Physiological characteristics of cassava tolerance to prolonged drought in the tropics: implications for breeding cultivars adapted to seasonally dry and semiarid environment.
ABSTRACT
The paper summarizes research conducted at International Center for Tropical Agriculture (CIAT) on responses of cassava to extended water shortages in the field aided by modern gas-exchange and water-relation techniques as well as biochemical assays. The aim of the research was to coordinate basic and applied aspects of crop physiology into a breeding strategy with a multidisciplinary approach.
Several physiological characteristics/traits and mechanisms underpinning tolerance of cassava to drought were elucidated using a large number of genotypes from the CIAT core germplasm collection grown in various locations representing ecozones where cassava is cultivated. Most notable among these characteristics are the high photosynthetic capacity of cassava leaves in favorable environments and the maintenance of reasonable rates throughout prolonged water deficits, a crucial characteristic for high and sustainable productivity. Cassava possess a tight stomatal control over leaf gas exchange that reduces water losses when plants are subjected to soil water deficits as well as to high atmospheric evaporative demands, thus protecting leaves from severe dehydration. During prolonged water deficits, cassava reduces its canopy by shedding older leaves and forming smaller new leaves leading to less light interception, another adaptive trait to drought.
Though root yield is reduced (but much less than the reduction in top growth) under water stress, the crop can recover when water becomes available by rapidly forming new canopy leaves with much higher photosynthetic rates compared to unstressed crops, thus compensating for yield losses with final yields approaching those in well-watered crops. Cassava can extract slowly water from deep soils, a characteristic of paramount importance in seasonally dry and semiarid environments where deeply stored water needs to be tapped. Screening large accessions under seasonally dry and semiarid environments showed that yield is significantly correlated with upper canopy leaf photosynthetic rates, and the association was attributed mainly to non–stomatal (anatomical/biochemical) factors. Parental materials with both high yields and photosynthetic rates were identified for incorporation into breeding and selection programs for cultivars adapted to prolonged drought coupled with high temperatures and dry air, conditions that might be further aggravated by global climate changes in tropical regions.
INTRODUCTION
The role of physiological research in crop improvement and cropping systems management has recently been reviewed (El-Sharkawy, 2006b). As a branch of basic science, plant physiological research has a fundamental role in advancing the frontier of knowledge that is essential for the better understanding of plants and their interactions with surrounding biophysical environments. It also plays a significant role in supporting other branches of science that deal with the practical application of knowledge and in the development of advanced technologies needed for improving biological systems in general and agricultural productivity in particular. Crop physiology deals with studying cultivated crops with the aim of increasing productivity by enhancing the inherent genetic capacities of crops as well as their adaptability to environments. To be effective in realizing such a goal, physiologists have to work within multidisciplinary research teams committed to a particular crop and/or to multi-cropping systems (El-Sharkawy, 2005). Furthermore, to be successful, the leader/manager of a multidisciplinary research team should not tolerate potential rivalries between disciplines involved nor the dominance of a particular discipline for research support and funds.
Although research conducted in laboratories and in controlled environments is useful in elucidating a specific plant physiological characteristic or mechanisms underlying certain biological processes and responses to environmental factors affecting growth and productivity, by themselves they are inadequate for creating some benefit to the farmer (Evans et al.,1985; Kramer, 1980, 1981). Field research under representative environments and in relevant cropping systems using a broad genetic base must be conducted not only to verify findings in laboratories and controlled environments but also to generate essential information and insights concerning the real potential of crops under natural conditions as well as their responses to a specific limiting environmental factor (El-Sharkawy et al., 1965; El-Sharkawy, 1993, 2004, 2005, 2006a,b; Long et al., 2006).
El-Sharkawy (2004, 2005, 2006a,b) and Long et al. (2006) have shown that research based only on potted plants grown in greenhouses and in controlled cabinets, without the proper calibration in the field, is a waste of time and resources since in most cases results cannot be extrapolated, or simulated by crop modeling, to describe what may take place in natural environments. Those authors concluded that field research is the only valid ecosystem research in studying plant water relations and crop photosynthesis in relation to productivity. Until recently, the controlled-environment and greenhouse potted-plant scientists (many of whom were members of national science academies) controlled the plant photosynthesis and water relations research and often invoked the misuse of the terminology “adaptation to stress” for “acclimation to stress” to protect their domination over the public-funded scientific establishment and to cover up their failure in contributing to the early discoveries of the C4 photosynthesis and its implications for plant water use efficiency (El-Sharkawy, 2005, 2006b; El-Sharkawy and Hesketh, 1965, 1986; Begonia and Begonia, 2007). This is not a very good example to teach young people about how science is done nor an efficient way to manage public-funded research. Still, there is a need for good reviews of field work similar to that reported on cassava research done in the tropics (El-Sharkawy et al., 1989; El-Sharkawy and Cock, 1990; El-Sharkawy, 1993, 2004, 2006a,b). Another good example of reviews of field work in the tropics is the one on coffee research recently published in the Brazilian Journal of Plant Physiology by DaMatta and Ramalho (2006).
In view of the ever increasing world human population, particularly in developing countries (Sasson, 1990; Cohen, 1997), increasing demands for food and feed in the coming few decades must be met by enhancing agricultural outputs as well as conserving dwindling natural resources, particularly arable lands and water. In developed and industrial countries of the temperate zone, the gap between the potential productivity of crops and actual yields had been largely closed aided by advanced public and private research that has resulted in improved technologies. On the other hand, in developing countries in general, and in the tropics in particular, agriculture productivity is still far below the potential. This situation will be further aggravated in light of the recognized global climate changes that might result in adverse effects on agricultural systems and food security in developing countries (Rosen
zweig and Parry, 1994; IPCC, 2006
The 2007 Nobel Prize for Peace was jointly awarded to IPCC and the former USA Vice President Al Gore in recognition of their efforts in raising public awareness of the implications of global climate changes for environmental conditions and its consequent adverse effects on agriculture, natural resources and ecosystems in general. With few exceptions, agricultural research in tropical countries has a short history and is inadequately supported by national financial resources due to unfavorable agricultural policies. Furthermore, the benefits from the so-called Green Revolution of the 1960’s, supported by international research on major cereal crops, were not equally distributed among continents, countries and crops (Evenson and Gollin, 2003). Moreover, the Green Revolution technologies were based on developing high-yielding new cultivars that required expensive production high-inputs, such as agrochemicals and irrigation facilities that are beyond the reach of the many resource-limited small farmers, particularly in the tropics.
Until three decades ago, crops other than wheat, rice and maize, were not included in the research agenda of the first two commodity-oriented international agriculture research centers, i.e. the International Rice Research Institute (IRRI) in the Philippines and the International Maize and Wheat Improvement Center (CIMMYT, from the Spanish acronym) in Mexico (Wortman, 1981). This situation was largely corrected by the creation of more international research laboratories and centers in Africa, Asia and Latin America dealing with various crops, ecosystems and natural resources management. The International Center for Tropical Agriculture (CIAT, from the Spanish acronym) in Colombia has a world mandate for research on cassava, while the International Institute of Tropical Agriculture (IITA) in Nigeria has a more regional mandate. Research on cassava has received attention and support from many developed countries and from various research/development granting agencies. Before CIAT, with a very few exceptions, cassava was a neglected crop as far as research was concerned in tropical countries where the crop was normally cultivated in marginal lands by resource-limited small farmers and constitutes a main staple for food and feed for both rural and urban habitants. Physiological information on cassava was, until recently, scarce (Hunt et al., 1977; Cock et al., 1979; Cock, 1985).
The cassava physiology section at CIAT conducted both basic and applied research in coordination with a breeding program and took advantage of the diverse genetic resources available within the extensive cassava germplasm collection as well as the diverse environments within Colombia where cassava is cultivated (El-Sharkawy, 1993; Madeley, 1994). The research covered a wide range of ecophysiological aspects of the crop. Cassava was the first cultivated crop to be considered as a C3-C4 intermediate species based on: (i) atypical leaf anatomy including the presence of conspicuous thin-walled bundle-sheath cells with large granal chloroplasts, which are less developed than those in the typical C4 Kranz leaf anatomy (e.g., El-Sharkawy and Hesketh,1965, 1986; Laetsch, 1974); (ii) close physical association of chloroplasts with numerous mitochondria and peroxisomes in bundle-sheath and mesophyll cells; (iii) low photorespiration (as determined by CO2 release from illuminated leaves in a rapid stream of CO2-free air, less than 10% of net photosynthesis) and low CO2 compensation point (G = 20-30 cm3 m-3); (iv) ability to recycle all internal respiratory CO2 within the palisade layer when abaxial stomata of amphistomatous leaves are closed under a wide range of irradiances and temperatures; (v) elevated activities of the C4 phosphoenolpyruvate carboxylase (PEPC) in leaf extracts (10-30% of activities in maize and sorghum); (vi) high percentage (30-60%) of 14C labeling in C4 dicarboxylic acids after 5-10 s exposure under illumination; (vii) and immunological analysis and DNA hybridization of PEPC from cassava and wild Manihot species against antibodies and ppc probes from maize (El-Sharkawy and Cock, 1987a; Cock et al., 1987; Riaño et al., 1987a,b; El-Sharkawy et al., 1989; El-Sharkawy and Cock, 1990; Bernal, 1991; López et al., 1993; Aguilar, 1995; El-Sharkawy, 2004, 2006a; El-Sharkawy and de Tafur, 2007). These characteristics, collectively, underpinned the high photosynthetic rate in normal air (PN > 40 µmol CO2 m-2 s-1) in high irradiances (> 1800 µmol m-2 s-1 of PAR), high leaf temperature from 30 to 40 ºC, and in high atmospheric humidities observed in cassava grown in favorable environments (El-Sharkawy et al., 1992a, 1993).
Moreover, leaf photosynthetic rates, as measured in the field, were significantly correlated with both total biomass and root yield of a wide range of cultivars grown across years and environments. The relations were attributed mainly to nonstomatal (biochemical/anatomical) factors (El-Sharkawy, 2004, 2006a). Cassava also tolerates prolonged drought that often exceeded five months aided by partial closure of stomata, deep rooting systems and small leaf canopy. These plant traits make cassava a desirable and adaptable crop, as source for food and feed, in the tropical regions that would be adversely affected by global climate changes (Rosenzweig and Parry, 1994; Kamukondiwa, 1996; El-Sharkawy, 2005; IPCC, 2006).
This paper summarizes some research findings, published and unpublished, on the responses of cassava to extended water shortages in the field where physiological mechanisms and characteristics related to the crop’s tolerance to drought were sorted out. Moreover, the research laid the foundations for selection and breeding of improved cultivars adapted to prolonged drought normally encountered in seasonally dry and semiarid environments.
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