African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 11, Num. 2, 2003, pp. 107-117

African Crop Science Journal, Vol. 11. No. 2, 2003,  pp. 107-117

LEAF CHLOROPHYLL CONTENT AND TUBEROUS ROOT YIELD OF CASSAVA IN INLAND VALLEY

M. T. LAHAI, I. J. EKANAYAKE¹ and J. B. GEORGE

Department of Crop Science, Njala University College, PMB, Freetown, Sierra Leone
¹International Institute of Tropical Agriculture (IITA), PMB 5320, Oyo Road, Ibadan, Nigeria (Current Address: c/o VVOB Secretariat, P. O. Box 13322, Paramaribo, Suriname)

(Received 12 April, 2002; accepted 25 March, 2003)

Code Number: cs03013

ABSTRACT

Two field trials were established in 1996 and 1997 to assess genotypic variability of four cassava (Manihot esculanta) cultivars for adaptability in the inland valley in terms of leaf chlorophyll content and tuberous root yield, using a 4 x 4 Latin square design with four replications arranged along the toposequence.  Leaf chlorophyll a, b and ab contents and root yield of the improved cultivars were similar, but were significantly greater than those of Isunikankiyan, the local cultivar.  Plants in the valley fringe had the highest concentrations of the three chlorophyll components and root yield.  Leaf chlorophyll a and ab contents and root yield were significantly higher in 1996 than in 1997, due partly to favourable weather conditions in 1996.  However, the reverse was the case for leaf chlorophyll b content, suggesting that chlorophyll b concentration may increase under stressful conditions.  Root yield in all cultivars increased with increase in concentration of the three leaf chlorophyll components, but chlorophyll a and ab were more correlated with yield than chlorophyll b.  Correlation between chlorophyll contents and root yield were strongest for TMS 91/02324 with the highest root yield, and weakest for Isunikankiyan, the lowest yielder.  Root yield of the four cultivars and concentrations of the three chlorophyll components decreased linearly as the groundwater table depth became shallow.  Therefore, selection of cassava cultivars that can maintain high leaf chlorophyll contents under moisture stress can lead to high root yield when combined with other yield determinants in inland valleys.

Key Words:  Ground water table, improved cultivars, moisture stress, Nigeria

RÉSUMÉ

Les essais de terrain étaient établis en 1996 et 1997 pour déterminer la variabilité génotypique de quatre variétés de manioc (Manihot esculenta) en adaptabilité dans les vallées en termes du contenu en chlorophile dans les feuilles et le rendement en tubercules, utilisant un arrangement 4x4 carré latin avec quatre replications sur la toposéquence. Les contenus de chlorophiles a, b, et ab dans les feuilles et dans les racines des variétés améliorées étaient similaires, mais étaient significativement grande par raport à la variété locale Isunikankiyan. Les plantes dans les vallées avaient des concentrations élévées de trois composantes du chlorophiles et le rendement en racines. Les contenues en chlorophiles a et ab et le rendement en racines étaient significativement élévés en 1996 par raport à 1997, probablement à cause des conditions climatiques en 1996. Cependant, l'inverse était observé pour le contenu en chlorophile b dans les feuilles, suggérant que la concentration en chlorophile peut augmenté dans les conditions de stress. Le rendement en racines a augmenté pour toutes les variétés avec une augmentation de concentration de composantes de chlorophiles, avec une corrélation élévée pour les chlorophiles a et ab. La corrélation entre le contenu en chlorophile  et le rendement en racine était plus élévée pour TMS 91/12324 qui avait aussi le plus grand rendement. Isunikankiyan avait une faible corrélation et un faible rendement. Le rendement en racine de quatre variétés et les concentrations de trois composantes de chlorophiles diminué linéairement avec l'abaissement du niveau d'eau dans le sol. Par conséquent, la sélection de variétés du manioc qui peut maintenir les niveaux de chlorophiles élévés sous des conditions d'humidité déficientes peut conduire à des rendements élévés quand combiné avec d'autres determinants de rendements dans les vallées.

Mots Clés: Niveau d'eau dans le sol, variété améliorée, stress d'humidité, Nigeria

INTRODUCTION

Cassava (Manihot esculenta Crantz) is a crop of great importance for the nutrition of over 500 million people in the tropical world (Bokanga, 1994).  It is predominantly an upland crop.  However, in West and Central Africa, it is a favourite crop grown in inland valleys (IVs) during the dry season to make use of the residual soil moisture (Carsky et al., 1993).  The IVs are the upper reaches of river systems comprising valley bottom, their hydromorphic fringes and continuous upland slopes (Andriesse et al., 1994).  They exhibit considerable potential for intensified and sustainable land use, due to better water availability throughout the growing season as compared to adjacent uplands (Ekanayake et al., 1994).  The IVs occupy about 25 million hectares of land in West Africa and 130 million hectares (7% of the total land area) in tropical sub-Saharan Africa with growing periods of more than 150 days.  However, only 10 to 25% of IVs are used for crop production (Andriesse et al., 1994).  The total world arable land is limited and as population increases in Africa, arable land available for cultivation is reducing at an alarming rate.  This reduction, coupled with unfavourable drought conditions in some areas may lead to a drastic decline in food production.  The IVs therefore, represent a large surface area with high fertility potential that should be exploited, without degradation of the environment and water resources.

Genetic improvement work on cassava so far has focused on the selection of varieties adapted to upland conditions and not much is known about the performance of the improved clones bred at the International Institute of Tropical Agriculture (IITA) in the IVs.  Moreover, little work has been done to assess the physiological traits associated with adaptability of cassava in IVs relative to that of the upland (Ekanayake et al., 1994).  One such trait is the leaf chlorophyll content which influences photosynthesis and light absorption and is often indirectly associated with growth and yield of cassava (Ekanayake et al., 1996; Oyetunji et al., 1998).  Decreases in photosynthetic activity are often paralleled by a reduction in leaf chlorophyll content (Ekanayake et al., 1998).  Ekanayake et al (1996) noted that low moisture condition, high air and soil temperatures at middays and very low temperatures at nights during the harmattan period negatively affect chlorophyll fluorescence activities, which contribute to reduction in growth rate of cassava during the dry season in the upland.

However, the constraints to production in the IVs are different from those on the uplands.  In particular, excess moisture is often a constraint to crop growth in the IVs.  Excess soil moisture or shallow groundwater table may result in restricted rooting depth, which leads to nutrient deficiency and poor growth (Jackson and Dew, 1984).  Young seedlings of bitter melon (Momordica charantia) under flooding conditions are shown to have exhibited reduced leaf CO₂ exchange rate, stomatal conductance, transpiration rate and chlorophyll content (Liao and Lin, 1994).  Deleterious effects on crop growth and tuber yield components of potato under flooded conditions have also been reported (Ekanayake, 1994).  However, possible differences between cultivars in adaptive mechanisms to waterlogging have not been fully explored for cassava.  This study was, therefore, undertaken to assess genotypic variability for adaptability of cassava in the IVs in terms of leaf chlorophyll content and tuberous root yield.

MATERIALS AND METHODS

Site description. The trials were conducted in a small inland valley on the IITA research farm at Ibadan (7° 30' N and 3° 54' E at altitude 243m) in southwestern Nigeria.  The bimodal character of rainfall distribution in this region results in two distinct growing seasons, one from April to July and the other from August to November.  Total annual rainfall varies from 1250 to 1750 mm with an average daily temperature of 26^oC.  The predominant upland soils in the area are Oxic Paleustalf and the valley bottom soils are Tropaquents (Moormann et. al., 1975).  Cassava was previously grown at the site from December to June followed by seven months of fallow before the first year's (1996) trial was established.  The second year's (1996/97) trial, on the same site followed after four months of fallow with natural vegetation.

Experimental design and plant establishment. The experimental design was a replicated 4 x 4 Latin square with four replications.  The replications were arranged along the toposequence and in each replication, three PVC pipes each 2m-long, 0.05m in diameter and buried to a depth of 1.5m to serve as modified piezometers were evenly installed to monitor groundwater table (GWT) depth.  In each replication, 16 flat-topped mounds each measuring 2m x 2m and 0.6m high were constructed 1m apart.  Each mound was regarded to constitute a plot and six stem cuttings (0.25-0.30m long), taken from mature healthy plants were planted on each at a spacing of 1m x 0.75m.  Planting was done on 12 February and 18 December 1996 during first and second cropping seasons, respectively.  Four cassava cultivars: Isunikankiyan (landrace), TMS 4(2)1425, TMS 91/02324 and TMS 91/02327 (improved IITA genotypes) were used.  The cuttings were treated with a pesticide 'perfekthion' in a ratio of 0.2ml to 20 litres of water before planting to prevent termite damage.  A day after planting, a pre-emergence herbicide, 'gramoxone' (containing 200g paraquat per litre) was applied to the entire experimental area at a rate of 100ml to 20 litres of water.  No fertilisers were applied.

Determination of leaf chlorophyll content.  Leaf chlorophyll content was determined on a two-monthly basis using the method described by Ekanayake and Adeleke (1996).  Fully expanded upper two or three leaves taken from four plants per plot were used for extraction.  Leaf tissue weighing 2g was crushed in a mortar and 80% acetone was added to it in sufficient quantities to allow the tissue to be thoroughly homogenised.  The supernatant was decanted through a filter paper into a 100ml volumetric flask.  Acetone was again added to the residue in the mortar and the extraction procedure repeated.  Additional acetone was used to wash off the chlorophyll until the 100ml mark was attained in the volumetric flask.  The resulting solution was thoroughly mixed and 5ml pipetted into a 50ml volumetric flask and made up to volume with 80% acetone.  The absorbance of the extract was then measured at 645, 663, and 652 nm wavelengths for chlorophyll a, b and ab, respectively, using 80% acetone as blank.  Concentrations of chlorophyll (mg /g fresh leaf weight) were calculated as follows: chlorophyll a = (20.2 x D₆₄₅) x (50/1000) x (100/5) x (1/2); chlorophyll b = (8.02 x D₆₆₃) x (50/1000) x (100/5) x (1/2); and total chlorophyll = (20.2 x D₆₄₅ + 8.02 x D₆₆₃) x (50/1000) x (100/5) x (1/2), where D = absorbance.

Final harvesting.  The 1996 and 1997 trials were harvested on 15 August 1996 and 23 July 1997, respectively.  All the plants in each plot (mound) were harvested.  Total fresh tuberous root weights were recorded and two sub-samples of fresh materials (250g) in each plot were dried in a ventilated oven at 70^oC for 48h to determine dry root weight.

Statistical analyses. The data were statistically analysed with SAS for Microsoft Windows, Release 6.10 (SAS Institute, 1989).  Both chlorophyll (averaged over sampling periods) and yield data were arranged based on the split-split-plot-like format with cassava cultivars as main plot, toposequence position as sub-plot and year as sub-sub-plot.  The analysis of variance (F-test) and comparisons between means were calculated with the mixed model procedure, using the restricted maximum likelihood method (REML) for the estimation of the random variance components.  In all cases rows and columns were taken as random effects and cultivars, toposequence positions and years considered as fixed effects.  Relationships between chlorophyll content, tuberous root yield and GWT depth were explored using conventional regression and correlation approaches.

RESULTS

Crop environment.  There were differences in climatic factors during the two growing seasons in the IVs.  On average, minimum temperature in 1996 (19.8°C) was higher than in 1997 (19.1°C), but the reverse was the case for maximum temperature (27.5°C and 28.4°C in 1996 and 1997, respectively).  Mean temperature was also slightly lower in 1996 (23.7°C) than in 1997 (23.8°C).  The minimum, maximum and mean relative humidity in 1996 (50.6%, 85.1% and 68.0%, respectively) were greater than in 1997 (41.9%, 84.5% and 63.1%, respectively) (Fig. 1a, b).  Total rainfall (955.9mm) and number of rainy days (92) were also higher in 1996 than in 1997 (642.6mm and 64 days), but evaporation and solar radiation were greater in 1997 (962.8mm and 13.49MJ/m/day) than in 1996 (891.1mm and 13.09MJ/m/day, respectively) (Fig. 1c, d).

The toposequence positions were designated based on the distance away from the upland.  Valley fringe was closest and valley bottom 2 farthest away from the upland.  There was a high water table gradient along IVs fringe as compared to the other toposequence positions (valley intermediate and valley bottom 1 and 2).  This was followed by valley bottom 1 position, while the lowest water table depth was observed in the valley intermediate during both years (Fig. 1e, f).  The GWT depth responded to rainfall, becoming shallow as rainfall increased.  The GWT depth in 1996 with higher rainfall was shallower than that in 1997 with lower rainfall (Fig. 1e,f).

Leaf chlorophyll content. Highly significant variations in leaf chlorophyll concentrations were observed among cassava cultivars in the IVs.  In general, chlorophyll a, b and ab (total chlorophyll) contents of the leaves of the improved TMS cultivars were similar, but were significantly greater than those of Isunikankiyan, the local check (Tables 1 and 2).

The effects of toposequence position and year on the concentration of the three chlorophyll components were also highly significant.  Plants in the valley fringe had the highest concentrations of the three chlorophyll components, but were only significantly different from plants in valley intermediate and valley bottom 2 positions.  Differences between valley intermediate and valley bottom positions were not significant (Tables 1 and 2).  Both chlorophyll a and total chlorophyll concentrations were significantly greater in 1996 than in 1997, but the reverse was the case for chlorophyll b concentration.  Generally, interactions between the main effects were not significant for all chlorophyll components (Table 2).

Dry tuberous root yield.  Differences in dry tuberous root yield were highly significant among cassava cultivars (P<0.0001).  Generally, TMS 91/02324 and TMS 91/02327 (more recently selected elite clones) produced similar tuberous root yields, but were significantly higher than those of TMS 4(2)1425 (old elite clone) and Isunikankiyan (local clone).  The yield of TMS 4(2)1425 was only slightly higher than Isunikankiyan (Tables 1 and 2).

Similarly, the effect of toposequence position on tuberous root yield was highly significant.  Plants in the valley fringe significantly gave the highest dry root yield, while no significant differences were observed among the other toposequence positions (Tables 1 and 2).  Moreover, root yield in 1996 was significantly greater (45%) than in 1997, but interactions between the main effects were not significant (Table 2).

Leaf chlorophyll content effect on dry tuberous root yield. Analysis by regression shows that cassava tuberous root yield increased with increase in concentration of the three chlorophyll components.  However, chlorophyll a and total chlorophyll were more correlated with storage root yield than chlorophyll b for all the cassava cultivars (Fig. 2). Also, correlation between chlorophyll concentration and root yield was higher for the improved (TMS) cultivars than the local cultivar for all chlorophyll components.  In addition, correlation between root yield and chlorophyll content was highest for TMS 91/02324 with the greatest root yield, and weakest for Isunikankiyan, with the lowest yield (Fig. 2).

GWT depth effects on leaf chlorophyll content and tuberous root yield. Regression analysis was also used to explore the responses of root yield and leaf chlorophyll content to GWT depth (Fig. 3).  Root yields of the four cultivars and the concentration of the three chlorophyll components significantly decreased linearly as GWT depth became shallow (Fig. 3a,b).

DISCUSSION

Several studies have shown cultivar differences for leaf chlorophyll contents in cassava in upland ecology (Ekanayake et al., 1996, 1998; Pereira et al, 1986; Oka and Matsuda, 1983).  Similar results were obtained in our study in the inland valley under different GWT depths.

Decreases in photosynthetic activity are paralleled by a reduction in leaf chlorophyll content (Ekanayake et al., 1998), and leaf chlorophyll content is often indirectly associated with growth and yield of cassava (Ekanayake et al., 1996; Oyetunji et al., 1998).  Our results are consistent with these findings in that the improved (TMS) cultivars, which gave the highest root yields also, had the highest concentrations of all the three chlorophyll components.  We also found that root yields of all cultivars increased linearly with leaf chlorophyll concentration and that the higher the correlation between root yield and chlorophyll concentration of a cultivar the higher the root yield.  Thus, as in the upland, increased concentration of leaf chlorophyll content could lead to high tuberous root yield in the IVs.  However, for all cultivars used chlorophyll a and total chlorophyll contents correlated more with root yield than chlorophyll b concentration.  This is in agreement with the results of Tankou and Lyonga (1994) who noted chlorophyll a to be more correlated with storage root weight than other chlorophyll components in four cassava cultivars.  Ezulike and Egwuatu (1990) attributed this to better light absorption by chlorophyll a than chlorophyll b.  Consequently, the high correlation between root yield and total chlorophyll content than chlorophyll b was likely largely due to the contribution of chlorophyll a.

Both chlorophyll a and total chlorophyll contents as well as root yield were greater in 1996 than in 1997.  This again supports the positive correlation observed between these chlorophyll components and root yield of cassava in the IVs.  Ekanayake et al (1996) observed that low moisture condition, high air and soil temperatures at mid-day and very low temperatures at night during the harmattan period negatively affect chlorophyll fluorescence activities, which contribute to reduction in growth rates of cassava during the dry season.  During the conduct of our study, there was virtually no rainfall for the first three months after planting in 1997 (Fig. 1d).  This, coupled with high evaporation rate and low mean minimum temperature and relative humidity, but high maximum temperature compared to 1996 (Fig. 1a,b,c,d) probably  exerted a lot of environmental stress on the cassava plants, resulting in low chlorophyll a and total chlorophyll concentrations and subsequently low root yield in 1997.  However, chlorophyll b concentration was higher and also greater than chlorophyll a in 1997 than in 1996.  Johnston and Onwueme (1998) showed that chlorophyll a:b ratio was less in plants grown under shade than in plants exposed to full sunlight, and that compared with chlorophyll a, chlorophyll b increased significantly under low light.  This suggests that chlorophyll b concentration may increase under stressful conditions compared to chlorophyll a.

During both years, root yield of all cultivars were significantly higher in valley fringe, followed by valley bottom 1 toposequence position, while the lowest was obtained in valley intermediate.  Generally, root yields of all cultivars decreased linearly as the GWT height increased towards the soil surface.  These results are consistent with those of several researchers (Carsky et al., 1993; Mohamoud, 1994; Jalloh, 1998; Lahai et al., 1999).  Therefore, the high root yield in the valley fringe was largely due to the deeper GWT depth in that toposequence representative of an upland site while the lowest yield was obtained in the valley intermediate with the shallowest GWT depth and highest waterlogging stress.  The shallow GWT depth and low yield in the valley intermediate as compared valley bottom were unexpected.  Lahai et al. (1999) reported deeper GWT depth and higher root yield (62%) in valley intermediate than in valley bottom, which is contrary to our results.  At our IVs site, there was a depression immediately after the valley fringe causing more water to accumulate in the valley intermediate than in the other toposequence positions.  The observation of GWT being more stressful to the cassava plants as the rooting zone had continuous standing water and poor aeration.  This supports the conclusion of Jalloh (1998) that productivity of cassava in the IVs is influenced more by GWT depth than any other environmental stress.  The differing GWT depth and root yield in the valley intermediate between the IVs site of Lahai et al. (1999) and ours was supported by Izac et al. (1991) who reported that inland valleys are complex and heterogeneous because physical, morphological and socio-economic conditions change between and within them.

As in the case of tuberous root yield, concentrations of all three chlorophyll components decreased linearly with shallowness of GWT depth.  Liao and Lin (1994) observed reduction of chlorophyll content in bitter melon subjected to flooding which also agrees with our results.  Therefore, low cassava root yield usually recorded in the IVs as compared to the upland (Lahai et al., 1999) could partly be attributed to decrease in leaf chlorophyll content and the subsequent reduction in light absorption and photosynthesis, conditions which normally occur as GWT depth becomes shallow.  This colloborates with results reported by Liao and Lin (1994) that seedlings of bitter melon under flooding conditions exhibited reduced leaf CO₂ exchange rate, stomatal conductance and transpiration rate, processes that influence the rate of photosynthesis.

CONCLUSIONS

Differences in tuberous root yield and leaf chlorophyll content among cassava cultivars exist in the inland valley and the improved cultivars (TMS 91/02324 and TMS 91/02327) with the highest root yield also virtually had the highest concentrations of the three chlorophyll (a, b and ab) components.  Root yield increased linearly with leaf chlorophyll concentration, and the better the correlation between root yield and chlorophyll content of a cultivar the higher the root yield.  Chlorophyll a and total chlorophyll contents correlated more with root yield than chlorophyll b content.  Root yield and leaf chlorophyll content also decreased linearly as GWT depth became shallow.  Therefore, selection of cassava cultivars that can maintain high chlorophyll (particularly chlorophyll a) content under excess moisture stress can lead to high tuberous root yield when combined with other yield determinants in inland valleys.

ACKNOWLEDGEMENT

This study was funded by IITA.  The senior author is grateful for the research fellowship awarded by IITA to study for  a PhD degree at Njala University College, Sierra Leone.

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