Effects of planting density on growth, light interception and yield of a photoperiod insensitive pigeon pea (Cajanus cajan)

Field Crops Research, 4 (1981) 201--213

201

Elsevier Scientific Publiching Company, Amsterdam -- Printed in The Netherlands

EFFECTS OF PLANTING DENSITY ON GROWTH, LIGHT INTERCEPTION A N D Y I E L D O F A P H O T O P E R I O D I N S E N S I T I V E P I G E O N P E A (CA J A N U S

CAJAN)

R. ROWDEN, D. GARDINER, P.C. WHITEMAN and E.S. WALLIS

Department of Agriculture, University of Queensland, St. Lucia, Qld. 4067 (Australia) (Accepted 13 April 1981)

ABSTRACT Rowden, R., Gardiner, D., Whiteman, P.C. and Wallis, E.S., 1981. Effects of planting density on growth, light interception and yield of a photoperiod insensitive pigeon pea (Cajanus cajan). Field Crops Res., 4: 201--213. A newly selected short-statured, early flowering, photoperiod insensitive line of pigeon pea was planted in February 1979 at four densities, viz. 2 x 10% 3 x 10 s, 5 x 10 s and 1 x 10 ~ plants ha -l. Maximum values of relative growth rate (RGR) (0.17 g g-~ day-l), net assimilation rate (NAR) (85 g m -2 week "l) and leaf area ratio (LAR) (1.6 dm 2 g-i) were achieved by 25 days from planting at the lower densities. Maximum NAR and RGR at 1 x 10 ~ density was delayed until day 36. By day 45 this density had almost complete light interception, and all densities had complete interception by day 85. This variety had a low extinction coefficient (0.3) suggesting that canopy structure was efficient in allowing light penetration. Since crop growth rate and leaf area index (LAI) were linearly related, highest dry matter yields were at the highest density. Highest seed yield per hectare (1300 kg) was at the 5 x 10 s density, but overall, yields were not significantly different. Pod number per plant was the main determinant of yield. F r o m the relationship of pod number per plant and density, an optimum density of 7.5 x 10 s plants ha -I was predicted. It was concluded on the basis of growth and leaf area data that this new material could support a higher photosynthetic demand if the objectives of a breeding program to increase seed size and/or seeds per pod could be achieved.

INTRODUCTION I n t h e p r o g r a m m e o f s c r e e n i n g a l a r g e n u m b e r o f Ca]anus cajan a c c e s s i o n s for adaptation to the Queensland environment, an unusual short statured early flowering single plant was isolated from an accession ICP7179 introduced from ICRISAT (India), (Wallis et al., 1979a). Further testing of progeny derived from selfing this plant showed that time to floral initiation was not affected by photoperiods in the range 8--16 h (L.V. Turnbull, personal communication, 1979) and the lines were early flowering in the field, in the r a n g e o f 50 t o 5 5 d a y s .

0378-4290/81/0000--0000/$0.2.50 © 1981 Elsevier Scientific Publishing Company

202

This type of plant contrasts markedly with the commercially grown varieties, which are strongly photoperiod sensitive, so that height and time to flowering vary with date of planting (Akinola and Whiteman, 1975a). In the cultivar Royes, released for commercial production in Queensland, plant height varies from 45 to 200 cm, and days to flowering from 85 to 120 days, depending on date of planting (Wallis et al., 1979b). Planting density has major effects on vegetative and seed yields of grain legumes, and is of particular importance to photoperiod sensitive crops where planting density must be adjusted with changing planting dates (Lawn et al., 1977; Wallis et al., 1979b). The experiment reported in this paper aimed to define the optimum planting density for the new photoperiod insensitive line, and to examine the effects of density on crop growth and light interception by the canopy. MATERIALS AND METHODS

The trials were established at the University of Queensland Research Farm at Redland Bay, 40 km southeast of Brisbane. This station has a mild frostfree climate with an average annual rainfall of 1250 mm mainly in summer. Soils are deep, well drained krasnozems (oxisols). Plots were machine planted on 22 February 1979 in rows 25 cm apart. Seed was inculated with Rhizobium strain CB756 before planting. The plots were hand thinned after 14 days to spacing within the row of 20, 13, 8 and 4 cm, giving plant densities of 2 X 10 s, 3 X l 0 s, 5 × 10 s and 1 X 106 plants ha -1. The experiment was designed as two randomized complete blocks each with three replicates. The experiment was irrigated as indicated from pan evaporation readings. Weeds were controlled by hoeing, and insects were controlled from flowering onwards by "Endosulphan" sprays. One set of plots was used for destructive harvesting for dry matter and leaf area measurements, the other set was for the light interception measurements. Plant population counts made after thinning showed that populations in both sets of plots were identical. Dry matter and leaf area samples were taken on 10 sampling occasions during the 105 days of the experiment at intervals of 7 to 13 days. Plots were large enough to allow three plants to be taken on each occasion, leaving sufficient plants so that subsequent samples could be taken without introducing border effects. For the first five harvests, plants were dug o u t in a trench to 0.5 m deep to allow an estimate of root weight. Plants were separated into roots, stem and leaves. Leaf area was measured with a optical planimeter and all material was dried at 60°C. Pod number per plant was recorded by hand picking 10 plants per plot. Pods were oven dried, and then threshed. Seed number per pod, 100 seed weight, and seed yield per plant and per hectare were then calculated. Interception of photosynthetically active radiation (PAR 400--700 nm) was measured between 1100 and 1300 on three occasions during crop growth;

203

13 April (49 days after sowing), 18 May (85 days), and 7 June (105 days). Radiation was measured at four heights in the canopy at eight sites in the interrow in each plot using a p h o t o m e t e r incorporating nine filtered DP-2 selenium photocells m o u n t e d in line on an aluminium probe (Muchow and Kerven, 1977). The probe was inserted into the canopy and a single reference photocell was held above the canopy in full sunlight. The ratio between the reference and the probe reading was recorded as % P.A.R. intercepted by the canopy. RESULTS

Plan t grow th

Effects of density on the growth of individual plants within the stand were assessed b y growth analysis calculated according to the equations of Watson (1952) and Radford (1967). The ratio of assimilatory leaf material per unit of total plant material, the leaf area ratio (LAR), reached a maximum for all densities 25 days after planting, and then declined steadily (Fig. 1). At most samplings the 1 × 106 density had a significantly higher L A R than the 2 × l 0 s density with the other densities intermediate. The net assimilation rate (NAR) expresses the rate of assimilation per unit leaf area. Maximum rates of 75 to 85 g m "2 week "1 were again achieved early in the growth of the crop, at 30 days for the three lower densities and after 36 days at the highest density (1 X 106), (Fig. 2). The N A R in all densities declined rapidly until 55 days from planting, after which they remained approximately constant, except for the 1 X 106 density where N A R increased significantly after 90 days. 1.6

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Fig. 3. Changes with time in relative growth rate of C. cajan sown at four densities. Since relative g r o w t h rate ( R G R ) is t h e p r o d u c t o f L A R × N A R , t h e trends w i t h t i m e in N A R and L A R r e i n f o r c e d each o t h e r so that m a x i m u m R G R was also a c h i e v e d by 25 d a y s in t h e three l o w e r densities and at 36 days in t h e highest d e n s i t y . R G R ' s d e c l i n e d f r o m m a x i m u m values o f 0 . 1 7 g g-' d a y -~ to values o f 0.01 t o 0 . 0 4 g g-1 day-1 at 55 days and r e m a i n e d c o n s t a n t l y l o w until t h e final harvest at 1 0 5 days (Fig. 3).

205 Leaf area and light interception Even though leaf area per plant declined linearly with increasing density (Fig. 4), this was more than compensated by increasing density so that the highest density achieved the highest leaf area index (LAI) and did so earlier than the other densities (Fig. 5). From a m a x i m u m LAI of 4.9 at 49 days, LAI declined to 3.5 at 100 days. The other densities reached maximum LAIs at about 85 days, but in the 5 X 10 s density the delay in attaining maximum LAI was caused by a depression in growth between day 49 and day 72. At day 62 there was a close linear relationship between LAI and density. (y = 1.70 + 3.16x; r = 0.99, where y is LAI and x is plant density X 106). The light interception profiles (Fig. 6) show that at the highest density at day 49, 50% of PAR was intercepted in the top 20 cm of the canopy. In the 5 X l 0 s density this was achieved in the top 30 cm and in the 3 X l 0 s in the upper 35 cm. However by day 85 there was less difference between densities so that over 90% of radiation had been absorbed at 20 cm above ground level. At 49 days the relationship between LAI and percentage interception of PAR measured at ground level was linear over the range of LAI 2.0 to 5.5, giving interceptions of 85 to 97% (Fig. 7). However at 85 and 105 days at LAI values above 2.5, interception measured at ground level was on average about 97%, suggesting that a change in canopy structure occurred after flowering. The extinction coefficient (K) was calculated from the Beer-Lambert Law

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Fig. 4. Effects of planting density on mean leaf area per plant measured over nine harvests in C. cajan.

206

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Fig. 6. Profiles of photosynthetically active radiation (PAR) interception in C. cajan in three p o p u l a t i o n densities measured at ( a ) 4 9 d a y s , ( b ) 8 5 d a y s , ( c ) 1 0 5 days from planting.

using t h e regression o f log n I/Io against LAI. T h e I/Io values w e r e i n t e r c e p t i o n profiles over t h r e e densities (1 X 106, 5 X l 0 s and 3 × l 0 s) f o r days 49 and 1 0 5 . . T h e data f r o m d a y 85 gave o v e r 95% P A R i n t e r c e p t i o n at all LAI values (Fig. 7). T h e regressions for t h e K values c a l c u l a t i o n s were: D a y 49

y = 3 . 2 5 5 - 0 . 3 0 2 x (r = 0 . 8 3 * * )

D a y 1 0 5 y = 1 . 9 4 6 - 0 . 1 7 7 x (r -- 0 . 6 5 n o t . sig.) w h e r e y is In ( 1 0 0 I/Io) and x is LAI.

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Fig. 7. The relationship between leaf area index and percentage interception of photosynthetically active radiation (PAR) measured at ground level in C. cajan over four population densities at 49, 85 and 105 days from planting. Thus the value for K at day 49 was 0.30, while the regression for day 105 was not significant and was similar to day 85 where there was greater than 90% light interception at all LAI values. Crop gro wth Dry matter yield per plant declined asymptotically with increasing density, from 21 g plant-' at the 2 X l 0 s density to 8 g plant "~ at 1 X 106, (Fig. 8). This relationship was linear when the reciprocal of dry matter yield for the harvest on day 105 was plotted against density. The dry matter yield per hectare relationship was described b y a parabolic curve (Fig. 8), with yields increasing from 4200 kg h a " to 8200 kg h a " at the highest density. Dry matter yield per hectare at any time is a function of the mean crop growth rate (CGR) over that time period. Over the whole experimental period CGR was linearly related to LAI (Fig. 9), but for short periods and particularly during the flowering period (49--60 days} this relationship was variable. Seed yield Total seed yield per hectare can be defined in terms of the following components: Seed yield ha -~ = (No. pods plant-ix No. seeds pod-~ × Mean dry weight seed-') X No. plants ha "1

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209 •

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Number of pods per pLent declined asymptotically from 32 to 7 at the highest density (Fig. 10) but number of seeds per pod was constant at 2.5, and mean weight per 100 seeds was also constant at 6.5 g. Thus seed yield per plant declined in parallel with pod number per plant. Seed yield per hectare increased to a m a x i m u m of 1300 kg ha "~ at the 5 X 10 s density, but differences between densities were not significant. With increasing density height at which pods were borne in the canopy increased. At 2 X l 0 s density the height of the lowest pod was 30 cm, at 3 X 10 s density 35 cm, at 5 X 10 s 37 cm, and at 1 X 106 42 cm. DISCUSSION

Initial values for RGR, NAR and LAR were quite high. Maximum NAR values recorded here of 75--85 g m -1 week 1 compare with m a x i m u m values recorded by Thorne (1960) for sugar beet 79, potato 73 and barley 40 g m -2 week -~, and by Watson (1947) for wheat 50, barley 55, and sugar beet 90 g m -2 week -~. Corresponding m a x i m u m RGR values recorded by Thorne (1960) were sugar beet 0.17, potato 0.17, and barley 0.14 g g-1 day -~ similar to m a x i m u m values recorded here. One effect of increasing density was to delay the attainment of m a x i m u m NAR and RGR at the highest density. These effects might be related to an earlier development of competition, leading to limitations of nutrients and water which may have delayed achieving m a x i m u m growth rates at the individual plant level. The later marked increase in NAR after 90 days at the 1 X 106 density was more apparent than real, due to the abscission of leaves lower in the canopy as a consequence of the shading effects. Leaf area ratio declined fairly uniformly with time, reflecting the steady increase with time of non-leaf material

210

such as stem, flower pods and seed. This general decline with time in LAR, and also in NAR and RGR has been recorded in other crops such as sugar beet, potato and barley (Thorne, 1960). In the present study the major decline in NAR and RGR occurred during the vegetative phase, and after the start of flowering at day 50, NAR and RGR remained low. The linear reduction in leaf area per plant with increasing density again reflects the effects of increasing competition on individual plant components and has been shown previously in pigeon peas (Akinola and Whiteman, 1975b) and maize (Nunez and Kamprath, 1969). Nevertheless, reductions in leaf area per plant were more than compensated by increasing plant population so that the highest density achieved the highest LAI and did so earlier than the other densities. The maximum value of LAI was 4.9 (at 62 days) at the highest population density. However, by the final harvest LAI had declined to 3.8 due to loss of lower leaves related to a longer duration of shading in the lower canopy and the highest LAI was recorded at the 5 X l 0 s density, which also had the highest seed yield. Akinola and Whiteman (1975b) also found that highest seed yield was attained at the highest LAI (5.5), while maximum dry matter yield, associated with larger plants and a greater stem fraction was achieved at a lower LAI of 3.5. Rachie and Roberts (1974) suggest that with the erect Cajanus plant, with comparatively small lanceolate leaves, optimum LAI values may be 7.0 or higher compared with 3.0 to 4.0 for other large leaved tropical pulses. The extinction coefficient (K) of 0.3 was low for a dicotyledonous species (Newton and Blackman, 1970). Since the leaves in this cultivar of C. cajan are relatively small, trifoliate, and are arranged spirally on the stem, this appears to allow better light penetration through the canopy during the vegetative stage. Even so, all densities had achieved complete light interception by the 85 day measurement, and probably had done so earlier with the onset of flowering at day 50 (see below), while the highest density had almost complete interception by day 45. The marked change in the relationship between LAI and light interception from linear at 49 days to asymptotic at 85 days was apparently due to the radiation interception by the profusion of flowers produced after day 50. Other changes in canopy structure and leaf arrangement may have also occurred to contribute to a greater light interception at a given LAI. Previous studies in pigeon pea (Akinola, 1973), soybean (Shibles and Weber, 1965) and maize (Williams et al., 1965) suggest that the radiation interception with increasing LAI approaches a maximum asymptotically. However, in these studies the exact relationship also depended on the stage of crop growth. Since crop growth rate and LAI were linearly related, highest dry matter yields were obtained at the highest density, which maintained highest LAIs throughout. Similar linear relationships have been found in sugar beet (Watson, 1958). Further increases in density should lead to higher dry matter yields as no maximum or plateaux in yield was obtained. These further increases in dry matter would be attained through earlier achievement of complete radiation

211

interception. The actual dry matter yields achieved (maximum 8200 kg ha -1), are much lower than 22,000 kg ha "1 reported by Akinola and Whiteman (1975b) for taller, more robust, cultivars. The lower dry matter yields in the present cultivar are related to the smaller plant type, a much shorter crop duration (105 days cf 203 days) and also to a lower mean crop growth rate (78 kg ha -1 day -1 cf. 108 kg ha "1 day "1). It was of particular interest that the response to increasing density in this short statured determinant type was exactly similar to the photoresponsive tall types grown by Akinola and Whiteman (1975b). In both cases, dry matter yield per plant declined linearly with increasing density, while yield per hectare increased in a parabolic cu~:ve. Seed yield was primarily affected by pod number per plant, since neither the number of seeds per pod nor mean weight per seed was affected by plant density, which is in agreement with previous density studies in C. ca]an by Hammerton (1971), Singh et al. (1971) and Akinola and Whiteman (1975a,b). Maximum seed yield of 1300 kg ha "1 was relatively low compared with earlier studies with this cultivar (Wallis et al., 1979a). However, the present crop was sown in late February and the main flowering and seed set period was in the dry cooler part of the year, while the yields reported by Wallis et al. (1979a) were obtained from September and January sowings (Table I). There was also TABLE I Comparison of seed yields, crop duration, yield/day (kg ha "~ day "~) and harvest index of

C. cajan Harvest index*

Reference

12.4

0.16

Present study (Feb sowing)

112

23.5

--

Wallis et al. (1979) Sept. sowing same cultivar as above

2890

132

21.9

--

Wallis et al. (1979) Jan. sowing same cultiv ar as above

2770

294

9.4

0.16

Akinola and Whiteman (1975b) (Cultivar UQ1)

1340

234

5.7

0.17

Akinola and Whiteman (1975c) (Cultivar UQ1)

1370

162

8.4

-

Singh et al. (1971) (Cultivar T21)

1710

133

12.8

0.25

Narayanan and Sheldrake (1979) (Cultivar C l l )

Maximum seed yield (kg ha "1)

Crop duration (days)

Yield/day

1300

105

2630

(kg ha -1 day "~)

*Harvest index = dry weight of seed/total dry weight above ground.

212

some evidence that the insecticide (Azodrin) which we used for the first time for Heliothis control caused leaf and flower shedding. The crop duration (days from sowing to harvest) was shorter than that recorded for other crops, giving a relatively high yield/day. However, the potential yield, and yield/day of this cultivar was better demonstrated by the data of Wallis et al. (1979), where much higher yields were achieved from earlier sowing. The yield/day was also exceeded by the cultivar C l l grown in the winter (short day) season in India by Narayanan and Sheldrake (1979). This crop also had a high harvest index, while the harvest index in the present study was similar to that for a range of other Ca]anus crops grown in Australia (Table I). Seed yield per hectare declined at the highest density due to the decline in pod number per plant which was not compensated by increasing plant population. Since mean weight per seed and number of seeds per pod remained constant with increasing density, analysis of data in Fig. 10 indicates that maximum yield per hectare of approximately 1,500 kg ha -1 should have been attained at a density of approximately 7.5 X 10 s plants ha". This assumes that pod number and seed yield per plant declines linearly, as in Fig. 10. Important objectives in the breeding programme presently in progress with this material are to retain its photoperiod insensitivity, early flowering, and short stature, while sttempting to increase mean seed weight which leads to a higher recovery of dhal after milling. ACKNOWLEDGEMENTS

We thank Mr T. Hughes, Farm Manager, and his staff at the University Research Farm, Redland Bay, for their competent technical assistance. We also thank Dr D. Keatinge, Grain Legume Project, University of West Indies, for his constructive criticisms of the manuscript. REFERENCES Akinola, J.O., 1973. An agronomic evaluation of pigeon pea. Ph.D. Thesis, University of Queensland (unpublished). Akinola, J.O. and Whiteman, P.C., 1975a. Agronomic studies on pigeon pea. I. Field responses to sowing time. Aust. J. Agric. Res. 23: 43--56. Akinola, J.O. and Whiteman, P.C., 1975b. Agronomic studies on pigeon pea. II. Responses to sowing density. Aust. J. Agric. Res., 23: 57--66. Akinola, J.O. and Whiteman, P.C., 1975c. Agronomic studies on pigeon pea. III. Responses to defoliation. Aust. J. Agric. Res., 23: 67--79. Hammerton, J.L., 1971. A spacing planting data trial with Cajanus cajan (L.) Millsp. Trop. Agric. (Trinidad), 48: 341--346. Lawn, R.J., Byth, D.E. and Mungomery, V.E., 1977. Response of soybeans to planting date in south-eastern Queensland. III. Agronomic and physiological response of cultivars to planting arrangements. Aust. J. Agric. Res., 28: 63--80. Muchow, R.C. and Kerven, G.L., i~77. A low cost instrument for measuring photosynthetically active radiation in field canopies. Agric. Meteorol., 18: 187--191.

213 Narayanan, A. and Sheldrake, A.R., 1979. Pigeon pea (Cajanus cajan) as a winter crop in peninsular India. Exp. Agric. 15: 91--95. Newton, J.E. and Blackman, G.E., 1970. The penetration of solar radiation through leaf canopies of different structure. Ann. Bot., 34: 329--336. Nunez, R. and Kamprath, E., 1969. The relationship between N response, plant population and row width on growth and yield of corn. Agron. J., 61 : 279--284. Rachie, K.O. and Roberts, L.M., 1974. Grain legumes of the lowland tropics. Adv. Agron., 26: 1--132. Radford, P.J., 1967. The use and abuse of growth analysis formulae. Crop Sci., I: 171--175. Shibles, R.M. and Weber, C.R., 1965. Leaf area, solar radiation interception and dry matter production by soybeans. Crop Sci., 5: 575--580. Singh, L., Maheshwari, S.K. and Sharma, D., 1971. Effect of date of planting and plant population on growth, yield components and protein content of pigeon pea (Cajanus cajan)(L.) Millsp.). Ind. J. Agric. Sci., 41: 535. Thorne, G.N., 1960. Variation with age in N.A.R. and other growth attributes of sugar beet, potato and barley in a controlled environment. Ann. Bot., 24: 356--371. Wallis, E.S., Whiteman, P.C. and Byth, D.E., 1979a. Pigeon pea (Cajanus cajan (L.) Millsp.) production systems in Australia. Proceedings of the Regional Workshop on Tropical Grain Legumes, University of West Indies, Trinidad, pp. 1--19. Wallis, E.S., Whiteman, P.C, and Byth, D.E., 1979b. Pigeon pea -- a new crop for Queensland. Queensl. Agric. J., 105(6): 487--492. Watson, D.J., 1947. Comparative physiological studies on growth of field crops. I. Variation in N.A.R. and leaf area between species and varieties within and between years. Ann. Bot., 2: 41--76. Watson, D.J., 1952. The physiological basis for variation in yield. Adv. Agron., 4: 101-144. Watson, D.J., 1958. The dependance of nett assimilation rate on leaf area index. Ann. Bot., 22: 37--54. Williams, W.A., Loomis, R.S. and Lepley, C.R., 1965. Vegetative growth of corn as affected by population density. I. Productivity in relation to interception of solar radiation. Crop Sci., 5: 211--214.