Photosynthesis and photosynthate partitioning in n(2)-fixing soybeans

Differences in photosynthetic behaviour and leaf senescence of soybean (Glycine max [L.] Merrill) dependent on N2 fixation or nitrate supply

Biological N(2) fixation can fulfil the N demand of legumes but may cost as much as 14% of current photosynthate. This photosynthate (C) sink strength would result in loss of productivity if rates of photosynthesis did not increase to compensate for the costs. We measured rates of leaf photosynthesis, concentrations of N, ureides and protein in leaves of two soybean cultivars (Glycine max [L.] Merrill) differing in potential shoot biomass production, either associated with Bradyrhizobium japonicum strains, or amended with nitrate. Our results show that the C costs of biological N(2) fixation can be compensated by increased photosynthesis. Nodulated plants shifted N metabolism towards ureide accumulation at the start of the reproductive stage, at which time leaf N concentration of nodulated plants was greater than that of N-fertilized plants. The C sink strength of N(2) fixation increased photosynthetic N use efficiency at the beginning of plant development. At later stages, although average protein concentrations were similar between the groups of plants, maximum leaf protein of nodulated plants occurred a few days later than in N-fertilized plants. The chlorophyll content of nodulated plants remained high until the pod-filling stage, whereas the chlorophyll content of N-fertilized plants started to decrease as early as the flowering stage. These results suggest that, due to higher C sink strength and efficient N(2) fixation, nodulated plants achieve higher rates of photosynthesis and have delayed leaf senescence.

Leaflet photosynthesis rate and carbon metabolite accumulation patterns in nitrogen-limited, vegetative soybean plants

Prolonged inorganic nitrogen (NO3 (-)+NH4 (+)) limitation of non-N2-fixing soybean plants affected leaflet photosynthesis rates, photosynthate accumulation rates and levels, and anaplerotic carbon metabolite levels. Leaflets of nitrogen-limited (N-Lim), 27-31-day-old plants displayed ≈ 15 to 23% lower photosynthesis rates than leaflets of nitrogen-sufficient (N-Suff) plants. In contrast, N-Lim plant leaflets displayed higher sucrose and starch levels and rates of accumulation, as well as higher levels of carbon metabolites associated with sucrose and starch synthesis, e. g., glycerate-3-phosphate and glucose phosphates, than N-Suff plant leaflets. Concurrently, levels of soluble protein, chlorophyll, and anaplerotic metabolites, e.g., malate and phosphoenolpyruvate, were lower in leaflets of N-Lim plants than N-Suff plants, suggesting that the enzymes of the anaplerotic carbon metabolite pathway were lower in activity in N-Lim plant leaflets. Malate net accumulation rates in the earliest part of the illumination period were lower in N-Lim than in N-Suff plant leaflets; however, by the midday period, malate accumulation rate in N-Lim plant leaflets exceeded that in leaflets of N-Suff plants. Further, soluble protein accumulation rates in leaflets of N-Suff and N-Lim plants were similar, and the rate of dark respiration, measured in the early part of the dark period, was higher in N-Lim plant leaflets than in N-Suff plant leaflets. It was concluded that during prolonged N-limitation, foliar metabolite conditions favored the channelling of a large proportion of the carbon assimilate into sucrose and starch, while assimilate flow through the anaplerotic pathway was diminished. However, in some daytime periods, there was a normal level of carbon assimilate channelled through the anaplerotic pathway for ultimate use in amino acid and protein synthesis.

Carbohydrate and carbon metabolite accumulation responses in leaves of ozone tolerant and ozone susceptible spinach plants after acute ozone exposure

The objective of this study was to determine whether exposure of plants to ozone (O3) increased the foliar levels of glucose, glucose sources, e.g., sucrose and starch, and glucose-6-phosphate (G6P), because in leaf cells, glucose is the precursor of the antioxidant, L-ascorbate, and glucose-6-phosphate is a source of NADPH needed to support antioxidant capacity. A further objective was to establish whether the response of increased levels of glucose, sucrose, starch and G6P in leaves could be correlated with a greater degree of plant tolerance to O3. Four commercially available Spinacia oleracea varieties were screened for tolerance or susceptibility to detrimental effects of O3 employing one 6.5 hour acute exposure to 25O nL O3 L(-1) air during the light. One day after the termination of ozonation (29 d post emergence), leaves of the plants were monitored both for damage and for gas exchange characteristics. Cultivar Winter Bloomsdale (cv Winter) leaves were least damaged on a quantitative grading scale. The leaves of cv Nordic, the most susceptible, were approximately 2.5 times more damaged. Photosynthesis (Pn) rates in the ozonated mature leaves of cv Winter were 48.9% less, and in cv Nordic, 66.2% less than in comparable leaves of their non-ozonated controls. Stomatal conductance of leaves of ozonated plants was found not to be a factor in the lower Pn rates in the ozonated plants. At some time points in the light, leaves of ozonated cv Winter plants had significantly higher levels of glucose, sucrose, starch, G6P, G1P, pyruvate and malate than did leaves of ozonated cv Nordic plants. It was concluded that leaves of cv Winter displayed a higher tolerance to ozone mediated stress than those of cv Nordic, in part because they had higher levels of glucose and G6P that could be mobilized during diminished photosynthesis to generate antioxidants (e.g., ascorbate) and reductants (e.g., NADPH). Elevated levels of both pyruvate and malate in the leaves of ozonated cv Winter suggested an increased availability of respiratory substrates to support higher respiratory capacity needed for repair, growth, and maintenance.

Influence of different nutrient regimes on the regulation of carbon metabolism in Norway spruce [Picea abies (L.) Karst.] seedlings

Phosphoenolpyruvate carboxylase (PEPC) activity, fructose 2, 6-bisphosphate (F26BP), starch and soluble sugar contents were determined m needles and roots of Norway spruce seedlings grown in a semi-hydroponic cultivation system under different nutrient regimes, tn needles, a surplus of nitrogen caused an increase in specific PEPC activity (up to six times control activity) and F26BP content (up to three times control level) while starch content was reduced. Sucrose contents were not affected. Basically, the responses in root samples were similar. Here, PEPC was highest at an imbalance in nutrition (+ N/ -P) F26BP, with root contents being 3- to 11 -times higher than those in needles, significantly exceeded control values at + N/+ P. The results show that alteration of nitrogen supply leads to marked changes in allocation of carbon between pathways, which is also influenced by P-nutrition. Pool sizes of F26BP and activity of PEPC are indicators for these changes in leaf as well as in root tissues of Norway spruce.

Photosynthate metabolism in the source leaves of n(2)-fixing soybean plants

Soybean plants (Glycine max [L.] Merr. cv Williams), which were symbiotic with Bradyrhizobium japonicum, and which grew well upon reduced nitrogen supplied solely through N(2) fixation processes, often exhibited excess accumulation of starch and sucrose and diminished soluble protein in their source leaves. Nitrate and ammonia, when supplied to the nodulated roots of N(2)-fixing plants, mediated a reduction of foliar starch accumulation and a corresponding increase in soluble protein in the source leaves. This provided an opportunity to examine the potential metabolic adjustments by which NO(3) (-) and NH(4) (+) (N) sufficiency or deficiency exerted an influence upon soybean leaf starch synthesis. When compared with soybean plants supplied with N, elevated starch accumulation was focused in leaf palisade parenchyma tissue of N(2)-fixing plants. Foliar activities of starch synthesis pathway enzymes including fructose-1,6-bisphosphate phosphatase, phosphohexoisomerase, phosphoglucomutase (PGM), as well as adenosine diphosphate glucose pyrophosphorylase (in some leaves) exhibited highest activities in leaf extracts of N(2)-fixing plants when expressed on a leaf protein basis. This was interpreted to mean that there was an adaptation of these enzyme activities in the leaves of N(2)-fixing plants, and this contributed to an increase in starch accumulation. Another major causal factor associated with increased starch accumulation was the elevation in foliar levels of fructose-6-phosphate, glucose-6-phosphate, and glucose-1-phosphate (G1P), which had risen to chloroplast concentrations considerably in excess of the K(m) values for their respective target enzymes associated with starch synthesis, e.g. elevated G1P with respect to adenosine diphosphate glucose pyrophosphorylase (ADPG-PPiase) binding sites. The cofactor glucose-1,6-bisphosphate (G1,6BP) was found to be obligate for maximal PGM activity in soybean leaf extracts of N(2)-fixing as well as N-supplemented plants, and G1,6BP levels in N(2)-fixing plant leaves was twice that of levels in N-supplied treatments. However the concentration of chloroplastic G1,6BP in illuminated leaves was computed to be saturating with respect to PGM in both N(2)-fixing and N-supplemented plants. This suggested that the higher level of this cofactor in N(2)-fixing plant leaves did not confer any higher PGM activation and was not a factor in higher starch synthesis rates. Relative to plants supplied with NO(3) (-) and NH(4) (+), the source leaf glycerate-3-phosphate (3-PGA) and orthophosphate (Pi) concentrations in leaves of N(2)-fixing plants were two to four times higher. Although Pi is a physiological competitive inhibitor of leaf chloroplast ADPG-PPiase, and hence, starch synthesis, elevated chloroplast 3-PGA levels in N(2)-fixing plant leaves apparently prevented interference of Pi with ADPG-PPiase catalysis and starch synthesis.

The soybean 94-kilodalton vegetative storage protein is a lipoxygenase that is localized in paraveinal mesophyll cell vacuoles

Soybean leaves contain three proteins (the vegetative storage proteins or VSPs) that respond to nitrogen status and are believed to be involved in the temporary storage of nitrogen. One of these proteins, with a molecular mass of 94 kD and termed vsp94, was microsequenced. Partial amino acid sequence indicated that vsp94 was highly homologous to the lipoxygenase protein family. Further evidence that vsp94 is a lipoxygenase was obtained by demonstrating that vsp94 cross-reacted with a lipoxygenase antibody. Also, a lipoxygenase cDNA coding region was able to detect changes in an mRNA that closely parallel changes in vsp94 protein levels resulting from alteration of nitrogen sinks. Extensive immunocytochemical data indicate that this vsp94/lipoxygenase is primarily expressed in the paraveinal mesophyll cells and is subcellularly localized in the vacuole. These observations are significant in that they suggest that plant lipoxygenases may be bifunctional proteins able to function enzymatically in the hydroperoxidation of lipids and also to serve a role in the temporary storage of nitrogen during vegetative growth.

Induction of soybean vegetative storage proteins and anthocyanins by low-level atmospheric methyl jasmonate

Soybean seedlings were exposed to atmospheric methyl jasmonate (MJ) to determine if low levels of this compound could regulate the expression and accumulation of the vegetative storage proteins (VSPs) in soybeans. Low levels of atmospheric MJ induced the accumulation of three VSPs with molecular masses of 27 kDa, 29 kDa, and 94 kDa (vsp27, vsp29, and vsp94, respectively). Atmospheric MJ caused vsp94 to be accumulated in all above-ground organs of the seedling uniformly after just 3 days of exposure. vsp27 preferentially accumulated in shoot tips and primary leaves, whereas vsp29 preferentially accumulated in the cotyledons. In addition to these effects, MJ also induced the biosynthesis of anthocyanins in light-grown seedlings but inhibited anthocyanin biosynthesis in etiolated seedlings. It is concluded that low levels of atmospheric MJ regulate anthocyanin biosynthesis and the organspecific accumulation of VSPs in developing soybean seedlings. The organ-specific differential accumulation may reflect changes in the pattern of nitrogen partitioning between various compounds and/or organs. These results lend substance to the hypothesis that volatile MJ may act as a gaseous messenger or growth regulator in plants.