James H. Cock

Response of cassava to water shortage I. Growth and yield

Abstract Rainfall was witheld from crops of two cultivars of cassava, M Col 22 and M Mex 59, for 10 weeks commencing 12 weeks after planting. The recuperation phase continued until the crops were 10 months old. Sequential harvests were made to define the effect of water shortage on rates of crop growth and the distribution of the biomass in the various component organs. Measurements were also made of the soil water content, fine root distribution and sugar and starch contents of stems and storage roots. Biomass production was substantially reduced especially during the second half of the stress period. M Mex 59 maintained the higher leaf area index during the stress and was least affected in total biomass production. The proportion of biomass in storage roots increased under stress, particularly in M Mex 59, and the modified distribution pattern persisted during the recovery. At ten months the M Col 22 control at 11.2 t/ha outyielded the stress plots of M Col 22 (7.6 t/ha) and the M Mex 59 control (7.3 t/ha). The previously stressed M Mex 59 (10.7 t/ha) outyielded the M Mex 59 control and was not significantly different from the M Col 22 control. The results are discussed in terms of two published models of assimilate allocation in cassava.

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.

Response of cassava to water shortage II. Canopy dynamics

Abstract Crops of two cultivars of cassava, M Col 22 and M Mex 59, were subjected to water shortage during the period 12–20 weeks of their growth cycle. The crops were then allowed to recover until a final harvest was made at 10 months. Measurements of the components of leaf area production, i.e. rate of apex production, rate of leaf production per apex and rate of leaf expansion were maintained along with the measurements of leaf senescence for the entire crop cycle. The loss of leaf area during water stress is shown to be dominated by restricted leaf area development and not by leaf loss. Leaf life was increased under stress. Leaf size was the most sensitive parameter of leaf area production and the most responsive following the release from stress, but apex production and the rate of leaf production per apex were also susceptible. The reliance on reduced leaf formation rather than leaf fall is an important feature of an efficient carbon balance under stress.

Branching habit as a yield determinant in cassava

Abstract Cassava was grown in the field and the ability of the roots to accept extra carbohydrate and the effects of branching habit on yield were observed. Apex and root were competing sinks; roots accepted more carbohydrate than was available in normal situations. Reduced sink did not limit total growth. Yield was increased by 75% by branch control. Good yields can be obtained by either late-branching types or continuous branching types with two branches at each branch level. Late-branching types appear to be the easiest to obtain in a breeding program.

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.

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.

The physiological basis of genotype—temperature interactions in cassava

Abstract Four different cassava varieties of low, medium, high and very high vigour were planted at three sites with mean temperatures of 20, 24 and 28°C. Harvests were taken at 8, 12 and 16 months. After 12 months Popayan, the most vigorous variety, yielded most (29 t/ha) at 20°C and least (9 t/ha) at 8°C. M Colombia 22, the least vigorous variety, yielded least (9 t/ha) at 20°C and most at 28°C (39 t/ha). Rate of root dry matter increase was maximum at LAI (leaf area index) of three in all sites. Above this value the rate of root dry matter increase decreased. The same phenotype yields well over a wide range of temperature but the genotype is different.

The Humidity Factor in Stomatal Cokttrol and Its Effect on Crop Productivity

Stomata of various woody and herbaceous plant species respond directly to changes in leaf-to-air vapor pressure difference (VPD). Closure of stomata upon exposure to dry air occurs in many species without changes in bulk leaf water status, suggesting an underlying mechanism different from the well-known closure through reduction in bulk leaf water potential. Recent studies in our laboratory on the response of cassava to water stress demonstrated that plants grown in pots or in the field, with and without soil water stress, were very sensitive to changes in atmospheric humidity. Both CO2 uptake rate and H2O loss decreased greatly as VPD increased. This decrease in gas exchange rate was associated with a reduction in leaf conductance in the absence of changes in leaf water potential. The strong stomatal response to changes in VPD may be of particular importance to perennial crops, such as cassava, that may have to endure a long period of drought. Under these conditions, and in the absence of stomatal response to humidity, both photosynthesis and transpiration will continue at relatively high rates until available soil water is depleted and leaf water potential drops to the level required to induce stomatal closure, at which time both photosynthesis and transpiration will approach zero. In such case, most of the transpirational loss will occur during periods of high VPD and low photosynthesis/transpiration ratio, resulting in a low dry matter accumulation per unit water transpired. On the other hand, with a direct stomatal response to changes in air humidity, available soil water will be depleted slowly, as most of the transpirational loss will occur during periods of the day when VPD is low and water use efficiency is highest. With a prolonged period of limited soil water, the greater water use efficiency will lead to a greater total accumulation of photosynthate over the stress period. Thus, the direct stomatal mechanism is beneficial for those crops that experience long period of drought. However, with nonlimiting soil water conditions or only short periods of soil water stress, optimizing water use efficiency would not be as important as maximizing photosynthesis and consequently crop productivity.