In a field rain-fed trial with 15 cassava cultivars, leaf gas exchanges and carbon isotope discrimination (Δ) of the same leaves were determined to evaluate genotypic and within-canopy variations in these parameters. From 3 to 7 months after planting leaf gas exchange was measured on attached leaves from upper, middle, and lower canopy layers. All gas exchange parameters varied significantly among cultivars as well as canopy layers. Net photosynthetic rate (PN) decreased from top canopy to bottom indicating both shade and leaf age effects. The same trend, but in reverse, was found with respect to Δ, with the highest values in low canopy level and the lowest in upper canopy. There were very significant correlations, with moderate and low values, among almost all these parameters, with PN negatively associated with intercellular CO2 concentration (Ci), ratio of C i to ambient CO2 concentration C i/C a, and Δ. Across all measured leaves, Δ correlated negatively with leaf water use efficiency (WUE = photosynthesis/stomatal conductance, gs) and with gs, but positively with Ci and Ci/Ca. The later parameters negatively correlated with leaf WUE. Across cultivars, both PN and correlated positively with storage root yield. These results are in agreement with trends predicted by the carbon isotope discrimination model. and M. A. El-Sharkawy, S. M. de Tafur.
Field trials with a large group of cassava germplasm were conducted at the seasonally-dry and hot environments in southwest Colombia to investigate photosynthetic characteristics and production under drought conditions. Measurement of net photosynthetic rate (PN), photosynthetic nitrogen use efficiency (PNUE), mesophyll conductance to CO2 diffusion (g m), and phosphoenolpyruvate carboxylase (PEPC) activity of upper canopy leaves were made in the field. All photosynthetic characteristics were significantly correlated with final dry root yield (Yield). Correlations among the photosynthetic traits were also significant. PEPC activity was highly significantly correlated with PN and PNUE, indicating the importance of the enzyme in cassava photosynthesis and productivity. Among a small selected group from the preliminary trial for yield performance, the second year Yield was highly significantly correlated with PN measured on the first year crop. Thus variations in the measured photosynthetic traits are genetically controlled and underpin variations in yield. One short-stemmed cultivar M Col 2215 was selected for high root dry matter content, high harvest index, and tolerance to drought. It was tested under the semi-arid conditions of the west coast of Ecuador; participating farmers evaluated cultivar performance. This cultivar was adopted by farmers and officially released in 1992 under the name Portoviejo 650. and M. A. El-Sharkawy, Y. Lopez, L. M. Bernal.
Earth’s climate has experienced notable changes during the past 50-70 years when global surface temperature has risen by 0.8°C during the 20th century. This was a consequence of the rise in the concentration of biogenic gases (carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and ozone) in the atmosphere that contribute, along with water vapor, to the so-called ‘greenhouse effect’. Most of the emissions of greenhouse gases have been, and still are, the product of human activities, namely, the excessive use of fossil energy, deforestations in the humid tropics with associated poor land use-management, and wide-scale degradation of soils under crop cultivation and animal/pasture ecosystems. General Circulation Models predict that atmospheric CO2 concentration will probably reach 700 μmol(CO2) mol-1. This can result in rise of Earth’s temperature from 1.5 to over 5°C by the end of this century. This may instigate 0.60-1.0 m rise in sea level, with impacts on coastal lowlands across continents. Crop modeling predicts significant changes in agricultural ecosystems. The mid- and
high-latitude regions might reap the benefits of warming and CO2 fertilization effects via increasing total production and yield of C3 plants coupled with greater water-use efficiencies. The tropical/subtropical regions will probably suffer the worst impacts of global climate changes. These impacts include wide-scale socioeconomic changes, such as degradation and losses of natural resources, low agricultural production, and lower crop yields, increased risks of hunger, and above all waves of human migration and dislocation. Due to inherent cassava tolerance to heat, water stress, and poor soils, this crop is highly adaptable to warming climate. Such a trait should enhance its role in food security in the tropics and subtropics., M. A. El-Sharkawy., and Obsahuje bibliografii
Almost four decades have passed since the new field of ecosystem simulation sprang into full force as an added tool for a sound research in an ever-advancing scientific front. The enormous advances and new discoveries that recently took place in the field of molecular biology and basic genetics added more effective tools, have strengthened and increased the efficiency of science outputs in various areas, particularly in basic biological sciences. Now, we are entering into a more promising stage in science, i.e. 'post-genomics', where both simulation modelling and molecular biology tools are integral parts of experimental research in agricultural sciences. I briefly review the history of simulation of crop/environment systems in the light of advances in molecular biology, and most importantly the essential role of experimental research in developing and constructing more meaningful and effective models and technologies. Such anticipated technologies are expected to lead into better management of natural resources in relation to crop communities in particular and plant ecosystems in general, that might enhance productivity faster. Emphasis is placed on developing new technologies to improve agricultural productivity under stressful environments and to ensure sustainable economic development. The latter is essential since available natural resources, particularly land and water, are increasingly limiting.
The review sums up research conducted at CIAT within a multidiscipline effort revolving around a strategy for developing improved technologies to increase and sustain cassava productivity, as well as conserving natural resources in the various eco-edaphic zones where the crop is grown, with emphasis on stressful environments. Field research has elucidated several physiological plant mechanisms underlying potentially high productivity under favourable hot-humid environments in the tropics. Most notable is cassava inherent high capacity to assimilate carbon in near optimum environments that correlates with both biological productivity and root yield across a wide range of germplasm grown in diverse environments. Cassava leaves possess elevated activities of the C4 phosphoenolpyruvate carboxylase (PEPC) that also correlate with leaf net photosynthetic rate (PN) in field-grown plants, indicating the importance of selection for high PN. Under certain conditions such leaves exhibit an interesting photosynthetic C3-C4 intermediate behaviour which may have important implications in future selection efforts. In addition to leaf PN, yield is correlated with seasonal mean leaf area index (i.e. leaf area duration, LAD). Under prolonged water shortages in seasonally dry and semiarid zones, the crop, once established, tolerates stress and produces reasonably well compared to other food crops (e.g. in semiarid environments with less than 700 mm of annual rain, improved cultivars can yield over 3 t ha-1 oven-dried storage roots). The underlying mechanisms for such tolerance include stomatal sensitivity to atmospheric and edaphic water deficits, coupled with deep rooting capacities that prevent severe leaf dehydration, i.e. stress avoidance mechanisms, and reduced leaf canopy with reasonable photosynthesis over the leaf life span. Another stress-mitigating plant trait is the capacity to recover from stress, once water is available, by forming new leaves with even higher PN, compared to those in nonstressed crops. Under extended stress, reductions are larger in shoot biomass than in storage root, resulting in higher harvest indices. Cassava conserves water by slowly depleting available water from deep soil layers, leading to higher seasonal crop water-use and nutrient-use efficiencies. In dry environments LAD and resistance to pests and diseases are critical for sustainable yields. In semiarid zones the crop survives but requires a second wet cycle to achieve high yields and high dry matter contents in storage roots. Selection and breeding for early bulking and for medium/short-stemmed cultivars is advantageous under semiarid conditions. When grown in cooler zones such and as in tropical high altitudes and in low-land sub-tropics, leaf PN is greatly reduced and growth is slower. Thus, the crop requires longer period for a reasonable productivity. There is a need to select and breed for more cold-tolerant genotypes. Selection of parental materials for tolerance to water stress and infertile soils has resulted in breeding improved germplasm adapted to both favourable and stressful environments.
The review is done to summarise the history of the discoveries of the many anatomical, agronomical, and physiological aspects of C4 photosynthesis (where the first chemical products of CO2 fixation in illuminated leaves are four-carbon dicarboxylic acids) and to document correctly the scientists at the University of Arizona and the University of California, Davis, who made these early discoveries. The findings were milestones in plant science that occurred shortly after the biochemical pathway of C3 photosynthesis in green algae (where the first chemical product is a three-carbon compound) was elucidated at the University of California, Berkeley, and earned a Nobel Prize in chemistry. These remarkable achievements were the result of ground-breaking pioneering research efforts carried out by many agronomists, plant physiologists and biochemists in several laboratories, particularly in the USA. Numerous reviews and books written in the past four decades on the history of C4 photosynthesis have focused on the biochemical aspects and give an unbalanced history of the multidisciplinary/multinstitutional nature of the achievements made by agronomists, who published much of their work in Crop Science. Most notable among the characteristics of the C4 species that differentiated them from the C3 ones are: (I) high optimum temperature and high irradiance saturation for maximum leaf photosynthetic rates; (II) apparent lack of CO2 release in a rapid stream of CO2-free air in illuminated leaves in varying temperatures and high irradiances; (III) a very low CO2 compensation point; (IV) lower mesophyll resistances to CO2 diffusion coupled with higher stomatal resistances, and, hence, higher instantaneous leaf water use efficiency; (V) the existence of the so-called "Kranz leaf anatomy" and the higher internal exposed mesophyll surface area per cell volume; and (VI) the ability to recycle respiratory CO2 by illuminated leaves.
Productivity of most improved major food crops showed stagnation in the past decades. As human population is projected to reach 9-10 billion by the end of the 21st century, agricultural productivity must be increased to ensure their demands. Photosynthetic capacity is the basic process underlying primary biological productivity in green plants and enhancing it might lead to increasing potential of the crop yields. Several approaches may improve the photosynthetic capacity, including integrated systems management, in order to close wide gaps between actual farmer’s and the optimum obtainable yield. Conventional and molecular genetic improvement to increase leaf net photosynthesis (P N) are viable approaches, which have been recently shown in few crops. Bioengineering the more efficient CC4 into C3 system is another ambitious approach that is currently being applied to the C3 rice crop. Two under-researched, yet old important crops native to the tropic Americas (i.e., the CC4 amaranths and the C3-CC4 intermediate cassava), have shown high potential P N, high productivity, high water use efficiency, and tolerance to heat and drought stresses. These physiological traits make them suitable for future agricultural systems, particularly in a globally warming climate. Work on crop canopy photosynthesis included that on flowering genes, which control formation and decline of the canopy photosynthetic activity, have contributed to the climate change research effort. The plant breeders need to select for higher P N to enhance the yield and crop tolerance to environmental stresses. The plant science instructors, and researchers, for various reasons, need to focus more on tropical species and to use the research, highlighted here, as an example of how to increase their yields., M. A. El-Sharkawy., and Obsahuje seznam literatury