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Leaf density profile and efficiency in partitioning dry matter among high and low yielding cultivars of cassava (Manihot esculenta crantz)
Field Crops Research, 10 (1985) 291--303
291
Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands
L E A F D E N S I T Y P R O F I L E A N D E F F I C I E N C Y IN P A R T I T I O N I N G DRY MATTER AMONG HIGH AND LOW YIELDING CULTIVARS OF CASSAVA (MANIHOT ESCULENTA CRANTZ)
T. RAMANUJAM
Central Tuber Crops Research Institute, Trivandrum 695 017, Kerala (India) (Accepted 21 November 1984)
ABSTRACT Ramanujam, T., 1985. Leaf density profile and efficiency in partitioning dry matter among high and low yielding cultivars of cassava (Manihot esculenta Crantz). Field Crops Res., 10: 291--303. Leaf area density, and efficiency in partitioning dry matter for tuber growth were studied using six cultivars of cassava which differ in their yielding ability. The cultivars of the low yielding group maintained either sub-optimal (< 2) or supra-optimal (> 4 ) levels of leaf area index during the major part of growth period and this adversely affected their light utilization efficiency. A significant negative correlation (r = -0.78) was observed between leaf area density and light transmission ratio. For better light interception and utilization, a leaf area index of 2.5--3.5 was found to be optimum f o r cassava. A significant difference in partitioning dry matter (DM) for tuber growth was noticed among the cultivars tested, which varied from 0.3 to 0.7 per unit of DM produced. The short-statured eultivar Ci 590 was more efficient in utilizing DM for storage root growth (harvesting efficiency = 70%). The leaf density profile suggested that the productivity of Ci 590 could be raised substantially by increasing plant density. The distribution of DM between shoot and tuber showed a linear relationship with total biomass. However, the harvest index reached a more or less constant value after the fourth month, with the onset of tuber bulking, tillfinalharvest.
INTRODUCTION
A b o u t 80% o f cassava p r o d u c t i o n (6.4 X 106 t) in I n d i a is f r o m Kerala State w h e r e t h e c r o p is extensively g r o w n . Even t h o u g h t h e average cassava yield in I n d i a (17.7 t h a -1) is far a b o v e t h e global yield (9.4 t h a - l ) , t h e p r e s e n t level o f p r o d u c t i v i t y is far b e l o w the e s t i m a t e d p o t e n t i a l yield o f 90 t ha -~ suggested b y C o c k et al. ( 1 9 7 9 ) . T h e p r o d u c t i v i t y o f c r o p plants largely d e p e n d s o n t h e i n t e r a c t i o n o f suitable genetic characters w i t h e n v i r o n m e n t a l factors. K n o w l e d g e o f this i n t e r a c t i o n f o r cassava is q u i t e meagre. L i m i t e d p h y s i o l o g i c a l investigations c o n d u c t e d at t h e Central T u b e r C r o p s R e s e a r c h I n s t i t u t e (C.T.C.R.I.) at T r i v a n d r u m suggested t h a t t h e p r o d u c t i v i t y o f cassava is limited b y leaf area d u r a t i o n (Sinha and
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© 1985 Elsevier Science Publishers B.V.
292 Nair, 1971). Further studies, on correlation and p a t h analysis of various physiological characters with tuber yield, revealed that the total biomass was positively correlated with tuber yield (Ramanujam and Biradar, 1985). Investigations carried o u t in .various places have suggested that tuber yield in cassava is largely influenced by leaf area development and duration (Enyi, 1972; Cock, 1976; Keating et al., 1982), and b y harvest index (Williams, 1972; San Jose and Mayobre, 1982). The breeding programme in cassava is time-consuming because of the long growing period of the crop and its slow rate of multiplication. Understanding the physiological basis of yield may accelerate the crop improvement programme in cassava b u t information in this regard is scanty. Comparing the growth physiology of diverse cultivars can improve understanding of the basis of yield in crop plants. The present paper reports the physiological basis of yield variation among high-, and low-yielding cultivars of cassava and also suggests suitable selection indices for mass-screening for improved productivity. MATERIALS AND METHODS Six cultivars of cassava (M4, H 165, H 1423, H 2304, Ce 22 and Ci 590) differing in yield potential were chosen for the study. Field trials on the yield performance of the genotype at the plant population of 12340 ha -1 r e c o m m e n d e d for this region have revealed that H 165 and H 2304 are high yielders (35--45 t ha -~) while M4, Ci 590, H 1423 and Ce 22 are low yielders (20--30 t ha-l). Field trials were carried o u t at C.T.C.R.I., Trivandrum (8 ° 29'N; 76 ° 57'E; 64 m altitude), for t w o consecutive seasons during 1978--80 in a randomized block design, with four replications (plot size = 81 m2). Uniform stem cuttings (20 cm long with 6--8 nodes) were planted vertically at 90 × 90 cm apart. R e c o m m e n d e d rates of FYM (12.5 t ha -~) and fertilizers (NPK = 1 0 0 : 1 0 0 : 1 0 0 kg ha -~) were applied. FYM and P were given as a basal application while N and K were applied in t w o equal doses at planting and 45 days later. Only two sprouts per cutting were allowed to grow. Observations on leaf area index (LAI) and light transmission ratio (LTR) were recorded at weekly intervals. The leaf area was calculated using the linear measurement m e t h o d of Ramanujam and Indira (1978), standardized for cassava. The L T R which explains the proportion of light being transmitted to the lower layer of the canopy was calculated using the formula Io/I × 100 where, I0 = light intensity at b o t t o m of the canopy and I = light intensity at the t o p of the canopy. Dry matter distribution in leaves, stems and tubers was recorded at monthly intervals up to the tenth month after planting by uprooting four plants at random from each replication. Crop growth rate (CGR), net assimilation rate (NAR) and harvest index (HI) were calculated at m o n t h l y intervals following the standard formulae (Watson, 1947; Radford, 1967) as indicated below.
293 W2 - - W1 CGR
dW
-
t2 - - tl
NAR -
HI =
W2 - - W1
L2 - - L 1
dt
×
(logeL2 - - logeLl ) t2 -- tl
t u b e r dry m a t t e r total d r y m a t t e r
where W2 and W1 are the total dry weights at time t2 and t~; L2 and L1 are t h e total leaf area at time t2 and tl respectively. The leaf area profile for a period of 10 m o n t h s was drawn for individual cultivars (per plant basis) to estimate c a n o p y size and its coverage over the land area at the normal spacing o f 90 cm × 90 cm. The form ul a o f L e m o n (1967) was used t o calculate leaf area density at m o n t h l y intervals b y c o m p u t i n g the total leaf area at time t and the corresponding c a n o p y volume (plant height × land area), expressed as cm 2 cm -3. The same procedure was used b y San Jose and Berrade (1983) under a savanna climate. Correlation and regression coefficients for partitioning o f dry m a t t e r (DM) b e t w een s h o o t and storage r o o t were w or ked o u t separately for each cultivar and a mo d el for harvest index was drawn by plotting the theoretical HI against whole plant dry weight. T he theoretical harvest indices were calculated using the formula HI = b (1 -- c / x ) developed by B o e r b o o m (1978) from the regression equat i on Y = b x - - a where, HI = harvest index, b = regression coefficient, c = initial plant weight at which storage r o o t p r o d u c t i o n starts, a = intercept with y axis, Y = t u b e r yield and x = total biomass. All the values were expressed on a d r y weight basis. The relative i m port ance of c for the harvest index steadily decreases as the plant grows older. Hence, the weight o f the m o t h e r stem (planting material) was n o t included in calculating the t o t a l biomass. Th e light intensity was measured using the q u a n t u m sensor m o u n t e d in a steady state p o r o m e t e r , model Li-cor 1600. T he p r o p o r t i o n o f land area covered with foliage {0.81 m 2 for individual plant) was recorded fortnightly f r o m t he first m o n t h till final harvest, for the purpose o f calculating percent light interception by the canopy. RESULTS AND DISCUSSION Canopy
development
L eaf area d e v e l o p m e n t was slow in all six cultivars until the third week after planting. Subsequently, significant differences in vegetative growth were noticed among the cultivars. Canopy d e v e l o p m e n t in Ce 22, H 1423 and H 2 3 0 4 was faster, and 90% o f the land area was covered with foliage
294
in these eultivars by 45 days after planting, while the other three cultivars t o o k 75--90 days to achieve the same level of ground coverage. Maximum canopy size for cassava in the acid laterite softs of Kerala was observed during the 3--6 month stage of the crop and thereafter, LAI was reduced drastically, due to dry weather (Ramanujam and Indira, 1983). The longevity of the individual leaf (interval between full leaf expansion and leaf fall) varied from 37 to 54 days, depending upon cultivar, when moisture was not limiting. Hence the most active vegetative growth period for cassava in our experiments was up to 6 months only in a crop duration of 10 months. In the remaining period, LAI was very low and sometimes (eighth and ninth month stages) it was below unity. However, significant differences in LAI were noticed among the six cultivars (Fig. 1). Cultivars Ce 22 and H 1 4 2 3 maintained higher mean LAI (4.9 and 4.0 respectively) 6-
~.
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I
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--
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Ce22 Ci1590
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III
rJT
"
'
2"
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,' .i
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I"
c MONTHS
AFTER
PLANTING
Fig. 1. Leaf area index o f the six cassava cultivate during the growth period o f 10 months.
295 during the active growth period (2--6 m o n t h stage) due to greater numbers o f leaves per plant, related to their characteristic multi-tier branching habit initiated around 45 days after planting. At the comparable period of growth, the high yielding cultivars H 165 and H 2304 averaged LAI of 2.4 and 3.0 respectively, while the two low yielding cultivars M4 and Ci 590 showed very low LAI of 1.8 and 1.7 respectively. Leaf area index is a function of mean leaf size, rate of new leaf formation, longevity of individual leaf, and branching habit, which are influenced by genetic and environmental factors. The cultivars H 165 and H 2304 possess broader leaves (300--325 cm 2) in comparison with the other four cultivars (160--290 cm2). The higher LAI recorded in H 2304 compared with that of its non-branching counterparts was associated with its longer leaf life (54 days compared with 37 to 41 days), which enables the cultivar to retain more leaves (80--86 per plant against 48--52) during the 3--6 m o n t h stage. The cumulative leaf area density profile for the 10-month growth period has been plotted for individual cultivars. There were significant differences in plant height (Table 1) as well as the leaf area density (Fig. 2). The initial low values of the profile correspond to the stage of cassava growth, before LAI reaches unity. The dense profile corresponds to the m a x i m u m canopy development period (3--6 m o n t h stage) and the decrease in leaf density at the top of the curves corresponds to the dry period. Leaf area density as a function of leaf area per unit of canopy volume is generally applied in the aerodynamic studies for CO2 exchange between the atmosphere and the plant canopy (Lemon, 1967). In cassava, San Jose and Berrade (1983) employed the leaf area density profile for energy transfer studies in a savanna climate. In the present study, the leaf area density profile was drawn mainly to determine the o p t i m u m canopy size for light utilization as well as the o p t i m u m plant density for morphologically contrasting cultivars of cassava to achieve m a x i m u m tuber yield.
TABLE 1 Plant height (cm) o f six cassava cultivars at different periods of crop growth Genotype
M4
H 2304 H 1423 H 165 Ce 22 Ci590
Months after planting
2
3
4
5
6
7
8
9
10
37 36 22 25 23 35
62 60 49 44 54 55
96 102 69 78 73 75
132 132 95 122 105 91
149 178 130 156 118 100
168 186 135 183 120 108
171 191 138 190 128 110
199 195 149 200 130 118
201 202 160 209 136 125
296
+0[
.
°..i
"
"
~1~.
+
y= 8'1--Z9- 1014""08 X
E ~ 20.
15o
~ %
~:cmeadensity 4,
.,;~ .,~, 4,, .,;, .6+ -~, .,;+
+.+
+.++
-
o
o
.o2 . ~
o
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Leaf
o .~ .;~ .& o
ar'ea
density
.&
.b4 .&
cm2/cm
o
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.'
3
Fig. 2. Leaf density profile for the six cassava cultivars. The arrow indicates the minimum leaf density required for 100% light interception. The inset, top right, indicates the relationship between leaf area density and light transmission ratio.
The cultivars Ce 22 and H 1423 {Fig. 3a) maintained a larger leaf density (dense canopy) for a longer period, as a result of their profuse branching habit compared with M4 and Ci 590 (Fig. 3b). These four cultivars were low yielders. The low leaf area density of M4 and Ci 590 grown at the normal plant density of 12340 ha -1 suggests that their yield potential per unit land area could be increased by increasing the plant density; the light interception by these cultivars was less than 50% during the major part of the plant growth. The high yielding varieties H 165 and H 2304 maintained leaf area density values of 0.03 to 0.04 cm 2 cm -3 during the 3--6 m o n t h stage after which their LAI was reduced significantly due to prolonged dry weather. The nature of the foliage cover is an important factor in determining the efficiency with which available solar radiation is used in primary production (Loomis and Williams, 1963). The a m o u n t of leaf area necessary to intercept 95% of noon sunlight has been considered as the critical leaf area index (Brougham, 1960). LAI may also be integrated over time to provide a measure of the persistence of the leaf area and this integral value
297 ti~ii/i ¸I? ! ~!i ~
i
/i ~ /~iiii!ii~!il
Fig. 3. The two extreme plant types in cassava: (a) profusely branching (H 1423); (b) non-branching (Ci 590).
has been termed leaf area duration (LAD) by Watson (1958). In the present study, the LAD of the cultivars differed significantly (Table 2). The LAD of the low yielding cultivars M4 and Ci 590 was very low (40 to 44). The cultivar Ce 22, which is also a low yielder, showed significantly higher LAD (111.7) when compared to other cultivars. The LAD of the high yielding cultivars H 165 and H 2304 was 60.3 and 84.4 respectively, which suggests that the maintenance of o p t i m u m LAI over a long period is important for maximum tuber yield, rather than LAD alone. Enyi (1972) TABLE 2 L e a f a r e a d u r a t i o n ( L A D ) , n e t a s s i m i l a t i o n r a t e ( N A R ) , c r o p g r o w t h r a t e ( C G R ) a n d y i e l d o f six cassava cultivars Cultivar L A D
N A R ( m g d m -2 leaf d a y - l ) 2nd month
M4 43.9 6.2 H 2304 8 4 . 4 10.1 H 1423 9 2 . 2 6.1 H 165 6 0 . 3 13.6 Ce 2 2 1 1 1 . 7 10.6 Ci 5 9 0 4 0 . 2 9.8 L S D (5%) 8.5
3.0
C G R (g m -2 d a y -I)
4th month
6th month
8th month
10th month
Whole plant
Shoot
Tuber
46.3 47.0 21.8 49.5 17.4 39.9
43.0 42.8 21.0 43.1 19.1 43.0
26.5 31.4 16.2 23.8 16.0 17.4
19.8 24.7 18.0 23.1 13.5 19.0
5.7 8.7 6.3 6.2 5.8 4.9
2.4 3.4 3.3 2.4 3.7 1.7
3.3 5.3 3.0 3.8 2.1 3.2
9.1
5.6
7.2
4.3
0.9
0.4
0.5
CGR tuber/ CGR shoot
Tuber yield (t h a -I)
1.38 1.56 0.91 1.58 0.57 1.88
27.8 41.6 28.4 42.2 21.1 27.4 6.1
298
found that 75% of yield variation in cassava was related to leaf area duration. However, in corn (Williams et al., 1965) and soybean (Shibles and Weber, 1966) it has been demonstrated that the time integral of percentage light interception m a y be a better index than LAD. The L T R showed a significant negative correlation (r = - 0 . 7 8 , P < 0.01) with leaf area density (Fig. 2). The L T R was 40--50% at an LAI of 3.0. The L T R o f the profusely-branching types (Ce 22 and H 1423) was only 20--30% during the 3--6 m o n t h stage at full canopy. Loomis and R a p o p o r t (1977) reported that the diurnal totals for solar radiation in the tropical climate usually fall in the range of 300 to 500 cal cm -2 day -~ (ca. 13.6 to 20.9 MJ m -2 day-~). Under controlled experiments, the light saturation for photosynthesis in cassava has been found to be around 1000 ~E m -2 s -1 (Palta, 1982), while in the present experiment the photosyntheticallyactive radiation (PAR) available under field conditions ranged from 1200 to 1800 uE m -2 s -~ (10--14 h).
Dry matter production and partitioning Significant differences in DM production and partitioning were noticed among the six cultivars (Fig. 4). The profusely-branching types, H 1423 and Ce 22, almost consistently accumulated more DM in the shoot than in the tuber (Table 2). In the other cultivars, which are non-branching (H 165 and Ci 590), or partially-branching (M4 and H 2304), the accumulation of DM in the tuber was more in proportion to that in the shoot at the c o m m e n c e m e n t of tuber bulking. The short-statured non-branching cultivar Ci 590 (Fig. 3b) demonstrated its superiority over the other five with respect to its efficiency in DM utilization (Table 2). The t u b e r growth rate of Ci 590 was 1.88 times greater than its shoot growth rate; hence its harvest index was very high (0.7). However, its t u b e r yield was low (27.4 t ha -1) mainly because of the sub-optimal LAI (1.7), which allowed less than 50% light interception during most of the growth period (Fig. 2). Other work at C.T.C.R.I. (1982) suggested that the yield of Ci 590 could be increased to 44 t ha -1 by increasing plant density from 12340 to 27770 plants ha -1. The profusely-branching types (H 1423 and Ce 22) achieved lower harvest indices (0.45 and 0.34 respectively) and hence were low yielding in spite of their high LAI. The two distinct productive phases of growth in cassava are: (a) the tuberization (tuber differentiation) phase (1--2 months); and (b) the tuberbulking phase (between third m o n t h and harvest). In our experiments the peak tuber-bulking rate was recorded between the fourth and seventh months (C.T.C.R.I., 1982). After this period the bulking rate slowed down because of the prolonged dry period, which was associated with more leaf fall. The N A R of all the cultivars was lower during the tuberization phase than during the tuber-bulking phase (Table 2). The highest values of N A R were recorded during the peak bulking period (fourth to sixth
299
month), which is consistent with earlier unpublished results of Ramanujam and Biradar showing that tuber growth rate exceeded shoot growth rate from the third month. The distribution of DM for tuber growth in cassava followed the pattern of phasic partitioning (as observed in potato and sweet potato) in contrast to the balanced partitioning noticed in sugar beet in which the growth of the storage organ begins early in the seedling stage and continues throughout the vegetative period of the plant (Loomis and Rapoport, 1977). Furthermore, tuberization in cassava requires an inductive environment (Bolhuis, 1966). In potato, which also requires an inductive environment for tuberization, Milthorpe (1967) reported higher NAR after tuberization. The work of Tsuno and Fujise (1965) on sweet potato, and of Burt c
:~ TUgER
m
J SHOOT
14-00
M.4
H.2304-J
H ' 165
Ci.590
Ce-22
H'H-25
c --~1000
g 600 o
200
1400
_~_I000
~
600
Q 200
1400
~I000
~ 600 Q
2~ 2
4
6
J
6 10 t V I O N T H S AFTER PLANTING
Fig. 4. T h e pattern o f dry matter a c c u m u l a t i o n in s h o o t and tuber a m o n g the six cultivars o f cassava.
300 (1964) and NSsberger and Humphries (1965) on potato, clearly demonstrated that the removal of tubers or the prevention of tuber growth resuited in lower N A R and depressed photosynthesis. Therefore the high N A R recorded in the present study during tuber bulking, when compared to the initial period of tuberization, is mainly a response to tuber growth. Reduction in N A R after the sixth month was because of decreased LAI (Fig. 1) due to dry weather. The N A R of the profusely-branching cultivars was very low when compared with that of the non-branching and partially-branching cultivars (Table 2). This may be due to the fact that the profusely-branching cultivars maintained a supra-optimal leaf area index (> 4), which could cause mutual shading; and in addition, their harvest indices were very low (0.34 and 0.45 for Ce 22 and H 1423 respectively). The cultivar H 165 recorded m a x i m u m N A R (49.5 mg dm -1 leaf day -1) at the fourth month stage. Among the six cultivars used in the present investigation, H 2304 achieved a significantly higher mean crop growth rate (8.7 g m -2 day -1) than the other cultivars. H 165 and H 2304 out-yielded the other cultivars, with fresh tuber yields of 42.2 and 41.6 t ha -1 respectively. The low yields in M4 and Ci 590 were a result of their small canopy size (Figs. 1 and 2) even though their harvest indices were high (0.59 and 0.70 respectively). Yields of these t w o cultivars could be increased by raising the plant density, b u t this is a subject for further investigation. The distribution of DM in shoot and tuber showed a significant linear relationship with total biomass in all six cultivars (Fig. 5). The necessary computations were also made to test whether a c o m m o n linear regression equation can be fitted to all six cultivars. This was not possible because the calculated value for testing the significance (F = 25.45) was higher than the expected limit. The regression coefficients (b) for storage r o o t growth also differed significantly among the cultivars (Table 3). Ci 590 recorded the highest value (b = 0.709) and hence is considered an efficient plant t y p e in utilizing the total biomass. Williams (1972) suggested that the differences in the yield of cassava cultivars cannot be explained TABLE 3
P a r a m e t e r s d e t e r m i n i n g t h e e f f i c i e n c y in dry m a t t e r u t i l i z a t i o n a m o n g t h e six cassava cultivars Cultivaz
M4 H 2304 H 1423 H 165 Ce 2 2 Ci 590
Correlation
Regression
c o e f f i c i e n t (r)
coefficient (b)
Tuber with biomass
Shoot with biomass
Tuber
0.9735 0.9779 0.9818 0.9774 0.9809 0.9847
0.9130 0.9216 0.9779 0.8815 0.9938 0.8499
0.59 0.60 0.46 0.65 0.35 0.71
Regression equat i o n for t u b e r y i e l d (dry wt.) (Y)
Sb
Initial dry wt.
at t u b e rization a
Shoot
(c) 0.41 0.40 0.54 0.35 0.65 0.29
Y Y Y Y Y Y
~ = = = = =
0.5919 0.5995 0.4529 0.6482 0.3478 0.7090
a w e i g h t o f m o t h e r s t e m (planting m a t e r i a l ) w a s n o t i n c l u d e d .
x x x x x x
+ + + + + +
13.72 23.15 14.21 8.28 28.93 19.35
0.0328 0.0302 0.0313 0.0331 0.0449 0.0296
23.18 38.62 31.38 12.77 83.18 27.31
301
completely by the properties of the assimilating apparatus alone and concluded that the ratio of tuber to total plant material which he termed as 'crop index' is also an important factor in determining tuber yield.
,S o~ o.i
7
o.s
H" 1425
/°.,y
at.
X
Oru~eR,es.oor
o'~
i
/-
6OO
40O --~ cR
0.
o.I
.
.
.
.
.
.
.
.
.
M.4
i
.
.
.
.
.
.
H'165
.
.
0
A ,
0.1
/ti.
Ce.22
~"
•
Xl
D R Y W E I G H T O F W H O L E P L A N T (g)
Fig. 5. Accumulation of dry matter in shoot and tuber in relation to total biomass and the pattern of harvest index for the six cultivars of cassava. The numbers 1, 2, 3, 4, and 5 are the observed harvest indices at 2nd, 4th, 6th, 8th and 10th month stages respectively.
The partitioning of DM for tuber growth reached a more or less constant value in all six cultivars with the onset of tuber bulking (Fig. 5). It is suggested therefore that large numbers of cultivars can be screened effectively for their harvesting efficiency between the fourth and sixth month after planting instead over a period of 10--12 months. In conclusion, the variation in the yield performance of cassava cultivars in our experiments could be explained on the basis of the significant differences in canopy size. This largely determines the light utilization efficiency, and also affects the partitioning of DM for storage root growth. ACKNOWLEDGEMENTS
The author is most grateful to Dr. S.P. Ghosh, Director, Central Tuber Crops Research Institute, Trivandrum, for the facilities provided and the
302 encouragement shown while preparing this manuscript. The services of M r . V . R a v i n d r a n , M r . C . S . A n t o n i s w a m y , Mrs. K . R . L a k s h m i a n d M r s . K.M. Fatima Beevi, C.T.C.R.I., Trivandrum, are gratefully acknowledged.
REFERENCES Boerboom, B.W.J., 1978. A model for dry matter distribution in cassava (Manihot esculenta Crantz). Neth. J. Agric. Sci., 26: 267--277. Bolhuis, G.G., 1966. Influence of length of the illumination period on root formation in cassava (Manihot utilissima Pohl). Neth. J. Agric. Sci., 14: 251--254. Brougham, R.W., 1960. The relationship between the critical leaf area, total chlorophyll content and maximum growth rate of some pasture and crop plants. Ann. Bot., 24: 463-- 474. Burr, R.L., 1964. Carbohydrate utilization as a factor in plant growth. Aust. J. Biol. Sci., 17: 867--877. Cock, J.H., 1976. Characteristics of high yielding cassava varieties. Exp. Agric., 12: 135--143. Cock, J.H., Franklin, D., Sandoval, G. and Juri, P., 1979. The ideal cassava plant for m a x i m u m yield. Crop Sci., 19: 271--279. C.T.C.R.I., 1982. Annual report of Central Tuber Crops Research Institute, Trivandrum, India. Enyi, B.A.C., 1972. Effect of shoot number and time of planting on growth, developm e n t and yield of cassava (Manihot esculenta Crantz). J. Hort. Sci., 47: 457---466. Keating, B.A., Evenson, J.P. and Fukai, S., 1982. Environmental effects on growth and development of cassava (Manihot esculenta Crantz). I. Crop development. Field Crops Res., 5 : 271--281. Lemon, E., 1967. Aerodynamic studies of CO 2 exchange between the atmosphere and the plant. In: A. San Pietro, F.A. Greer and T.J. A r m y (Editors), Harvesting the Sun: Photosynthesis in Plant Life. Academic Press, New York, NY., pp. 263--290. Loomis, R.S. and Rapoport, H., 1977. Productivity of r o o t crops. In: J.H. Cock, R. MacIntyre and M. Graham (Editors), Proc. IVth Syrup. International Society for Tropical Root Crops. CIAT, Cali, Colombia, pp. 70--84. Loomis, R.S. and Williams, W.A., 1963. Maximum crop productivity: an estimate. Crop Sci., 3: 67--72. Milthorpe, F.L., 1967. Some physiological principles determining the yield of r o o t crops. In: E.A. Tai et al. (Editors), Proc. International Symposium on Tropical Root Crops. Univ. West Indies, Trinidad, Section II. pp. 1--19. N~sberger, J. and Humphries, E.C., 1965. The influence of removing tubers on dry matter production and net assimilation rate of potato plants. Ann. Bot., 2 9 : 5 7 9 --588. Palta, J.A., 1982. Gas exchange of four cassava cultivars in relation to light intensity. Exp. Agric., 18: 375--382. Radford, P.J., 1967. Growth analysis formulae - - their use and abuse. Crop Sci., 7 : 171--175. Ramanujam, T. and Biradar, R.S., 1985. Growth analysis in cassava (Manihot esculenta Crantz). Field Crops Research, (submitted). Ramanujam, T. and Indira, P., 1978. Linear measurement and weight methods for estimation of leaf area in cassava and sweet potato. J. Root Crops., 4: 47--50. Ramanujam, T. and Indira, P., 1983. Canopy structure on growth and development of cassava (Manihot esculenta Crantz). Turrialba, 33: 321--326. San Jose, J.J. and Berrade, F., 1983. Transfer of mass and energy in a cassava (Manihot
303
esculenta Crantz cv. Cubana) community. 1. Microclimate and water vapour exchange in a savanna climate. Ann. Bot., 52: 507--520. San Jose, J.J. and Mayobre, F., 1982. Quantitative growth relationships of cassava (Manihot esculenta Crantz): crop development in a savanna wet season. Ann. Bot., 50: 309--316. Shibles, R.M. and Weber, C.R., 1966. Interception of solar radiation and dry matter production by various soybean planting patterns. Crop Sci., 6: 55--59. Sinha, S.K. and Nair, T.V.R., 1971. Leaf area during growth and yielding capacity of cassava. Indian J. Genet., 31: 16--20. Tsuno, Y. and Fujise, K., 1965. Studies on the production of the sweet potato. VIII. The factors influencing the photosynthetic activity of sweet potato leaf. Proc. Crop Sci. Soc. Jpn., 33: 230--235. Watson, D.J., 1947. Comparative physiological studies on the growth of field crops. I. Variation in NAR and leaf area between species and varieties and within and between years. Ann. Bot., 11: 41--76. Watson, D.J., 1958. The dependence of net assimilation rate on leaf area index. Ann. Bot., 22: 37--54. Williams, C.N., 1972. Growth and productivity of tapioca (Manihot utilissima Pobl). III. Crop ratio, spacing and yield. Exp. Agric., 8: 15--23. Williams, W.A., Loomis, R.S. and Lepley, C.R., 1965. Vegetative growth of corn as affected by population density. I. Components of growth, net assimilation rate and leaf area index. Crop Sci., 5: 215--219.
291
Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands
L E A F D E N S I T Y P R O F I L E A N D E F F I C I E N C Y IN P A R T I T I O N I N G DRY MATTER AMONG HIGH AND LOW YIELDING CULTIVARS OF CASSAVA (MANIHOT ESCULENTA CRANTZ)
T. RAMANUJAM
Central Tuber Crops Research Institute, Trivandrum 695 017, Kerala (India) (Accepted 21 November 1984)
ABSTRACT Ramanujam, T., 1985. Leaf density profile and efficiency in partitioning dry matter among high and low yielding cultivars of cassava (Manihot esculenta Crantz). Field Crops Res., 10: 291--303. Leaf area density, and efficiency in partitioning dry matter for tuber growth were studied using six cultivars of cassava which differ in their yielding ability. The cultivars of the low yielding group maintained either sub-optimal (< 2) or supra-optimal (> 4 ) levels of leaf area index during the major part of growth period and this adversely affected their light utilization efficiency. A significant negative correlation (r = -0.78) was observed between leaf area density and light transmission ratio. For better light interception and utilization, a leaf area index of 2.5--3.5 was found to be optimum f o r cassava. A significant difference in partitioning dry matter (DM) for tuber growth was noticed among the cultivars tested, which varied from 0.3 to 0.7 per unit of DM produced. The short-statured eultivar Ci 590 was more efficient in utilizing DM for storage root growth (harvesting efficiency = 70%). The leaf density profile suggested that the productivity of Ci 590 could be raised substantially by increasing plant density. The distribution of DM between shoot and tuber showed a linear relationship with total biomass. However, the harvest index reached a more or less constant value after the fourth month, with the onset of tuber bulking, tillfinalharvest.
INTRODUCTION
A b o u t 80% o f cassava p r o d u c t i o n (6.4 X 106 t) in I n d i a is f r o m Kerala State w h e r e t h e c r o p is extensively g r o w n . Even t h o u g h t h e average cassava yield in I n d i a (17.7 t h a -1) is far a b o v e t h e global yield (9.4 t h a - l ) , t h e p r e s e n t level o f p r o d u c t i v i t y is far b e l o w the e s t i m a t e d p o t e n t i a l yield o f 90 t ha -~ suggested b y C o c k et al. ( 1 9 7 9 ) . T h e p r o d u c t i v i t y o f c r o p plants largely d e p e n d s o n t h e i n t e r a c t i o n o f suitable genetic characters w i t h e n v i r o n m e n t a l factors. K n o w l e d g e o f this i n t e r a c t i o n f o r cassava is q u i t e meagre. L i m i t e d p h y s i o l o g i c a l investigations c o n d u c t e d at t h e Central T u b e r C r o p s R e s e a r c h I n s t i t u t e (C.T.C.R.I.) at T r i v a n d r u m suggested t h a t t h e p r o d u c t i v i t y o f cassava is limited b y leaf area d u r a t i o n (Sinha and
0378-4290/85/$03.30
© 1985 Elsevier Science Publishers B.V.
292 Nair, 1971). Further studies, on correlation and p a t h analysis of various physiological characters with tuber yield, revealed that the total biomass was positively correlated with tuber yield (Ramanujam and Biradar, 1985). Investigations carried o u t in .various places have suggested that tuber yield in cassava is largely influenced by leaf area development and duration (Enyi, 1972; Cock, 1976; Keating et al., 1982), and b y harvest index (Williams, 1972; San Jose and Mayobre, 1982). The breeding programme in cassava is time-consuming because of the long growing period of the crop and its slow rate of multiplication. Understanding the physiological basis of yield may accelerate the crop improvement programme in cassava b u t information in this regard is scanty. Comparing the growth physiology of diverse cultivars can improve understanding of the basis of yield in crop plants. The present paper reports the physiological basis of yield variation among high-, and low-yielding cultivars of cassava and also suggests suitable selection indices for mass-screening for improved productivity. MATERIALS AND METHODS Six cultivars of cassava (M4, H 165, H 1423, H 2304, Ce 22 and Ci 590) differing in yield potential were chosen for the study. Field trials on the yield performance of the genotype at the plant population of 12340 ha -1 r e c o m m e n d e d for this region have revealed that H 165 and H 2304 are high yielders (35--45 t ha -~) while M4, Ci 590, H 1423 and Ce 22 are low yielders (20--30 t ha-l). Field trials were carried o u t at C.T.C.R.I., Trivandrum (8 ° 29'N; 76 ° 57'E; 64 m altitude), for t w o consecutive seasons during 1978--80 in a randomized block design, with four replications (plot size = 81 m2). Uniform stem cuttings (20 cm long with 6--8 nodes) were planted vertically at 90 × 90 cm apart. R e c o m m e n d e d rates of FYM (12.5 t ha -~) and fertilizers (NPK = 1 0 0 : 1 0 0 : 1 0 0 kg ha -~) were applied. FYM and P were given as a basal application while N and K were applied in t w o equal doses at planting and 45 days later. Only two sprouts per cutting were allowed to grow. Observations on leaf area index (LAI) and light transmission ratio (LTR) were recorded at weekly intervals. The leaf area was calculated using the linear measurement m e t h o d of Ramanujam and Indira (1978), standardized for cassava. The L T R which explains the proportion of light being transmitted to the lower layer of the canopy was calculated using the formula Io/I × 100 where, I0 = light intensity at b o t t o m of the canopy and I = light intensity at the t o p of the canopy. Dry matter distribution in leaves, stems and tubers was recorded at monthly intervals up to the tenth month after planting by uprooting four plants at random from each replication. Crop growth rate (CGR), net assimilation rate (NAR) and harvest index (HI) were calculated at m o n t h l y intervals following the standard formulae (Watson, 1947; Radford, 1967) as indicated below.
293 W2 - - W1 CGR
dW
-
t2 - - tl
NAR -
HI =
W2 - - W1
L2 - - L 1
dt
×
(logeL2 - - logeLl ) t2 -- tl
t u b e r dry m a t t e r total d r y m a t t e r
where W2 and W1 are the total dry weights at time t2 and t~; L2 and L1 are t h e total leaf area at time t2 and tl respectively. The leaf area profile for a period of 10 m o n t h s was drawn for individual cultivars (per plant basis) to estimate c a n o p y size and its coverage over the land area at the normal spacing o f 90 cm × 90 cm. The form ul a o f L e m o n (1967) was used t o calculate leaf area density at m o n t h l y intervals b y c o m p u t i n g the total leaf area at time t and the corresponding c a n o p y volume (plant height × land area), expressed as cm 2 cm -3. The same procedure was used b y San Jose and Berrade (1983) under a savanna climate. Correlation and regression coefficients for partitioning o f dry m a t t e r (DM) b e t w een s h o o t and storage r o o t were w or ked o u t separately for each cultivar and a mo d el for harvest index was drawn by plotting the theoretical HI against whole plant dry weight. T he theoretical harvest indices were calculated using the formula HI = b (1 -- c / x ) developed by B o e r b o o m (1978) from the regression equat i on Y = b x - - a where, HI = harvest index, b = regression coefficient, c = initial plant weight at which storage r o o t p r o d u c t i o n starts, a = intercept with y axis, Y = t u b e r yield and x = total biomass. All the values were expressed on a d r y weight basis. The relative i m port ance of c for the harvest index steadily decreases as the plant grows older. Hence, the weight o f the m o t h e r stem (planting material) was n o t included in calculating the t o t a l biomass. Th e light intensity was measured using the q u a n t u m sensor m o u n t e d in a steady state p o r o m e t e r , model Li-cor 1600. T he p r o p o r t i o n o f land area covered with foliage {0.81 m 2 for individual plant) was recorded fortnightly f r o m t he first m o n t h till final harvest, for the purpose o f calculating percent light interception by the canopy. RESULTS AND DISCUSSION Canopy
development
L eaf area d e v e l o p m e n t was slow in all six cultivars until the third week after planting. Subsequently, significant differences in vegetative growth were noticed among the cultivars. Canopy d e v e l o p m e n t in Ce 22, H 1423 and H 2 3 0 4 was faster, and 90% o f the land area was covered with foliage
294
in these eultivars by 45 days after planting, while the other three cultivars t o o k 75--90 days to achieve the same level of ground coverage. Maximum canopy size for cassava in the acid laterite softs of Kerala was observed during the 3--6 month stage of the crop and thereafter, LAI was reduced drastically, due to dry weather (Ramanujam and Indira, 1983). The longevity of the individual leaf (interval between full leaf expansion and leaf fall) varied from 37 to 54 days, depending upon cultivar, when moisture was not limiting. Hence the most active vegetative growth period for cassava in our experiments was up to 6 months only in a crop duration of 10 months. In the remaining period, LAI was very low and sometimes (eighth and ninth month stages) it was below unity. However, significant differences in LAI were noticed among the six cultivars (Fig. 1). Cultivars Ce 22 and H 1 4 2 3 maintained higher mean LAI (4.9 and 4.0 respectively) 6-
~.
/
I
,,'!1 ,'
d, H 2 3 0 4
*. . . . . . . . . .
o--
--
~ H165 -O
Ce22 Ci1590
!
iiii /
I
IAI
•
/
I
III
rJT
"
'
2"
? /?
,' .i
I/#l f
\1' / .."
I"
c MONTHS
AFTER
PLANTING
Fig. 1. Leaf area index o f the six cassava cultivate during the growth period o f 10 months.
295 during the active growth period (2--6 m o n t h stage) due to greater numbers o f leaves per plant, related to their characteristic multi-tier branching habit initiated around 45 days after planting. At the comparable period of growth, the high yielding cultivars H 165 and H 2304 averaged LAI of 2.4 and 3.0 respectively, while the two low yielding cultivars M4 and Ci 590 showed very low LAI of 1.8 and 1.7 respectively. Leaf area index is a function of mean leaf size, rate of new leaf formation, longevity of individual leaf, and branching habit, which are influenced by genetic and environmental factors. The cultivars H 165 and H 2304 possess broader leaves (300--325 cm 2) in comparison with the other four cultivars (160--290 cm2). The higher LAI recorded in H 2304 compared with that of its non-branching counterparts was associated with its longer leaf life (54 days compared with 37 to 41 days), which enables the cultivar to retain more leaves (80--86 per plant against 48--52) during the 3--6 m o n t h stage. The cumulative leaf area density profile for the 10-month growth period has been plotted for individual cultivars. There were significant differences in plant height (Table 1) as well as the leaf area density (Fig. 2). The initial low values of the profile correspond to the stage of cassava growth, before LAI reaches unity. The dense profile corresponds to the m a x i m u m canopy development period (3--6 m o n t h stage) and the decrease in leaf density at the top of the curves corresponds to the dry period. Leaf area density as a function of leaf area per unit of canopy volume is generally applied in the aerodynamic studies for CO2 exchange between the atmosphere and the plant canopy (Lemon, 1967). In cassava, San Jose and Berrade (1983) employed the leaf area density profile for energy transfer studies in a savanna climate. In the present study, the leaf area density profile was drawn mainly to determine the o p t i m u m canopy size for light utilization as well as the o p t i m u m plant density for morphologically contrasting cultivars of cassava to achieve m a x i m u m tuber yield.
TABLE 1 Plant height (cm) o f six cassava cultivars at different periods of crop growth Genotype
M4
H 2304 H 1423 H 165 Ce 22 Ci590
Months after planting
2
3
4
5
6
7
8
9
10
37 36 22 25 23 35
62 60 49 44 54 55
96 102 69 78 73 75
132 132 95 122 105 91
149 178 130 156 118 100
168 186 135 183 120 108
171 191 138 190 128 110
199 195 149 200 130 118
201 202 160 209 136 125
296
+0[
.
°..i
"
"
~1~.
+
y= 8'1--Z9- 1014""08 X
E ~ 20.
15o
~ %
~:cmeadensity 4,
.,;~ .,~, 4,, .,;, .6+ -~, .,;+
+.+
+.++
-
o
o
.o2 . ~
o
.oz .;+ .&
Leaf
o .~ .;~ .& o
ar'ea
density
.&
.b4 .&
cm2/cm
o
.'o~.'o+ -'o~ .'o8 o .'
.'
3
Fig. 2. Leaf density profile for the six cassava cultivars. The arrow indicates the minimum leaf density required for 100% light interception. The inset, top right, indicates the relationship between leaf area density and light transmission ratio.
The cultivars Ce 22 and H 1423 {Fig. 3a) maintained a larger leaf density (dense canopy) for a longer period, as a result of their profuse branching habit compared with M4 and Ci 590 (Fig. 3b). These four cultivars were low yielders. The low leaf area density of M4 and Ci 590 grown at the normal plant density of 12340 ha -1 suggests that their yield potential per unit land area could be increased by increasing the plant density; the light interception by these cultivars was less than 50% during the major part of the plant growth. The high yielding varieties H 165 and H 2304 maintained leaf area density values of 0.03 to 0.04 cm 2 cm -3 during the 3--6 m o n t h stage after which their LAI was reduced significantly due to prolonged dry weather. The nature of the foliage cover is an important factor in determining the efficiency with which available solar radiation is used in primary production (Loomis and Williams, 1963). The a m o u n t of leaf area necessary to intercept 95% of noon sunlight has been considered as the critical leaf area index (Brougham, 1960). LAI may also be integrated over time to provide a measure of the persistence of the leaf area and this integral value
297 ti~ii/i ¸I? ! ~!i ~
i
/i ~ /~iiii!ii~!il
Fig. 3. The two extreme plant types in cassava: (a) profusely branching (H 1423); (b) non-branching (Ci 590).
has been termed leaf area duration (LAD) by Watson (1958). In the present study, the LAD of the cultivars differed significantly (Table 2). The LAD of the low yielding cultivars M4 and Ci 590 was very low (40 to 44). The cultivar Ce 22, which is also a low yielder, showed significantly higher LAD (111.7) when compared to other cultivars. The LAD of the high yielding cultivars H 165 and H 2304 was 60.3 and 84.4 respectively, which suggests that the maintenance of o p t i m u m LAI over a long period is important for maximum tuber yield, rather than LAD alone. Enyi (1972) TABLE 2 L e a f a r e a d u r a t i o n ( L A D ) , n e t a s s i m i l a t i o n r a t e ( N A R ) , c r o p g r o w t h r a t e ( C G R ) a n d y i e l d o f six cassava cultivars Cultivar L A D
N A R ( m g d m -2 leaf d a y - l ) 2nd month
M4 43.9 6.2 H 2304 8 4 . 4 10.1 H 1423 9 2 . 2 6.1 H 165 6 0 . 3 13.6 Ce 2 2 1 1 1 . 7 10.6 Ci 5 9 0 4 0 . 2 9.8 L S D (5%) 8.5
3.0
C G R (g m -2 d a y -I)
4th month
6th month
8th month
10th month
Whole plant
Shoot
Tuber
46.3 47.0 21.8 49.5 17.4 39.9
43.0 42.8 21.0 43.1 19.1 43.0
26.5 31.4 16.2 23.8 16.0 17.4
19.8 24.7 18.0 23.1 13.5 19.0
5.7 8.7 6.3 6.2 5.8 4.9
2.4 3.4 3.3 2.4 3.7 1.7
3.3 5.3 3.0 3.8 2.1 3.2
9.1
5.6
7.2
4.3
0.9
0.4
0.5
CGR tuber/ CGR shoot
Tuber yield (t h a -I)
1.38 1.56 0.91 1.58 0.57 1.88
27.8 41.6 28.4 42.2 21.1 27.4 6.1
298
found that 75% of yield variation in cassava was related to leaf area duration. However, in corn (Williams et al., 1965) and soybean (Shibles and Weber, 1966) it has been demonstrated that the time integral of percentage light interception m a y be a better index than LAD. The L T R showed a significant negative correlation (r = - 0 . 7 8 , P < 0.01) with leaf area density (Fig. 2). The L T R was 40--50% at an LAI of 3.0. The L T R o f the profusely-branching types (Ce 22 and H 1423) was only 20--30% during the 3--6 m o n t h stage at full canopy. Loomis and R a p o p o r t (1977) reported that the diurnal totals for solar radiation in the tropical climate usually fall in the range of 300 to 500 cal cm -2 day -~ (ca. 13.6 to 20.9 MJ m -2 day-~). Under controlled experiments, the light saturation for photosynthesis in cassava has been found to be around 1000 ~E m -2 s -1 (Palta, 1982), while in the present experiment the photosyntheticallyactive radiation (PAR) available under field conditions ranged from 1200 to 1800 uE m -2 s -~ (10--14 h).
Dry matter production and partitioning Significant differences in DM production and partitioning were noticed among the six cultivars (Fig. 4). The profusely-branching types, H 1423 and Ce 22, almost consistently accumulated more DM in the shoot than in the tuber (Table 2). In the other cultivars, which are non-branching (H 165 and Ci 590), or partially-branching (M4 and H 2304), the accumulation of DM in the tuber was more in proportion to that in the shoot at the c o m m e n c e m e n t of tuber bulking. The short-statured non-branching cultivar Ci 590 (Fig. 3b) demonstrated its superiority over the other five with respect to its efficiency in DM utilization (Table 2). The t u b e r growth rate of Ci 590 was 1.88 times greater than its shoot growth rate; hence its harvest index was very high (0.7). However, its t u b e r yield was low (27.4 t ha -1) mainly because of the sub-optimal LAI (1.7), which allowed less than 50% light interception during most of the growth period (Fig. 2). Other work at C.T.C.R.I. (1982) suggested that the yield of Ci 590 could be increased to 44 t ha -1 by increasing plant density from 12340 to 27770 plants ha -1. The profusely-branching types (H 1423 and Ce 22) achieved lower harvest indices (0.45 and 0.34 respectively) and hence were low yielding in spite of their high LAI. The two distinct productive phases of growth in cassava are: (a) the tuberization (tuber differentiation) phase (1--2 months); and (b) the tuberbulking phase (between third m o n t h and harvest). In our experiments the peak tuber-bulking rate was recorded between the fourth and seventh months (C.T.C.R.I., 1982). After this period the bulking rate slowed down because of the prolonged dry period, which was associated with more leaf fall. The N A R of all the cultivars was lower during the tuberization phase than during the tuber-bulking phase (Table 2). The highest values of N A R were recorded during the peak bulking period (fourth to sixth
299
month), which is consistent with earlier unpublished results of Ramanujam and Biradar showing that tuber growth rate exceeded shoot growth rate from the third month. The distribution of DM for tuber growth in cassava followed the pattern of phasic partitioning (as observed in potato and sweet potato) in contrast to the balanced partitioning noticed in sugar beet in which the growth of the storage organ begins early in the seedling stage and continues throughout the vegetative period of the plant (Loomis and Rapoport, 1977). Furthermore, tuberization in cassava requires an inductive environment (Bolhuis, 1966). In potato, which also requires an inductive environment for tuberization, Milthorpe (1967) reported higher NAR after tuberization. The work of Tsuno and Fujise (1965) on sweet potato, and of Burt c
:~ TUgER
m
J SHOOT
14-00
M.4
H.2304-J
H ' 165
Ci.590
Ce-22
H'H-25
c --~1000
g 600 o
200
1400
_~_I000
~
600
Q 200
1400
~I000
~ 600 Q
2~ 2
4
6
J
6 10 t V I O N T H S AFTER PLANTING
Fig. 4. T h e pattern o f dry matter a c c u m u l a t i o n in s h o o t and tuber a m o n g the six cultivars o f cassava.
300 (1964) and NSsberger and Humphries (1965) on potato, clearly demonstrated that the removal of tubers or the prevention of tuber growth resuited in lower N A R and depressed photosynthesis. Therefore the high N A R recorded in the present study during tuber bulking, when compared to the initial period of tuberization, is mainly a response to tuber growth. Reduction in N A R after the sixth month was because of decreased LAI (Fig. 1) due to dry weather. The N A R of the profusely-branching cultivars was very low when compared with that of the non-branching and partially-branching cultivars (Table 2). This may be due to the fact that the profusely-branching cultivars maintained a supra-optimal leaf area index (> 4), which could cause mutual shading; and in addition, their harvest indices were very low (0.34 and 0.45 for Ce 22 and H 1423 respectively). The cultivar H 165 recorded m a x i m u m N A R (49.5 mg dm -1 leaf day -1) at the fourth month stage. Among the six cultivars used in the present investigation, H 2304 achieved a significantly higher mean crop growth rate (8.7 g m -2 day -1) than the other cultivars. H 165 and H 2304 out-yielded the other cultivars, with fresh tuber yields of 42.2 and 41.6 t ha -1 respectively. The low yields in M4 and Ci 590 were a result of their small canopy size (Figs. 1 and 2) even though their harvest indices were high (0.59 and 0.70 respectively). Yields of these t w o cultivars could be increased by raising the plant density, b u t this is a subject for further investigation. The distribution of DM in shoot and tuber showed a significant linear relationship with total biomass in all six cultivars (Fig. 5). The necessary computations were also made to test whether a c o m m o n linear regression equation can be fitted to all six cultivars. This was not possible because the calculated value for testing the significance (F = 25.45) was higher than the expected limit. The regression coefficients (b) for storage r o o t growth also differed significantly among the cultivars (Table 3). Ci 590 recorded the highest value (b = 0.709) and hence is considered an efficient plant t y p e in utilizing the total biomass. Williams (1972) suggested that the differences in the yield of cassava cultivars cannot be explained TABLE 3
P a r a m e t e r s d e t e r m i n i n g t h e e f f i c i e n c y in dry m a t t e r u t i l i z a t i o n a m o n g t h e six cassava cultivars Cultivaz
M4 H 2304 H 1423 H 165 Ce 2 2 Ci 590
Correlation
Regression
c o e f f i c i e n t (r)
coefficient (b)
Tuber with biomass
Shoot with biomass
Tuber
0.9735 0.9779 0.9818 0.9774 0.9809 0.9847
0.9130 0.9216 0.9779 0.8815 0.9938 0.8499
0.59 0.60 0.46 0.65 0.35 0.71
Regression equat i o n for t u b e r y i e l d (dry wt.) (Y)
Sb
Initial dry wt.
at t u b e rization a
Shoot
(c) 0.41 0.40 0.54 0.35 0.65 0.29
Y Y Y Y Y Y
~ = = = = =
0.5919 0.5995 0.4529 0.6482 0.3478 0.7090
a w e i g h t o f m o t h e r s t e m (planting m a t e r i a l ) w a s n o t i n c l u d e d .
x x x x x x
+ + + + + +
13.72 23.15 14.21 8.28 28.93 19.35
0.0328 0.0302 0.0313 0.0331 0.0449 0.0296
23.18 38.62 31.38 12.77 83.18 27.31
301
completely by the properties of the assimilating apparatus alone and concluded that the ratio of tuber to total plant material which he termed as 'crop index' is also an important factor in determining tuber yield.
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Fig. 5. Accumulation of dry matter in shoot and tuber in relation to total biomass and the pattern of harvest index for the six cultivars of cassava. The numbers 1, 2, 3, 4, and 5 are the observed harvest indices at 2nd, 4th, 6th, 8th and 10th month stages respectively.
The partitioning of DM for tuber growth reached a more or less constant value in all six cultivars with the onset of tuber bulking (Fig. 5). It is suggested therefore that large numbers of cultivars can be screened effectively for their harvesting efficiency between the fourth and sixth month after planting instead over a period of 10--12 months. In conclusion, the variation in the yield performance of cassava cultivars in our experiments could be explained on the basis of the significant differences in canopy size. This largely determines the light utilization efficiency, and also affects the partitioning of DM for storage root growth. ACKNOWLEDGEMENTS
The author is most grateful to Dr. S.P. Ghosh, Director, Central Tuber Crops Research Institute, Trivandrum, for the facilities provided and the
302 encouragement shown while preparing this manuscript. The services of M r . V . R a v i n d r a n , M r . C . S . A n t o n i s w a m y , Mrs. K . R . L a k s h m i a n d M r s . K.M. Fatima Beevi, C.T.C.R.I., Trivandrum, are gratefully acknowledged.
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