Plant and Soil 44, 575-586 (1 976)
Ms. 2733
SOURCE-SINK INTERRELATIONSHIPS IN LOWLAND RICE by B. VENKATESWARLU All-India Coordinated Rice Improvement Project, Rajendranagar, Hyderabad-500030 SUMMARY Present studies indicate t h a t leaf could determine the degree of plants' performance and condition the level of unfilled spikelets. Environmental constraints like low light intensity itself can considerably affect the function of the leaf resulting in greater proportion of unfilled spikelets or sterility, which appears to be a post-fertilization problem. This suggests that 'source' (leaf) is unable to cater to the requirements of the panicle. Further, the LAI studies have revealed the existence of a wide gap between spikelet production and grain filling. Panicle-tailoring studies pointed out that sterility could be reduced if panicle size is reduced. I n the light of all these evidences, it was concluded t h a t 'source' is limiting at the present level of evolution in rice under I n d i a n conditions. INTRODUCTION
In sixties, morphological architecture of tile plant served as a route for augmenting yields. In several crop plants, particularly in cereals like wheat, rice, maize and sorghum, dwarfism contributed for higher yields through avoiding lodging, increasing harvest index and better canopy performance. This resulted in attaining a plateau in crop yields. Particularly in rice, the change in plant type concept mainly covered by a plethora of characters associated with a dwarfing gene had brought transformation in rice yields from a maximum of 5.0 to 10.0 tons per ha. Considering the further possibility of increasing the yields of cereal crops like rice, attempts are being made to break the present yield barrier. The approaches mainly directed towards 'source-sink' dynamics are likely to determine the future yield levels. Since several parameters concerning the physical frame of the plants are almost
576
B. VENKATESWARLU
e x h a u s t e d , i t is l i k e l y t h a t t h e m a n i p u l a t i o n of f u n c t i o n a l t r a i t s m a y h e l p t o b r i n g a n o t h e r p l a t e a u in rice y i e l d s . T h e r e f o r e , i t is n e c e s s a r y t o d e t e r m i n e w h i c h o r g a n of t h e p r o d u c t i o n s y s t e m is l i m i t i n g a t t h e p r e s e n t l e v e l of e v o l u t i o n i n rice. I n v i e w of t h e i m p o r t a n c e of t h i s p r o b l e m , a t t e m p t s a r e m a d e in t h e p r e s e n t s t u d y t o e l u c i d a t e t h e ' s o u r c e ' (leaf) a n d ' s i n k ' (panicle) i n t e r r e l a t i o n s h i p s s u c h t h a t f u t u r e breeding programs could be monitored to improve the functional e f f i c i e n c y of p l a n t s .
MATERIALS AND METHODS L Lea/clipping
Twenty five-day aged seedlings of J a y a and Sona were t r a n s p l a n t e d at a spacing of 20 × 15 cm in 4 × 3 m plots. The fertilizers applied were 100 kg N, 25 kg P and 40 kg K/ha. F i f t y per cent of N and the rest of the fertilizers were added basally. The remaining N was applied 25% each at mid-tillering and at panicle initiation stage. The following treatments were given when the plants just flowered: a) All leaves clipped off, b) All leaves clipped + stem covered with a paper roll and fastened with rubber bands, c) All leaves clipped and the panicle covered with paper bags and tied with rubber bands, d) All leaves clipped and the whole plant covered with paper roll and fastened, and e) Control without a n y d i p p i n g and covering. I n all cases where paper-roll bagging was used, small holes were made with a pin so as to allow aeration. The contribution of each organ to yield was calculated based on d a t a obtained from each of the treatments. The yield obtained ill control p l a n t was taken as base and the yield obtained in t r e a t m e n t 'a' was deducted from the former and the product thus obtained was considered as contribution of leaf. In t r e a t m e n t 'd', the grain yield obtained could be only due to the accumulated carbohydrate already present in the p l a n t since the stem and panicle photosynthesis was arrested. The yield under such a situation was considered to be due to the contribution of reserve food material. Likewise, the yield obtained ill trea*ment 'a' becomes the contribution of stem, panicle, and reserve. Similarly, the yield obtained in t r e a t m e n t 'c' becomes the contribution of stem and reserve and the grain obtained in treatment 'b' becomes the contribution of panicle and reserve. Now, deduction of yield of 'c' from 'a' gives the contribution of panicle. Similarly, deduction of contribution of panicle and reserve from treatment 'a' gives the contribution of stem.
SOURCE-SINK INTERRELATIONSHIPS IN LOWLAND RICE
577
2. Shading trial Twenty five-day seedlings of Sona and Jaya were transplanted in 4 × 3 m plots adopting a spacing of 20 × 15 cm. The plots were shaded by thin kora cloth such t h a t 40% of natural light is received on the crop canopy. The cloth was fastened tight to the wire which served as a lining connecting the wooden supports Mround. The light intensity was measured with a Toshiba luxmeter. The shading treatments are - (i) planting to panicle initiation, (if) panicle initiation to flowering, (iii) flowering to maturity, and (iv) control (unshaded). The shading t r e a t m e n t was started 15 days after planting. Nitrogen was apptied at the rate of 120 kg/ha and P and K at the rates of 26 kg and 40 kg/ha respectively. The study was conducted in triplicate. O b s e r v a t i o n s. Dry-matter distribution, tillers, pollen fertility b y iodine test; and the n u m b e r of fertilized spikelets was counted b y the indication of the development of dark scar in the spikelets 7 days after flowering.
3. 'Source' and 'sink' manipulation The seedlings of Sona (25 days old) were transplanted in 5 × 4 m plots adopting spacings - (i) 10 × 10 cm, (if) 20 × 20 em, (iii) 30 × 30 cm, (iv) 50 x 50cm, a n d ( v ) 100 × 1 0 0 c m - a t 1 0 0 k g N a n d 2 0 0 k g N l e v e l s . P a n d K were applied at 25 kg and 40 kg per hectare for 100-N plots and 50 kg and 80 kg per hectare for 200-N plots respectively. Fifty per cent N was applied 1S days after planting and the rest split in two top applications of 25% each. This approach with different levels of N and spacings facilitated manipulation of the 'source' (leaf) and the 'sink' to different degrees. The trial was laid out on split plot design with 3 replications and conducted during Kharif (wet) and Rabi (dry) seasons. O b s e r v a t i o n s . Leaf area index at flowering, panicles, spikelet and grain number.
4. Tailoring o/the panicle I n order to vary the size of the panicle and study the effects on sterility and filled-grain proportions, the panicle-tMloring approach was adopted. The seedlings of Jaya were transplanted in 3 × 4 m plots at the spacing of 20 × 15 cm. The N, P and Jd were applied at the rate of 100 kg, 25 kg and 40 kg/ha respectively. The panicles were tailored in the following manner when the plants have just flowered: (a) Approximately half bottom portion of the panicle retained and the rest severed off, (b) Half top branches of the panicle retained and the lower spikelets only scissored off, retaining the rest of the part of the panicle. (c) The control (all the spikelets in tact). Twelve hills were chosen for each of the mentioned treatments and all panicles of each hill were given the required treatment as and when the panicles emerged. G e n e r a l o b s e r v a t i o n . Grain yield, chaff, partially filled and well-filled grains were calculated in all the experiments b y adopting the specific gravity method.
578
B. VENKATESWARLU
A s a m p l e of t h e spikelets h a r v e s t e d were p l a c e d in 1.06 specific g r a v i t y sol u t i o n a n d t h o s e t h a t h a v e s u b m e r g e d were c o n s i d e r e d as grains a n d t h o s e f l o a t e d w e r e t a k e n as unfilled. The unfiUed spikelets were s e p a r a t e d m a n u a l l y i n t o p a r t i a l l y filled a n d chaff. TvFo 20-g s a m p l e s were t a k e n for e a c h replication. RESULTS AND DISCUSSION
The leaf clipping and shading (covering) study revealed that removal of all leaves at flowering resulted in only 30% of grains maturing compared with 50% in the control, and in a reduction in grain yield of 50% (Table 1), the maturity being reduced to 4% TABLE 1 Influence of leaf clipping and different shading (covering) treatments on sterility and filled grain proportions of Jaya and Sona, Rabi (dry) season, 1973 Jaya
Treatments
Grain yiled/hill (g)
(a) All leaves clipped off (b) All leaves clipped + stem covered (c) All leaves clipped + panicle covered (d) All leaves clipped + whole plant covered (e) Control
Sona
Chaff Partially Filled (%) filled grain (%) (%)
Grain yield/hill (g)
Chaff Partially Filled (%) filled grain (%) (%)
6.25
49
20
31
6.33
54
20
26
4.58
70
22
8
4.00
66
28
6
3.91
68
26
6
4.51
78
17
5
1.75 11.91
75 26
21 24
4 50
1.76 13.90
70 27
26 32
4 41
The figures are the mean of 12 hills.
when the whole plant was covered or shaded with paper bags. Interestingly, the contributions of reserve food, stem, panicle and leaf to yield was 14.7, 18.1, 19.7, and 47.5 per cent respectively in Jaya; and 12.7, 19.7, 13.1, and 54.5 per cent respectively in Sona. These data indicate that leaf, by and large, determines the yield and conditions the formation of chaffy and partially filled grains. Some of the workers like L u c a s and A s a n a 5 and R a w s o n and E v a n s ¢ in wheat; and Sri R a n g a s a i and V e n k a t e s w a r l u 7 in finger millet, have reported that the yield decrease due to leaf clipping was little and the mobilization of stem sugars and flag leaf photosynthesis compensated for the yield losses. But the present study indicates
SOURCE--SINK I N T E R R E L A T I O N S H I P S IN LOWLAND RICE
579
that the yield loss depends not only on the type of crop but on the degree of leaf clipping effeeted. The estimated contribution of ear photosynthesis to the grain ranges from 8 to 23°//o for rice ( E n y i z) and 10 to 49% for wheat ( B o o n s t r a 1 and K r i e d e m a n n 4 ) , indicating that panicle contribution to yield is greater in wheat than in rice. Obviously, leaf is the major determinant of yield in rice since the contribution of it is greater than each of the rest of the organs (Table 1). In India, 60°/0 rice is grown during monsoon (wet) season when the light intensity is 40 to 60% less than that of Rabi (dry) season. The shading trial conducted with 60% of light cut gave very interesting information as seen from Table 2. The stem weight, total dry TABLE 2 Effect of continuous shading (40% of natural light) on dry matter distribution, tillers, filled grains and grain yield, Rabi (dry) season, 1973 Sona
Leaf weight/hill (g) Stem weight/hill (g) TDW/hill (g) Tillers/hill (no.) Grain yield (g/sq m) Sterility (%) Filled grain (%)
Control
Shade
2.5 13.1 45.0 18 820.0 23.0 77.0
2.4 6.5 22.3 10 275.0 41.0 59.0
matter content, tillers, grain yield and sterility have been severely affected although the change in leaf weight was insignificant. It was clear that the leaf was unable to assimilate more food material at the required level and as a result the tiller number is affected very much, as it depends upon the degree of carbohydrate accumulation. The figures on sterility show that grain filling is a problem for higher yields in addition to filleting. Thus clearly the function of leaf is a serious limitation under low light intensity conditions. The data collected on sterility from several experiments have shown that it ranged from 20 to 45%. The data obtained from different cooperating centres in the country also have shown that the sterility is of high order. Therefore, some studies were conducted in
580
B. V E N K A T E S W A R L U
order to find out whether the sterility is a pre- or post-fertilization problem. The data presented in Table 3 show that the pollen fertility is of high order beyond 90% in all excepting the bottom spikelets of Vijaya. However, no change occurred in pollen fertility irrespective of shading at any growth stage. This illustrates that the pollen fertility is not a problem under low light intensity irrespective of growth stage. The question of fertilization is still not clear to precisely say that it was a post-fertilization problem. Hence, data (Table 3) were collected from the shading trial on the number of dark Scars developed in the spikelets. Generally, dark scars develop within 4 to 7 days after flowering in the spikelets of rice. The scars are nothing but the extensions of ovary which is developing into a kernel. The scar bulging will be clear from the tip of the spikelets after fertilization in rice. The data indicated that more than 94% of the spikelets have exhibited dark scars 7 days after flowering. It clearly suggests that fertilization is no more a problem irrespective of the low light intensity prevalent at a given stage. It suggests that the sterility problem, by and large, is only a post-fertilization one and m a y be even dependent more on the leaf. Though sterility could be caused by other factors such as rain at the time of anthesis, low temperature, insect damage and diseases, in the present context, it could be by leaf only since one of the important inhibiting factors like temperatures were within the safe limits (maximum 37-25°C and minimum 27-20°C) during crop growth and there was no infestation of pests and diseases. The point that sterility could be caused by environmental constraints like low light intensity is very clear from the data presented in Table 4. The maturity percentage has fallen sharply from 78.5% to 37.9% in Sona and from 79.6% to 40.0% in Jaya, when the crop canopy was shaded from flowering to maturity. Even the grain number has decreased under shade Conditions. This clearly illustrates that sterility in terms of chaffiness can be caused to high proportions even due to low light intensity which might have been manifested due to the inhibition of the function of the leaf. In order to manipulate the 'source' and the 'sink', different plant densities were maintained. The dimension of the source was varied from 1 to 8 LAI and the sink size in terms of panicle number ranged from 53 to 687 (Table 5). Though the source size did not vary much
S O U R C E - S I N K I N T E R R E L A T I O N S H I P S IN L O W L A N D RICE
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B. V E N K A T E S W A R L U TABLE 4
Effect of shading (40% of natural light) from flowering to maturity on sterility and yield components, Kharif (wet) season, 1973 Sona
Jaya
Component
Control
Shade (flowering to maturity)
Chaff (%) Partially filled grain (%) Filled grain (%) Panicles per hill (no.) Grains per panicle (no.) Grain yield (g/sq. m)
10.6 11.2 78.5 9.8 126 630.0
45.7 16.4 37.9 9.0 87 261.0
Shade (flowering to maturity)
Control 8.8 12.8 79.6 9.8 103 533.0
39.5 20.5 40.0 10.6 65 328.0
TABLE 5 Leaf Area Index and sterility, maturity, panicles, yield interrelationships Kharif 1973
Rabi 1974
Sterility
Sterility
Spacing
LAI
Chaff (%)
Partially filled grain (%)
100-N: 10 x 10 cm 20 × 2 0 c m 30 × 3 0 c m 50 x 50 cm 100 × 100cm
7.6 4.1 2.4 1.8 1.3
17 19 19 18 20
13 14 11 14 11
70 67 70 68 69
455 257 154 122 53
736 685 394 314 114
7.0 2.7 3.1 2.7 1.3
20 18 16 15 16
15 18 16 16 21
65 64 68 69 63
674 334 257 180 85
785 655 567 420 137
200-N: I0 × 10cm 20 × 2 0 c m 30 x 3 0 c m 50 × 50 cm I00 x 100cm
8.6 4.9 4.2 3.3 1.5
24 24 19 20 25
15 18 16 13 15
61 58 65 67 60
558 298 225 165 58
701 699 559 374 134
8.0 5.2 4.1 3.6 1.4
20 24 18 23 17
15 17 16 16 17
65 59 66 61 66
687
913 790 712 554 187
Fig. 2.
Filled grain (%)
Panicles/ sq m (no.)
Grain yield (g/ sq m)
LAI
Chaff (%)
Partially filled grain (%)
(%)
Panicles/ sq m (no.)
sq m)
Filled grain
Inter-relationships between LAI and spikelet and grain number per sq. m , R a b i 1974, H y d e r a b a d .
385 283 236 88
Grain yield
(g/
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B. VENKATESWARLU
584
in Kharif (wet) and Rabi (dry) seasons, the sink size considerably differed. The sterility percentage varied from 30 to 41 indicating the presence of greater proportion of unfilled grains (in both the seasons). The spikelet and grain number were plotted against LAI and the quadratic regression was fitted for both the seasons at Hyderabad centre. The increase in LAI was not reflected in increased grain number which was far dower than that of the spikelet production. Tile gap between spikelet number and grain number was very wide (Figs. 1 and 2) indicating the greater possibility for obtaining higher yields, had there been pr0per, support f r o m t h e source. It also infers that the increased I/AI is leading to considerable production of spikelets but a sizable number of them are not getting filled up. Similar type of data from other important centres of India viz Cuttack, Pattambi and Mandya also confirmed that in spite of in=o
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S O U R C E - S I N K I N T E R R E L A T I O N S H I P S IN L O W L A N D RICE
585
creased LAI at 100 and 200-N levels, the gap between spikelet number and grain number remained very wide (Fig. 3). This d e a r l y illustrates that under Indian conditions, emphasis is to be put more on the improvement of 'source'. C o c k and Y o s h i d a 3 reported under Los Banos conditions, there was no change in grain number with their CO2 enrichment studies, suggesting that source is not a limitation. But under Hyderabad conditions, although sink is being formed considerably, the source seemed to he limiting as there were m a n y unfilled spikelets. Thus the present findings are contrary to the observations made by these workers. However, if one considers to achieve greater yields, it is but natural greater sink size would be more advantageous. But the question was whether at the present level of sink size, is the capacity of the source sufficient? Here, the difference between spikelet and grain number (Figs. 1, 2 and 3) suggests that greater capability of leaf is called for to fill all the spikelets. TABLE 6 Influence of tailoring of panicle on sterility, well filled grains and 1000-grain weight, Variety: J a y a Treatment
Half b o t t o m portion of the panicle retained Half top branches of the panicle retained Control
Grain yield/ hill (gm)
Chaff (%)
Partially filled grains (%)
Filled grains (%)
1000grain weight (gm)
9.8
8
7
85
22.2
11.3 16.2
10 20
5 18
85 62
21.8 19.4
The figures are the mealz of 12 hills.
The evidence obtained from the studies on panicle tailoring (Table 6) show that by reducing half tile size of the panicle, sterility could be reduced by 230/0 . This point suggests that moderate-sized panicles m a y be sufficient for higher maturity. Further, it appears that at the present evolution, leaf is not efficient to cater to the demand of all the spikelets in the panicle. Even if we consider that certain amount of sterility is bound to occur ranging from 5 to 8% due to plants' functional constitution and climatic constraints, it should be possible to reduce the present level of unfilled spikelets.
586
SOURCE-SINK INTERRELATIONSHIPS IN LOWLAND RICE
Obviously, a high proportion of sterility or unfilled spikelets caused by leaf clipping treatments; greater sterility, low dry matter accumulation and more partially filled grains caused by shading; existence of a wide gap between spikelet number and grain number at different LAI values at 100 and 200 kg N/ha levels, impressive decrease in sterility when panicle size was tailored - all compel us to conclude that 'source' is a limitation at the present level of evolution. Our plant improvement programs all these years were mainly guided by the selection procedures directed on panicles rather than on the elastic abilities of leaves. Therefore, naturally there is good improvement in panicle size. Now it appears clearly that our efforts are to be diverted towards the identification and selection of leaves endowed with capabilities to function well under natural stresses such as low light intensities, CO2 tensions and water stresses not only to stabilize but to raise the present yield plateau. ACKNOWLEDGEMENTS T h e a u t h o r is p e r s o n a l l y g r a t e f u l t o Dr. L. T. E v a n s , Chief, Division of P l a n t I n d u s t r y , C S I R O , Canberra, A u s t r a l i a for his critical c o m m e n t s a n d s u g g e s t i o n s on t h e m a n u s c r i p t . Received 10 January 1975
REFERENCES 1" B o o n s t r a , A. E. H. R., Meded. Landbouwhogesch. Wageningen 33, 3-21 (1929). 2 E n y i , B. A. C., The contribution of different organs to grain weight in upland and swamp rice. Ann. Bot. N.S. 26, 529-31 (1962). 3 Cock, J a m e s H. and Y o s h i d a , S h o u i c h i , Changing sink and source relationships in rice (Oryza sativa L.) using carbon dioxide enrichment in the field. Soil Sci. Plant Nutrition 19, 229-234 (1973). 4 K r i e d e m a n n , P., The photosynthetic activity of the wheat Ear. Ann. Bot. N.S. 30, 349-63 [1966). 5 Lucas, D. and Asana, R. D., Effect of defoliation on the growth and yield of wheat. Physiol. Plantarum 21, 1217-1223 (1968). 6 R a w s o n , H. M. and E v a n s , L. T., The contribution of stem reserves to grain development, in a range of wheat varieties of different height. Australian J. Agr. Research 22, 851-863 (1971). 7 Sri R a n g a s a y i , I. and V e n k a t e s w a r i u , B., Some aspects of plant type in finger millet, Etusine coracana, G. In Press. Special issue o] Indian J. Gen. and Plant Breeding, under the auspices of SABRAO (1972).
* Original not seen.
