Jann P. Conroy

Faster Rubisco Is the Key to Superior Nitrogen-Use Efficiency in NADP-Malic Enzyme Relative to NAD-Malic Enzyme C4 Grasses

In 27 C4 grasses grown under adequate or deficient nitrogen (N) supplies, N-use efficiency at the photosynthetic (assimilation rate per unit leaf N) and whole-plant (dry mass per total leaf N) level was greater in NADP-malic enzyme (ME) than NAD-ME species. This was due to lower N content in NADP-ME than NAD-ME leaves because neither assimilation rates nor plant dry mass differed significantly between the two C4 subtypes. Relative to NAD-ME, NADP-ME leaves had greater in vivo (assimilation rate per Rubisco catalytic sites) and in vitro Rubisco turnover rates (kcat; 3.8 versus 5.7 s−1 at 25°C). The two parameters were linearly related. In 2 NAD-ME (Panicum miliaceum and Panicum coloratum) and 2 NADP-ME (Sorghum bicolor and Cenchrus ciliaris) grasses, 30% of leaf N was allocated to thylakoids and 5% to 9% to amino acids and nitrate. Soluble protein represented a smaller fraction of leaf N in NADP-ME (41%) than in NAD-ME (53%) leaves, of which Rubisco accounted for one-seventh. Soluble protein averaged 7 and 10 g (mmol chlorophyll)−1 in NADP-ME and NAD-ME leaves, respectively. The majority (65%) of leaf N and chlorophyll was found in the mesophyll of NADP-ME and bundle sheath of NAD-ME leaves. The mesophyll-bundle sheath distribution of functional thylakoid complexes (photosystems I and II and cytochrome f) varied among species, with a tendency to be mostly located in the mesophyll. In conclusion, superior N-use efficiency of NADP-ME relative to NAD-ME grasses was achieved with less leaf N, soluble protein, and Rubisco having a faster kcat.

Nitrogen and phosphorus requirements of cotton and wheat under changing atmospheric CO2 concentrations

The influence of increasing atmospheric CO2 on shoot growth, leaf nitrogen and phosphorus concentrations and carbohydrate composition was investigated in cotton and wheat. Shoot dry weight of both species was generally higher at elevated CO2, especially at high rates of available soil N and P. Critical leaf N concentration was reduced but critical P concentration was increased in both species at high CO2.

The effect of drought on plant water use efficiency of nine NAD–ME and nine NADP–ME Australian C4 grasses

We investigated the response to drought of nine NAD-malic enzyme (NAD-ME) and nine NADP-malic enzyme (NADP-ME) C4 grasses. Species were grown from seeds in potted soil in a glasshouse. Seedlings were either watered regularly or exposed to two successive drying cycles of 8-10 d each, after which plants were harvested. Under well-watered conditions, average water use efficiency (WUE; dry mass gain per unit water transpired) was similar for NAD-ME and NADP-ME C4 grasses, and ranged between 6.0 and 8.7 g dry mass kg-1 H2O. Drought enhanced WUE of most species, but to a significantly greater extent in NAD-ME (1.20-fold) than NADP-ME (1.11-fold) grasses. Inhibition of dry matter accumulation (average of 12%) and shoot elongation under drought was similar among the C4 grasses. Leaf dry matter carbon (δ13C) and oxygen (δ18O) isotope compositions were significantly different between the two C4 subtypes. Leaf δ13C averaged -13.3 and -12.2, and leaf δ18O averaged 26.0 and 26.9 in well-watered NAD-ME and NADP-ME grasses, respectively. Drought significantly reduced leaf δ13C in most C4 grasses by an average 0.5. Leaf δ18O was not significantly affected by drought, indicating that leaf δ18O does not reflect drought-induced changes in leaf transpiration of C4 grasses. In the experiment reported here, NAD-ME grasses increased their WUE under drought to a greater extent than their NADP-ME counterparts. Increased WUE of the C4 grasses under drought was primarily related to control of water loss relative to carbon gain at the leaf, rather than the plant, level.

Photosynthetic responses of two eucalypts to industrial-age changes in atmospheric [CO2] and temperature.

The unabated rise in atmospheric [CO(2)] is associated with increased air temperature. Yet, few CO(2)-enrichment studies have considered pre-industrial [CO(2)] or warming. Consequently, we quantified the interactive effects of growth [CO(2)] and temperature on photosynthesis of faster-growing Eucalyptus saligna and slower-growing E. sideroxylon. Well-watered and -fertilized tree seedlings were grown in a glasshouse at three atmospheric [CO(2)] (290, 400, and 650 µL L(-1)), and ambient (26/18 °C, day/night) and high (ambient + 4 °C) air temperature. Despite differences in growth rate, both eucalypts responded similarly to [CO(2)] and temperature treatments with few interactive effects. Light-saturated photosynthesis (A(sat)) and light- and [CO(2)]-saturated photosynthesis (A(max) ) increased by ∼ 50% and ∼ 10%, respectively, with each step-increase in growth [CO(2)], underpinned by a corresponding 6-11% up-regulation of maximal electron transport rate (J(max)). Maximal carboxylation rate (V(cmax)) was not affected by growth [CO(2)]. Thermal photosynthetic acclimation occurred such that A(sat) and A(max) were similar in ambient- and high-temperature-grown plants. At high temperature, the thermal optimum of A(sat) increased by 2-7 °C across [CO(2)] treatments. These results are the first to suggest that photosynthesis of well-watered and -fertilized eucalypt seedlings will remain strongly responsive to increasing atmospheric [CO(2)] in a future, warmer climate.

Changes in source-sink relations during development influence photosynthetic acclimation of rice to free air CO2 enrichment (FACE)

Relationships between photosynthetic acclimation and changes in the balance between source-sink supply and demand of carbon (C) and nitrogen (N) were tested using rice (Oryza sativa L. cv. Akitakomachi). Plants were field-grown in northern Japan at ambient CO2 partial pressure [p(CO2)] or free air CO2 enrichment (FACE; p(CO2) ~ 26-32 Pa above ambient) with low, medium or high N supplies. Leaf CO2 assimilation rates (A) and biochemical parameters were measured at 32-36 (eighth leaf) and 76-80 (flag leaf) d after transplanting, representing stages with a contrasting balance between C and N supply and demand in sources and sinks. Acclimation due to FACE was pronounced in flag leaves at each N supply. This was not fully accounted for by reductions in leaf N concentrations, because A/N and Vcmax/N were lower in FACE-grown flag leaves. Acclimation did not occur in the eighth leaf, and A/N and Vcmax/N was not significantly increased in FACE-grown leaves. Soluble protein / sucrose and amino acid / sucrose concentrations decreased under FACE, whereas sucrose phosphate synthase protein levels increased. At flag leaf stage, there was a discrepancy between the demand and supply of N, which was resolved by enhanced leaf N remobilization, associated with the lower Rubisco concentrations under FACE. In contrast to the early growth stage, enhanced growth of rice plants was accompanied by increased plant N uptake in FACE. We conclude that photosynthetic acclimation in flag leaves occurs under FACE because there is a large demand for N for reproductive development, relative to supply of N from root uptake and remobilization from leaves.

Accelerated early growth of rice at elevated CO2 : Is it related to developmental changes in the shoot apex?

The influence of elevated CO2 on the development of the shoot apex and on subsequent vegetative growth and grain yield was investigated using rice (Oryza sativa L. cv Jarrah) grown in flooded soil at either 350 or 700 [mu]L CO2 L-1. At 8 d after planting (DAP), elevated CO2 increased the height and diameter of the apical dome and lengths of leaf primordia and tiller buds but had no effect on their numbers. By 16 DAP, there were five tiller buds in the apex at 700 [mu]L CO2 L-1 compared with only three tiller buds at 350 [mu]L CO2 L-1. These changes in development of the shoot apex at high CO2 were forerunners to faster development of the vegetative shoot at elevated CO2 between 11 and 26 DAP as evidenced by increases in the relative growth rates of the shoot and tillers. Accelerated development at high CO2 was responsible for the 42% increase in tiller number at the maximum tillering stage and the 57% enhancement of grain yield at the final harvest. The link between high CO2 effects on development during the first 15 DAP and final tiller number and grain yield was demonstrated by delaying exposure of plants to high CO2 for 15 d. The delay totally inhibited the tillering response to high CO2, and the increase in grain yield of 20% arose from a greater number of grains per panicle. Consequently, it can be concluded that accelerated development in the shoot apex early in development is crucial for obtaining maximum increases in grain yield at elevated atmospheric CO2 concentrations.

Diurnal Regulation of Leaf Blade Elongation in Rice by CO2 (Is it Related to Sucrose-Phosphate Synthase Activity?).

The relationship between leaf blade elongation rates (LER) and sucrose-phosphate synthase (SPS) activity was investigated at different times during ontogeny of rice (Oryza sativa L. cv Jarrah) grown in flooded soil at either 350 or 700 [mu]L CO2 L-1. High CO2 concentrations increased LER of expanding blades and in vivo activity (Vlimiting) SPS activity of expanded blades during the early vegetative stage (21 d after planting [DAP]), when tiller number was small and growing blades were strong carbohydrate sinks. Despite a constant light environment, there was a distinct diurnal pattern in LER, Vlimiting SPS activity, and concentration of soluble sugars, with an increase in the early part of the light period and a decrease later in the light period. The strong correlation (r = 0.65) between LER and Vlimiting SPS activity over the diurnal cycle indicated that SPS activity played an important role in controlling blade growth. The higher Vlimiting SPS activity at elevated CO2 at 21 DAP was caused by an increase in the activation state of the enzyme rather than an increase in Vmax. Fructose and glucose accumulated to a greater extent than sucrose at high CO2 and may have been utilized for synthesis of cell-wall components, contributing to higher specific leaf weight. By the mid-tillering stage (42 DAP), CO2 enrichment enhanced Vlimiting and Vmax activities of source blades. Nevertheless, LER was depressed by high CO2, probably because tillers were stronger carbohydrate sinks than growing blades.

Photosynthetic responses of temperate species to free air CO2 enrichment (FACE) in a grazed New Zealand pasture

A New Zealand temperate pasture is currently exposed to either ambient air or air enriched to 475 µbar CO2 using free-air CO2 enrichment (FACE) technology. Sheep graze the site regularly, which results in heterogeneity in nutrient return. To investigate leaf photosynthetic responses, leaf gas exchange characteristics and nitrogen (N) content were measured in two consecutive years in spring under standard conditions on Lolium perenne L. and Trifolium subterraneum L. and on Trifolium repens L. and Paspalum dilatatum Poir. in the second year only. Leaves of the three C3 species growing under FACE conditions had lower (up to 37% in 1998 and 22% in 1999) photosynthetic rates than leaves growing under ambient conditions, when measured at the same standard conditions of high light and 360-380 µbar CO2. Differences in photosynthetic rates were correlated with leaf N content and stomatal conductance when measured under these conditions. There was no difference in photosynthetic capacities between ambient or FACE grown P. dilatatum, a C4 grass. Photosynthetic N use efficiency (A/N) differed among species. For the C3 species A/N was on average 25% greater under FACE conditions and L. perenne had the highest (240 µmol CO2 mol N -1 s -1 ) and T. repens the lowest A/N (142 µmol CO2 mol N -1 s -1 ) under ambient CO2 partial pressure (p(CO2)). A/N of L. perenne was similar to that of P. dilatatum measured under ambient p(CO2) but 21% greater under FACE conditions. In the second year, leaf stable carbon isotope compositions (δ 13 C) were determined for P. dilatatum, L. perenne and T. repens to assess long-term responses of leaf transpiration efficiency. Using the difference in δ 13 C between ambient and FACE-grown P. dilatatum as a reference to difference in δ 13 C in ambient and FACE air, we concluded that the ratio of leaf intercellular to ambient p(CO2) (Ci/Ca) was similar between FACE and ambient grown L. perenne and T. repens.

A possible plant-mediated feedback between elevated CO2, denitrification and the enhanced greenhouse effect

Natural abundances (d )o f 15 N were used to detect eAects of elevated atmospheric CO2 concentration ([CO2]) and soil wetness on soil N transformations in the presence or absence of plants. An elevated [CO2] of 1000 m ll ˇ1 reduced water use by the perennial C4 grass Panicum coloratum and stimulated root and whole-plant growth. Soil remained wetter between infrequent irrigations than in soil supporting P. coloratum grown in an ambient [CO2] (350 m ll ˇ1 ). The d 15 N value of soil nitrate increased fromˇ2.4 to +9.6- as nitrate was depleted from the soil, but remained unchanged in unplanted soil. The change in d 15 No f soil nitrate was greatest in frequently watered soil regardless of [CO2], and in infrequently watered soil only in elevated [CO2]. It was least in the infrequently watered, ambient [CO2] treatment. Isotope mass balances and 15 N/ 14 N fractionation theory identified denitrification as the most probable cause of this eAect, through the eAect of elevated [CO2] on soil wetness. Nitrification, nitrogen assimilation, leaching or ammonia volatilisation were unlikely causes. The data suggest a positive, plantinduced eAect of elevated atmospheric [CO2] on denitrification. The possibility exists, therefore, for a positive feedback between elevated atmospheric [CO2], a greater soil-to-atmosphere N2O flux and an exacerbation of the enhanced greenhouse eAect. # 1998 Elsevier Science Ltd. All rights reserved.

Plant water use efficiency of 17 Australian NAD-ME and NADP-ME C₄ grasses at ambient and elevated CO₂ partial pressure

This study investigates the response to elevated CO2 partial pressure (pCO2) of C4grasses belonging to different biochemical subtypes (NAD–ME and NADP–ME), and taxonomic groups (main Chloroid assemblage, Paniceae and Andropogoneae). Seventeen C4 grasses were grown under well-watered conditions in two glasshouses maintained at an average dailyppCO2 of 42 (ambient) or 68 (elevated) Pa. Elevated pCO2 significantly increased plant water-use efficiency (WUE; dry matter gain per unit water transpired) in 12 out of the 17 C4 grasses, by an average of 33%. In contrast, only five species showed a significant growth stimulation. When all species are considered, the average plant dry mass enhancement at elevated pCO2 was 26%. There were no significant subtype (or taxa) × pCO2 interactions on either WUE or biomass accumulation. When leaf gas exchange was compared at growth pCO2 but similar light and temperature, high pCO2-grown plants had similar CO2 assimilation rates (A) but a 40% lower stomatal conductance than their low pCO2-grown counterparts. There were no signs of either photosynthetic or stomatal acclimation in any of the measured species. We conclude that elevated pCO2 improved WUE primarily by reducing stomatal conductance.