Changes in temperature, CO2, and precipitation under the scenarios of climate change for the next 30 yr present a challenge to crop production. This review focuses on the impact of temperature, CO2, ...and ozone on agronomic crops and the implications for crop production. Understanding these implications for agricultural crops is critical for developing cropping systems resilient to stresses induced by climate change. There is variation among crops in their response to CO2, temperature, and precipitation changes and, with the regional differences in predicted climate, a situation is created in which the responses will be further complicated. For example, the temperature effects on soybean Glycine max (L.) Merr. could potentially cause yield reductions of 2.4% in the South but an increase of 1.7% in the Midwest. The frequency of years when temperatures exceed thresholds for damage during critical growth stages is likely to increase for some crops and regions. The increase in CO2 contributes significantly to enhanced plant growth and improved water use efficiency (WUE); however, there may be a downscaling of these positive impacts due to higher temperatures plants will experience during their growth cycle. A challenge is to understand the interactions of the changing climatic parameters because of the interactions among temperature, CO2, and precipitation on plant growth and development and also on the biotic stresses of weeds, insects, and diseases. Agronomists will have to consider the variations in temperature and precipitation as part of the production system if they are to ensure the food security required by an ever increasing population.
ABSTRACT
Photosynthesis is inhibited by high temperatures that plants are likely to experience under natural conditions. Both increased thylakoid membrane ionic conductance and ...ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) deactivation have been suggested as the primary cause. The moderately heat‐tolerant crop Pima S‐6 cotton (Gossypium barbadense) was used to examine heat stress‐induced inhibition of photosynthesis. Previous field‐work indicated that moderate heat stress (T = 35–45 °C) is associated with very rapid leaf temperature changes. Therefore, a system was devised for rapidly heating intact, attached leaves to mimic natural field heat‐stress conditions and monitored Rubisco activation, carbon‐cycle metabolites, thylakoid ionic conductance, and photosystem I activity. As a proxy for NADPH and stromal redox status the activation state of NADP‐malate dehydrogenase (NADP‐MDH) was measured. In dark‐adapted cotton leaves, heating caused an increase in thylakoid permeability at temperatures as low as 36 °C. The increased permeability did not cause a decline in adenosine 5′‐triphosphate (ATP) levels during steady‐state or transient heating. Rapid heating caused a transient decline in ribulose 1,5‐bisphosphate without a decrease in Rubisco activation. Sustained heating caused a decline in Rubisco activation and also oxidized the stroma as judged by NADP‐MDH activation and this is hypothesized to result from increased cyclic photophosphorylation, explaining the maintenance of ATP content in the face of increased thylakoid membrane ion leakiness.
Photosynthesis in warm-climate plants is substantially reduced after chilling. Tropical and subtropical species offer the opportunity to study the effects of low temperature on photosynthetic ...processes undisguised by the myriad of protective responses observed in temperate species. In this article, we highlight the primary components of photosynthesis that are affected by a short chill, in both the dark and the light, and discuss what is known of the mechanisms involved. Recent work implicates impaired redox and circadian regulation among other processes.
The C4 grass Zea mays (maize or corn) is the third most important food crop globally in terms of production and demand is predicted to increase 45% from 1997 to 2020. However, the effects of rising ...CO2 upon C4 plants, and Z. mays specifically, are not sufficiently understood to allow accurate predictions of future crop production. A rainfed, field experiment utilizing free-air concentration enrichment (FACE) technology in the primary area of global corn production (US Corn Belt) was undertaken to determine the effects of elevated CO2 on corn. FACE technology allows experimental treatments to be imposed upon a complete soil–plant–atmosphere continuum with none of the effects of experimental enclosures on plant microclimate. Crop performance was compared at ambient CO2 (354 μ mol mol-1) and the elevated CO2 (549 μmol mol-1) predicted for 2050. Previous laboratory studies suggest that under favorable growing conditions C4 photosynthesis is not typically enhanced by elevated CO2. However, stomatal conductance and transpiration are decreased, which can indirectly increase photosynthesis in dry climates. Given the deep soils and relatively high rainfall of the US Corn Belt, it was predicted that photosynthesis would not be enhanced by elevated CO2. The diurnal course of gas exchange of upper canopy leaves was measured in situ across the growing season of 2002. Contrary to the prediction, growth at elevated CO2 significantly increased leaf photosynthetic CO2 uptake rate (A) by up to 41%, and 10% on average. Greater A was associated with greater intercellular CO2, lower stomatal conductance and lower transpiration. Summer rainfall during 2002 was very close to the 50-year average for this site, indicating that the year was not atypical or a drought year. The results call for a reassessment of the established view that C4 photosynthesis is insensitive to elevated CO2 under favorable growing conditions and that the production potential of corn in the US Corn Belt will not be affected by the global rise in CO2.
ABSTRACT
Mesophyll conductance (gm) generally correlates with photosynthetic capacity, although the causal relationship between the two is unclear. The response of gm to various CO2 regimes was ...measured to determine its relationship to environmental changes that affect photosynthesis. The overall effect of CO2 growth environment on gm was species and experiment dependent. The data did not statistically differ from the previously shown A–gm relationship and was unaffected by CO2 treatment. The consequences of the CO2 effect on gm for interpreting photosynthesis in individual cases were investigated. Substantial effects of assumed versus calculated gm on leaf properties estimated from gas‐exchange measurements were found. This differential error resulted in an underestimation in ratio of maximum carboxylation to electron transport, especially in plants with high photosynthetic capacity. Including gm in the calculations also improved the agreement between maximum carboxylation rates and in vitro Rubisco measurements. It is concluded that gm is finite and varies with photosynthetic capacity. Including gm when calculating photosynthesis parameters from gas‐exchange data will avoid systematic errors.
A lower than theoretically expected increase in leaf photosynthesis with long‐term elevation of carbon dioxide concentration (CO2) is often attributed to limitations in the capacity of the plant to ...utilize the additional photosynthate, possibly resulting from restrictions in rooting volume, nitrogen supply or genetic constraints. Field‐grown, nitrogen‐fixing soybean with indeterminate flowering might therefore be expected to escape these limitations. Soybean was grown from emergence to grain maturity in ambient air (372 µmol mol−1CO2) and in air enriched with CO2 (552 µmol mol−1CO2) using Free‐Air CO2 Enrichment (FACE) technology. The diurnal courses of leaf CO2 uptake (A) and stomatal conductance (gs) for upper canopy leaves were followed throughout development from the appearance of the first true leaf to the completion of seed filling. Across the growing season the daily integrals of leaf photosynthetic CO2 uptake (A′) increased by 24.6% in elevated CO2 and the average mid‐day gs decreased by 21.9%. The increase in A′ was about half the 44.5% theoretical maximum increase calculated from Rubisco kinetics. There was no evidence that the stimulation of A was affected by time of day, as expected if elevated CO2 led to a large accumulation of leaf carbohydrates towards the end of the photoperiod. In general, the proportion of assimilated carbon that accumulated in the leaf as non‐structural carbohydrate over the photoperiod was small (< 10%) and independent of CO2 treatment. By contrast to A′, daily integrals of PSII electron transport measured by modulated chlorophyll fluorescence were not significantly increased by elevated CO2. This indicates that A at elevated CO2 in these field conditions was predominantly ribulose‐1,5‐bisphosphate (RubP) limited rather than Rubisco limited. There was no evidence of any loss of stimulation toward the end of the growing season; the largest stimulation of A′ occurred during late seed filling. The stimulation of photosynthesis was, however, transiently lost for a brief period just before seed fill. At this point, daytime accumulation of foliar carbohydrates was maximal, and the hexose:sucrose ratio in plants grown at elevated CO2 was significantly larger than that in plants grown at current CO2. The results show that even for a crop lacking the constraints that have been considered to limit the responses of C3 plants to rising CO2 in the long term, the actual increase in A over the growing season is considerably less than the increase predicted from theory.
1. AT-402 (20 g/kg), a proprietary blend of Aloe ferox leaf extract and calcium bentonite, had no adverse effect on poult body weights at 3 weeks of age. 2. Lower body weights and lower feed ...consumption were observed in 4-week-old poults given 10 and 20 g/kg of AT-402 compared to poults given 0 or 5 g/kg. 3. Floor-reared poults, given either 5 or 10 g/kg AT-402 to 4 weeks of age, were significantly heavier than controls (0 g/kg AT-402), and feed conversion ratios for AT-402 given poults were improved. 4. Xylose uptake in 5 g/kg AT-402-fed poults was significantly greater than in poults given AT-402 at 0 and 10 g/kg, reflecting increased body weights of 5 g/kg AT-402-fed poults. 5. Delayed access by poults to the AT-402 until 10 d of age also improved 4-week body weights, suggesting that the AT-402 might improve performance as soon as it is given. 6. AT-402 at 5 g/kg was most efficacious as demonstrated by improved body weights and feed conversions.
1. Four experiments were conducted to evaluate the effects of temperature (TEM) and oxygen (O2) concentrations during the last 4 d of incubation on bone development. Fertile eggs from two strains ...were obtained that either exhibited Low or High eggshell conductance (G). 2. Four experimental cabinets provided either four TEM (36, 37, 38 or 39°C) or four O2 concentrations (17, 19, 21 or 23% O2). Data were analysed as a 2 x 2 factorial design. In the fourth experiment, two temperatures (36 and 39°C), two O2 concentrations (17 and 23%) and the same Low and High G strains were evaluated in a 2 x 2 x 2 factorial design. 3. Body weights (BW) and residual yolks were obtained, both legs were dissected. Femur, tibia and shank weights, length and thickness were recorded. Relative asymmetry (RA) of each leg section was calculated. 4. The results indicated that elevated TEM during incubation increased RA between the two legs, mainly in the Low G strain. Chickens at the lowest O2 concentrations had lighter and shorter tibias, lighter shanks, and increased RA of femur length compared to chickens in the 23% O2. In the fourth experiment no interactions were observed between O2 and TEM. High TEM depressed BW of Low G broilers, but no significant effect of treatments was observed on BW of High G broilers. Nevertheless, the high TEM or low O2 independently caused reduced femur and tibia weights and length, shank length and thickness, and both low O2 and high TEM together increased RA in shank weight. 5. These results suggest that late incubation conditions affect long bone development in broilers.