James A. Bunce

Influence of increasing carbon dioxide concentration on the photosynthetic and growth stimulation of selected C4crops and weeds

Plants of six weedy species (Amaranthus retroflexus, Echinochloa crus-galli, Panicum dichotomiflorum, Setaria faberi, Setaria viridis, Sorghum halapense) and 4 crop species (Amaranthus hypochondriacus, Saccharum officinarum, Sorghum bicolor and Zea mays) possessing the C4type of photosynthesis were grown at ambient (38 Pa) and elevated (69 Pa) carbon dioxide during early development (i.e. up to 60 days after sowing) to determine: (a) if plants possessing the C4photosynthetic pathway could respond photosynthetically or in biomass production to future increases in global carbon dioxide and (b) whether differences exist between weeds and crops in the degree of response. Based on observations in the response of photosynthesis (measured as A, CO2assimilation rate) to the growth CO2condition as well as to a range of internal CO2(Ci) concentrations, eight of ten C4species showed a significant increase in photosynthesis. The largest and smallest increases observed were for A. retroflexus (+30%) and Z. mays (+5%), respectively. Weed species (+19%) showed approximately twice the degree of photosynthetic stimulation as that of crop species (+10%) at the higher CO2concentration. Elevated carbon dioxide also resulted in significant increases in whole plant biomass for four C4weeds (A. retroflexus, E. crus-galli, P. dichotomiflorum, S. viridis) relative to the ambient CO2condition. Leaf water potentials for three selected species (A. retroflexus, A. hypochondriacus, Z. mays) indicated that differences in photosynthetic stimulation were not due solely to improved leaf water status. Data from this study indicate that C4plants may respond directly to increasing CO2concentration, and in the case of some C4weeds (e.g. A. retroflexus) may show photosynthetic increases similar to those published for C3species.

Carbon dioxide effects on stomatal responses to the environment and water use by crops under field conditions

Reductions in leaf stomatal conductance with rising atmospheric carbon dioxide concentration ([CO2]) could reduce water use by vegetation and potentially alter climate. Crop plants have among the largest reductions in stomatal conductance at elevated [CO2]. The relative reduction in stomatal conductance caused by a given increase in [CO2] is often not constant within a day nor between days, but may vary considerably with light, temperature and humidity. Species also differ in response, with a doubling of [CO2] reducing mean midday conductances by <15% in some crop species to >50% in others. Elevated [CO2] increases leaf area index throughout the growing season in some species. Simulations, and measurements in free air carbon dioxide enrichment systems both indicate that the relatively large reductions in stomatal conductance in crops would translate into reductions of <10% in evapotranspiration, partly because of increases in temperature and decreases in humidity in the air around crop leaves. The reduction in evapotranspiration in crops is similar to that in other types of vegetation which have smaller relative reductions in stomatal conductance, because of the poorer aerodynamic coupling of the canopy to the atmosphere in crops. The small decreases in evapotranspiration at elevated [CO2] may themselves be important to crop production in dry environments, but changes in climate and microclimate caused by reduced stomatal conductance could also be important to crop production.

Influence of Drought-Induced Water Stress on Soybean and Spinach Leaf Ascorbate-Dehydroascorbate level and Redox Status.

We examined the influence of water stress (water deficit) induced by drought on the steady state levels of ascorbic acid (ASC), dehydroascorbate (DHA), and the ASC:DHA redox status in leaflets of Glycine max (soybean) and leaves of Spinacia oleracea (spinach). Two soybean cultivars (cv. Essex and cv. Forrest) and one spinach cultivar (cv. Nordic) were grown in high‐light growth chambers (≈1000–1200 μmol m−2 s−1) or in the greenhouse during May, June, and July 1999. The cultivars were supplied with water until ≈25–29 d postemergence, at which time one‐half of the plants were not watered for a period of from 4.5 to 7.5 d; the other half of the plants were provided water daily and served as controls. On designated days, leaf water potential (&PSgr;Leaf) was measured, and leaf disks of constant area were excised in the period between ≈1230 and 1330 hours. Leaf disk samples were immediately frozen in liquid N2, samples were extracted, and ASC and DHA levels were measured and expressed as μmol per gram dry mass per time point. For the soybean cultivars, low &PSgr;Leaf values (≈−3.00 to −3.95 MPa) were accompanied by slight decreases in ASC levels and slight increases in DHA levels per gram dry mass. In some cases, leaflet ASC levels of water‐stressed soybeans were similar to controls or were even increased by as much as 1.2 times. In soybeans, the mole fraction of ASC remained at 93–99 mol% of the total ascorbate ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Are Annual Plants Adapted to the Current Atmospheric Concentration of Carbon Dioxide

\mathrm{ASC}\,+\mathrm{DHA}\,

Photosynthetic characteristics of leaves developed at different irradiances and temperatures: an extension of the current hypothesis

\end{document} ), indicating that most of the total ascorbate remained in the reduced form even at low water potential. In spinach plants subjected to water stress (−1.8 to −2.6 MPa), leaf ASC decreased as much as 38%, but the ASC remained at 96–99 mol% of the total ascorbate. It is concluded that during water stress, enzymes of the ascorbate‐glutathione cycle in leaf mesophyll cells, as well as in the system that generates reductant to support DHA to ASC recycling, e.g., photosynthetic electron transport in chloroplasts, is able to remain active enough to maintain reduction of DHA to ASC.

EVIDENCE THAT PREMATURE SENESCENCE AFFECTS PHOTOSYNTHETIC DECLINE OF WHEAT FLAG LEAVES DURING GROWTH IN ELEVATED CARBON DIOXIDE

Although genotypic differences among soybean (Glycine max (L.) Merr.) cultivars in their response to future CO2 partial pressures have been observed in the glasshouse, it is unclear if similar responses would occur among cultivars when grown under field conditions at normal stand densities. To determine variation in the sensitivity of soybean growth and seed yield to CO2, we grew two contrasting cultivars of the same maturity group, Ripley (semi-dwarf, determinate) and Spencer (standard, indeterminate), to reproductive maturity at ambient and elevated (30 Pa above ambient) CO2 partial pressures for two field seasons. Spencer had been previously selected in glasshouse trials as responsive to increased CO2. Significant cultivar x CO2 interaction was observed for both vegetative biomass and seed yield, with Spencer demonstrating a consistently greater yield enhancement at elevated CO2 than Ripley (60 vs 35%, respectively). Differences in CO2 sensitivity between cultivars were not evident in measurements of single leaf photosynthesis taken during anthesis, nor early or late pod-fill. Analysis of reproductive characteristics indicated that the sensitivity of the seed yield response to CO2 in Spencer was associated with the ability to form additional seed on axillary branches in response to elevated CO2. Data from this experiment suggest that screening of soybean germplasm at the glasshouse level, when combined with field trials, may be an effective strategy to begin selecting soybean lines that will maximize yield in a future, higher CO2 environment.

Photosynthesis at ambient and elevated humidity over a growing season in soybean

The concentration of carbon dioxide [CO2] in the atmosphere has risen from about 280 μmol mol−1 in 1870 to about 370 μmol mol−1 currently, and this concentration continues to increase rapidly. In planning for future, higher atmospheric [CO2], the question arises whether genetic modifications of crop plants are required in order to fully exploit the increased availability of this often growth‐limiting resource, as does the question of whether genetic changes are likely to result from natural selection in non–crop species as atmospheric [CO2] rises. Based on the concept that adaptation to a given resource level is reflected in how resource‐use efficiency changes with the availability of that resource, we examined various aspects of plant growth response to [CO2] from 90 μmol mol−1 below to 90 μmol mol−1 above the current atmospheric [CO2] in four annual weedy herbaceous species. By several measures, the efficiency at which plants used carbon dioxide decreased abruptly just above the current atmospheric concentration of carbon dioxide. For example, total plant leaf area increased up to, but not above, the current [CO2], and leaf area per unit of plant dry mass was constant up to the current [CO2] and decreased at higher [CO2]. Down‐regulation of photosynthesis occurred in three of the four species when grown above the current [CO2]. These patterns occurred for two different growth light regimes. These responses indicate that these annual weedy species are adapted to the current atmospheric [CO2], but not to higher concentrations.

Low humidity effects on photosynthesis in single leaves of C4 plants

We will review the status of photosynthetic field crop research contributing to the prediction of crop behavior, aspects of laboratory photosynthetic research that can be used to predict behavior in the field, interactions between crop photosynthesis and other plant processes, and aspects of related modeling problems. We will build on other CRC reviews related to this subject area. We will also emphasize physiological aspects of genotypic differences in photosynthetic rates.