carbon reduction cycle enzymes, electron transport components, and proteins and pigments associated with light harvesting. This adaptation/acclimation is often characterized by a redistribution of resources among these components of the photosynthetic apparatus, and is dominated by the capacity of the plant to change the proportion of leafN dedicated to Rubisco protein (for review, see ref. 6). Since as much as 20 to 25% of total N in a leaf may be contained in Rubisco (5), changes in the activity and/or regulation of this enzyme associated with light adaptation/ acclimation could have a considerable impact on photosynthetic NUE. Rubisco activity is light-dependent, both because production of the substrate RuBP is dependent upon ATP and NADPH production, and because mechanisms for the control of this enzyme's activity are linked to PFD (for review, see refs. 9 and 11). These mechanisms, carbamylation-decarbamylation, Rubisco activase, and CA 1P metabolism, affect the efficiency of Rubisco use. At low PFDs, where the capacity for RuBP regeneration typically limits photosynthesis, the efficiency of Rubisco use is potentially low, as evidenced by the fact that the activity of the enzyme is generally reduced by these regulatory mechanisms to match the reduced capacity for RuBP regeneration (3, 8). Plants which grow at low PFD might be expected to produce less Rubisco per unit area than plants growing at high PFD, and regulate its activity in such a way that it was fully active at lower PFDs than plants growing at high PFDs. In this paper, a hypothesis is proposed concerning how plants which grow at different PFDs might adjust the regulatory characteristics of Rubisco in order to maximize photosynthetic NUE. To test this hypothesis, the photosynthetic characteristics and Rubisco regulatory properties oftwo species which are adapted to grow at substantially different PFDs were examined. Glycine max (soybean) is adapted for growth at relatively high PFDs. Alocasia macrorrhiza (an Australian tropical understory species) is generally considered to be adapted for growth at low PFDs. The results reported in this paper demonstrate that each species possesses a significant capacity for acclimation of photosynthesis to different PFDs within the bounds of the PFD ranges to which they are adapted. Photosynthetic NUE of both species was conserved during light acclimation, primarily through adjustments in the level of Rubisco protein. However, photosyn-