Mabrouk A. El-Sharkawy

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

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 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.

Long-term effects of mulch, fertilization and tillage on cassava grown in sandy soils in northern Colombia

Abstract Cassava (Manihot esculenta Crantz) is often cultivated in sandy soils that are very low in nutrients and organic matter. Under such conditions, yields often decline when the crop is grown successively without fertilizer application. An 8-year on-farm trial was conducted on sandy soils in northern Colombia to study effects of (1) surface mulching with residues of the grass Panicum maximum, (2) moderate applications of N, P and K fertilizer and (3) tillage on crop productivity, root quality and soil properties. Mulch applications significantly increased root and top biomass, increased root dry matter content while reducing its yearly variation, and decreased root HCN, particularly in the absence of fertilizer. Mulch applications also significantly reduced soil temperatures within the top 20 cm and increased soil organic carbon, K, P, Ca and Mg. Without mulch, soil pH decreased over the years. Annual applications of 21 kg ha−1 P resulted in a build-up of soil P, whereas no build-up of soil K was observed with applications of 41 kg K ha−1. The application of N, P and K fertilizer significantly increased root and top biomass and reduced root HCN, especially if no mulch was applied. Where both mulch and fertilizer applications were absent, root yield and top biomass declined over the years. Tillage, except when combined with fertilizer application, had no effect on root yield, top biomass, root dry matter or HCN contents. Neither were effects of tillage observed in any of the studied soil parameters. The trial indicated that, to sustain cassava productivity in poor sandy soils, applications of plant mulch and/or chemical fertilizer are highly desirable.

Stomatal response to air humidity and its relation to stomatal density in a wide range of warm climate species

The gas exchange of 19 widely different warm climate species was observed at different leaf to air vapour pressure deficits (VPD). In all species stomata tended to close as VPD increased resulting in a decrease in net photosynthesis. The absolute reduction in leaf conductance per unit increase in VPD was greatest in those species which had a large leaf conductance at low VPDs. This would be expected even if stomata of all species were equally sensitive. However the percentage reduction in net photosynthesis (used as a measure of the relative sensitivity of stomata of the different species) was also closely related to the maximal conductance at low VPD. Similarily the relative sensitivity of stomata to changes in VPD was closely related to the weighted stomatal density or ‘crowding index’.The hypothesis is presented that stomatal closure at different VPDs is related to peristomatal evaporation coupled with a high resistance between the epidermis and the mesophyll and low resistance between the stomatal apparatus and the epidermal cells. This hypothesis is consistent with the greater relative sensitivity of stomata on leaves with a high crowding index.The results and the hypothesis are discussed in the light of selection, for optimal productivity under differing conditions of relative humidity and soil water availablility, by observation of stomatal density and distribution on the two sides of the leaf.

Response of cassava to water stress

SummaryCassava (Manihot esculenta Crantz) is a staple food for a large sector of human population in the tropics. It is widely produced for its starchy roots by small farmers over a range of environments on poor infertile soils with virtually no inputs. It is highly productive under favorable conditions and produces reasonably well under adverse conditions where other crops fail. The crop, once established, cansurvive for several months without rain. There is a wide variation within the cassava germplasm for tolerance to prolonged drought and the possibility to breed and select for stable and relative high yields under favorable and adverse conditions does indeed exist. Research with several cassava clones at CIAT has shown that high root yield under mid—term stress is not incompatible with high yield under nonstress conditions. Plant types with high yield potential under both conditions (e.g. the hybrid CM 507-37) are characterized by having slightly higher than optimum leaf area index under nonstress conditions, higher leaf area ratio and more intensive and extensive fine root system.

Relationships between biomass, root-yield and single-leaf photosynthesis in field-grown cassava

Abstract Preliminary field screening with 127 cultivars of cassava ( Manihot esculenta Crantz) was conducted in 1986/1987 to determine the relationships among single-leaf photosynthesis, shoot and total biomass, and storage-root tield. Gas exchange (CO 2 uptake and H 2 O loss) of individual leaves was monitored on three different occasions at 4–6 months after planning. There were significant correlations among leaf photosynthesis, total biomass and root yield across all cultivars. When the cultivars were grouped on the basis of top weight (as a proxy for leaf area), the correlations were significant only with average and high top-weight cultivars. The same trends were observed for correlations among mesophyll conductance, total biomass and root yield. There were no significant correlations between biomass or root yield and leaf conductance. The results suggest that, when light interception is not limiting, selection for high leaf photosynthesis is likely to lead to higher yield. The results of a second-year trial (1988/1989) with 16 cultivars, selected on the basis of their yield, showed a significant correlation between leaf photosynthesis, measured only once at 4 months after planting, and final root yield. Root yield was positively correlated with mesophyll conductance, and negatively correlated with intercellular CO 2 , but not significantly correlated with leaf conductance. Furthermore, root yield in the 1988/1989 season was significantly correlated with leaf photosynthesis measured in the 1986/1987 season. It is suggested that screening for high leaf photosynthetic rate under field conditions could be used as a selection criterion for parent materials to obtain progeny with high yield.

Yield Stability of Cassava During Prolonged Mid-Season Water Stress

Yield stability in cassava requires genotypes that produce well under the variable moisture conditions encountered during the growth cycle. Plant characteristics related to yield stability were studied in two cassava clones subjected to 105 days of water stress in a field drainage lysimeter. Stress conditions commenced 117 days after planting, and the plants were allowed to recover at the end of the stress period for the rest of the growth cycle. Water stress restricted the growth of leaves and stems, but root yields were increased or remained unaffected. Leaf water potential varied little with stress, but gas exchange rates were about 75% those of the control throughout the stress period in both cultivars. Under stress, the plants partially closed their stomata and extracted deep soil moisture slowly. A high yield in both wet and stressed environments was associated with high mean LAI, better leaf retention, and greater partitioning of shoot biomass into leaf formation.

The effect of pre-harvest pruning of cassava upon root deterioration and quality characteristics

One of the major constraints of cassava as a crop is its perishability. Physiological deterioration, parenchymal blue–black vascular streaking, often starts within 24 h after harvest. This paper presents the results of a detailed study of the effects of pre-harvest pruning upon post-harvest physiological deterioration (PPD) and some other quality characteristics. Six cultivars, grown at CIAT (Centro International de Agricultura Tropical), with varying intrinsic susceptibility to PPD, were assessed at pruning–harvest intervals of 0, 2, 4, 6, 8, 10, 15, 20, 25, 28 and 39 days. After harvesting, the roots were analysed. For the unpruned plants a low susceptibility was found to coincide with a low dry matter content and a high sugar content. After pruning, the susceptibility for all cultivars was drastically reduced, reaching a minimum of around 25% of the original value for a pruning–harvest interval of up to 25 days. Beyond this interval the plants slowly develop new leaf canopy, normal assimilation sets in again and the starch content increases. Analysis of the cassava roots revealed a relationship between the combined sugar and starch contents and the interval duration, and that sugar and starch contents were inversely related to each other. The sugar content increased with the interval period, probably as a result of starch hydrolysis. Other properties such as the contents of dry matter, cyanogen, scopoletin, amylose and reducing sugars and the starch pasting properties were not affected by pruning to a comparable, interval-dependent, extent. It is concluded that the sugar content, ie the sugar/starch ratio, of cassava roots is positively related to their resistance to post-harvest physiological deterioration. © 2000 Society of Chemical Industry

Photosynthesis of Cassava ( Manihot esculenta )

In recent studies of cassava at CIAT, net CO 2 uptake rates of 20 to 35 μmol CO 2 m −2 s −1 were commonly observed. Cassava photosynthesis has a high optimum temperature (35°C) and a wide plateau (25 to 35°C) corresponding to the temperature range under which cassava is cultivated. Leaf photosynthesis requires high saturation irradiance (1500 μmol m −2 s −1 ) and the rates are greatly reduced by leaf-air vapour pressure differences above 1.5 kPa; this reduction is associated with stomatal closure. Cassava leaves have low photorespiration, low CO 2 compensation point, high percentage of carbon fixation in C 4 acids and a high PEP-carboxylase activity (15–35% of that in maize), but cassava does not have the typical ‘C 4 -Kranz’ anatomy. Field measurements of single leaf photosynthesis among a wide range of cultivars grown under rain-fed conditions showed that when light interception was not limiting, there were significant correlations between leaf photosynthesis, total biomass and root yield. This suggests that the use of parental materials with high photosynthetic capacity, in combination with other yield determinants, could be a successful strategy for developing high yielding cultivars. This might be done by exploiting any genetical variations in leaf anatomy and biochemistry that could enhance photosynthesis efficiency and hence productivity.

Cassava varietal response to phosphorus fertilization. I. Yield, biomass and gas exchange

Abstract Cassava ( Manihot esculenta Crantz) yield can be limited by phosphorus (P) supply. Previous trials conducted at CIAT indicated a wide range of response to P fertilization in cassava germplasm. The purpose of this study was to elucidate possible mechanisms and morphological traits underlying cassavas differential response to P fertilization. Four contrasting varieties were grown under rainfed conditions for 10 months in the field for two consecutive seasons under four fertilizer treatments. There were three dosages of P fertilizer at 0, 50 and 100 kg P ha −1 with 100 kg NK ha −1 , plus a control with no fertilizer. Sequential harvests that included biomass determination were conducted. Single-leaf gas exchange was monitored by portable infrared gas analysis systems. All varieties showed a significant positive response in total and aerial biomass as well as mean leaf area index (LAI) to P fertilization, at least in the second year. The variety M Col 1684 was the only one that did not significantly respond in terms of dry root yield and number of storage roots to P fertilization over the years. This variety was among the most responsive for peak LAI, number of aerial apices and reproductive organs (flowers and fruits). Yield and storage root number correlated positively with net photosynthetic rates and negatively with reproductive organs. Variety M Col 1684 could not maintain a high harvest index with increasing aerial growth. In contrast, CM 489-1 balanced aerial growth enhancement with intensive root formation, augmenting root sink strength and yield. It is concluded that yield response to P fertilization depended upon the balance between aerial growth potential of varieties and storage root sink strength.

C3-C4 intermediate photosynthetic characteristics of cassava (Manihot esculenta Crantz)

Cassava, bean and maize leaves were fed with(14)CO2 in light and the primary products of photosynthesis identified 5 and 10 seconds after assimilation. In maize, approximately three quarters of the labelled carbon was incorporated in C4 acids, in beans about two thirds in PGA, and in cassava approximately 40-60% in C4 acids with 30-50% in PGA. These data indicate that cassava possesses the C4 photosynthetic cycle, however due to the lack of typical Kranz anatomy appreciable carbon assimilation takes place directly through the Calvin-Benson-Bassham cycle.Cassava, bean and maize leaves were fed with14CO2 in light and the primary products of photosynthesis identified 5 and 10 seconds after assimilation. In maize, approximately three quarters of the labelled carbon was incorporated in C4 acids, in beans about two thirds in PGA, and in cassava approximately 40–60% in C4 acids with 30–50% in PGA. These data indicate that cassava possesses the C4 photosynthetic cycle, however due to the lack of typical Kranz anatomy appreciable carbon assimilation takes place directly through the Calvin-Benson-Bassham cycle.