International research on cassava photosynthesis, productivity, eco-physiology, and responses to environmental stresses in the tropics

Abstract

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 C 4 phosphoenolpyruvate carboxylase (PEPC) that also correlate with leaf net photosynthetic rate (P N ) in field-grown plants, indicating the importance of selection for high P N . Under certain conditions such leaves exhibit an interesting photosynthetic C 3 -C 4 intermediate behaviour which may have important implications in future selection efforts. In addition to leaf P N , 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 P N , 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 as in tropical high altitudes and in low-land sub-tropics, leaf P N 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 ...

Utility of basic research in plant/crop physiology in relation to crop improvement: a review and a personal account

“…Screening for cold-tolerance among a larger group (107 clones) of accessions was conducted under representative field conditions at highland locations (mean annual temperature | 18 o C) using portable infrared gas analyzers. Several clones were identified with significantly higher leaf photosynthetic rates, as compared with the trial mean, due mainly to nonstomatal factors (i.e., biochemical/anatomical leaf traits) (El-Sharkawy, 2006). These selected clones were used in a crossing program for breeding for new more cold-tolerant cultivars.…”

“…Unfortunately, this short-sighted vision on the part of some research managers and funding agencies still prevails today, even in international research centers that are supposedly committed to the development of improved technologies needed by developing countries (Blake, 1994;Kawano and Cock, 2005;El-Sharkawy, 2006). A case in point is what has been taking place in centers like CIAT, where in the past decade it suffered from a severely reduced research budget that led to dispersion of committed and productive scientists as well as in dismantling interdisciplinary-based research programs (El-Sharkawy, 2006).…”

“…A case in point is what has been taking place in centers like CIAT, where in the past decade it suffered from a severely reduced research budget that led to dispersion of committed and productive scientists as well as in dismantling interdisciplinary-based research programs (El-Sharkawy, 2006). Steadfast funding in support of science in general and of maintenance research in particular, as was the case with the Green Revolution, is crucial and should not be subject to political manipulation, fashionable trends and bandwagons (Simmonds, 1991;El-Sharkawy, 2006). Simmonds (1991) stated: a bandwagon is merely the obvious response to a new idea or technique which promises well; if you can t beat em, join em.…”

“…J. Plant Physiol, 18(4): 2006 1993; El-Sharkawy, 2006). Genetic variations in leaf anatomy and activities of photosynthesis enzymes of wild Manihot should be evaluated in breeding programs (El-Sharkawy, 2004.…”

“…Under prolonged water shortage in the field, C 4 PEPC activity is little changed or increased while the C 3 Rubisco activity is significantly decreased resulting in a higher PEPC/Rubisco ratio. Under drought with stomata partially closed, PEPC effectively recycles internal respiratory CO 2 (El- Sharkawy, 2004Sharkawy, , 2006, a mechanism that may partially explain the reasonable yields under semiarid environments (Fukuda et al, 1992(Fukuda et al, -1993El-Sharkawy, 1993;de Tafur et al, 1997).…”