STUDIES IN PHYSIOLOGICAL ANALYSIS OF YIELu- VIII. COMPARISON OF DEVELOPMENT OF UPPER AND BASAL GRAINS OF SPIKELETS OF TWO VARIETIES OF WHEAT R. D. AsANA

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VIII. COMPARISON OF DEVELOPMENT OF UPPER AND BASAL GRAINS OF SPIKELETS OF TWO

VARIETIES OF WHEAT

R. D. AsANA and A. K. BAGGA

Division of Botany, Indian Agricultural Research Institute, Delhi-12

SUMMARY

The development of basal and upper grains of ten central spikelets and of grains of the rest of the spikelets of the ear of two wheat varieties, NP 720 and Pb C 281, was compared. The two basal grains (of the central spikelets) of Pb C 281 had greater length, volume, water content, and dry weight than those of NP 720 earlier than a fortnight from an thesis, but the dry weight of upper grains and of grains of other spikelets of the former was slightly less than in the latter variety until about four weeks from anthesis. The grain weight per ear was thus equal in the two varieties for about four weeks from anthesis. The number of cells in the endosperm of the basal grain was slightly smaller, but their size was larger in Pb C 281 than in NP 720. The pentosan and the lipid contents of these grains were also different in the two varieties. During the first fortnight from anthesis, the supply of photosynthate is in excess of its utilization in grain development as indicated by increase in the sugar content of the stem. The larger size of the basal grain of Pb C 281 may, there-fore, depend on other factors besides the rate of supply of substrate through photosynthetic activity.

2

grain weight. It is also likely that the earlier decline in the moisture status of the grain of NP 720 might have led to an earlier cessation of synthetic activity.

INTRODUCTION

Earlier investigations in this laboratory showed that grain weight per ear of wheat varieties, NP 720 and Pb C 281, increased at the same rate for about four weeks from anthesis, but subsequently it increased more in the latter variety (Asana and Saini, 1958, 1962). Asana and Joseph ( 1964) kept the entire shoot in dark from the 26th day from an thesis until maturity and found that Pb C 281 still produced a higher grain weight than NP 720, although the grain weight per ear of the two varieties was less than in shoots exposed to sunlight. They concluded, therefore, that in dark the grains grew at the expense of stem sugars and that varietal differences in grain yield may partly depend upon the synthetic activities in the grain and not entirely upon differential photosynthetic activity. Asana and Williams ( 1965) also referred to some evidence in support of this conclusion from the study of two other cultivars, Ridley and Diadem.

In the work referred to above, observations were made on the rates of increase in dry matter and starch content of the grains, and it was found that the higher grain weight of Pb C 281 was largely due to its higher starch content. It was considered desirable to extend observations to other growth attributes such as increase in length, width, volume, water content, and pentosans and lipids of the grain. Since the size of a grain varies with a spikelet and in different regions of an ear, it was necessary to compare the development ofthe grains situated at these locations. There are usually three to nine flowers in a spikelet, the upper one or more being imperfect and" sterile. The first to open are the basal flowers of the spikelets situated in the middle region of the ear. Anthesis occurs in regular succession from these spikelets towards the apex and the base of the ear and from the basal to the upper flowers within a spikelet. The two basal flowers in the middle spikelets usually open on the same day and form the largest grains of the spikelet as well as of the ear (Percival, 1921). The basal grains from the ten middle spikelets were used for detailed study. Relevant observations were also made on the upper grains of the ten middle spikelets and on the grains from the other spikelets.

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Three sowings were made at 15-day intervals in order to assess the effects of different temperature conditions.

ExPERIMENTAL PROCEDURE

Plants of wheat varieties NP 720 and Pb C 281 were grown in pots according to the procedure described earlier by Asana and Saini ( 1962). Observations were taken on main shoots only (roots excluded). The dates of sowing and anthesis on main shoots are given below :

Variety Date of

sowing

Date of an thesis

Pb C 281 was sown later than NP 720 by four days in order to synchronize the time of anthesis.

Sampling.-In each sowing, the main shoots, in which

anthesis occurred on the same day, were tagged and used as samples. In view of the time taken in isolating the grains and recording other observations, the number of ears had to be restricted. Three shoots per variety in sowings I and II, and four shoots per variety in sowing III were sampled on each sampling day. At complete maturity (final harvest) 20 main shoots were sampled in each treatment. Only one main shoot of each variety was cut at a time and brought immediately to the laboratory for observations. The stem and leaves were cut into small pieces and kept separately for drying in an electric oven at 80°C .

the upper grains. The upper grains were immediately kept for drying. The following observations were taken on the basal grains which usually numbered 20.

Length and width.-Immediately after removal from the

spikelets the grains were placed in a small butter paper envelope and weighed on a Mettler balance. The length and width were measured by arranging the same grains in a row lengthwise and breadthwise respectively.

Measurement of volume.-A simple water-displacement method was used for quick measurement of grain volume. A small fiat-bottomed pyrex-glass flask (diameter of the bulb 2 · 5 em and length of the neck 4 · 0 em) was calibrated to a 5 ml mark with distilled water run from a micro-burette, reading up to 0 · 01 ml. A fine wire with a loop at one end and a constant length up to the open end of the neck of the flask was also inserted into the flask while calibrating. The loop effectively prevented the grains from floating. For each measurement a separate flask, cleaned with chromic, sulphuric acid mixture and dried beforehand, was used. The grains were dropped into an empty flask, the wire was kept in position, and water was run from the burette up to the 5 ml mark. The difference between 5 ml and the burette reading gave the volume of the grain sample. The measurement of the volume of a sample took one to two minutes. The grains were then dried at 80°C.

Estimation of pentosans, lipids and DNA.-Pentosans and lipids

of the grain were estimated according to the methods described

resp~ctively by Loska and Shellenberger (1949) and Jennings and Morton (1963b). For the estimation of DNA the grain material was prepared according to the method of Schneider (1945) and deoxyribose estimated by the method of Webb and Levy (1955).

Sectioning of the grain.-Basal grains from the middle ten

1

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Sections passing through the central portion of the endosperm were measured along the three planes mentioned in Tables VII and VIII, with the help of an ocular micrometer.

EXPERIMENTAL RESULTS

Grain weight per em·.-On the 15th day from anthesis grain weights of the three sowings varied as III>II>I (Fig. lA). The two varieties had more or less equal grain weights up to 28 days from anthesis in sowings II and III and up to 36 days in sowing I. Pb C 281 had higher grain weight than NP 720 on the 32nd day in sowings II and III and on the 44th day in sowing I. It is doubtful if grain weight increased appreciably after 32 days in sowings II and III and after 44 days in sowing I. The weights on the 60th, 48th, and 44th day for sowings I, II, and III respectively, will be considered as final since they represent mean values of 20 ears in each case. The data in Table I show that the final grain weight of Pb C 281 was consis-tently higher than that of NP 720 and grain weight in sowing I was significantly higher than in sowings II and III.

Grain number per ear.-The number of grains per ear was more in NP 720 than in Pb C 281 in all the three sowings (Table I) and it was the highest in sowing III. From the data on grain number of different samples it appeared that grain setting was more or less complete on the 9th day from anthesis .

Table I. Final grain weight, grain number per ear, and 1,000-grain weight

-Sowing

Grain weight/ear Grain no./ 1,000-grain

(gm) car weight (gm)

- - - M e a n - - - Mean---Mean

NP Pb C NP Pb C NP Pb C

720 281 720 281 720 281

-I II III

2·91 2·64 2·68

3·13 3·02 63·1 59·6 61·3 46·1 52·5 49·3 2·90 2·77 60·8 57·6 59·2 43·3 50·3 46·8 2·92 2·80 71·2 63·7 67·4 37·6 45·9 41·7

- - - -- - - · - - -

-1-1ean 2·74 2·98 65·0 60·3 42·3 49·6

L.S.D. at 5% P Variety 0·112 2·3 0·647

Sowing 0·143 2·8 0·792

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DAYS FROM ANTHES/5

Fro. 1. A-C. Changes in dry weight of different organs etc .

In sowings II and III Pb C 281 had higher 1,000-grain weight than NP 720 consistently from the 15th day from anthesis onward. The final 1,000-grain weight varied with sowing as I>II>III and Pb C 281 had a higher 1,000-grain weight than NP 720 (Table I).

Chaff weight.-The chaff showed some increase in dry weight after anthesis, but subsequently it remained more or less constant. There was no consistent difference between the chaff weights of the two varieties.

Stem weight.-The stem weight of Pb C 281 was, on the whole, higher than that of NP 720. It increased or remained constant up to 15 to 20 days from anthesis and then decreased sharply (Fig. 1 C).

Leaf weight.-The leaf weight per shoot was slightly more in Pb C 281 than in NP 720. There occurred a gradual decrease in leaf weight with time, the final decrease amounting to 25 to 30 per cent of the initial value.

Length of ovary.-The ovary at anthesis was longer in Pb C 281 than in NP 720. There was no effect of time of sowing on the ovary length of the two varieties (Table II).

Table II. Length of ovary of basal flower from camera lucida drawings zn em

NP 720

Pb C 281

l'viean

L.S.D. at 5% P

Sowing I Sowing II Mean

4·65 4·84

4·74

Variety

4·71 4·85

4·78 0·147

4·68 4·84

that of NP 720 in all the sowings. The length varied with sowing as I> II> III. The differences were statistically signi-ficant (exception : NP 720, sowing I=sowing II, Table III).

Grain width was measured a little later in sowing II (Fig. 2B). In sowing III width was similar in the two varieties up to day 28, the maximal values having been attained by this time. The grain width decreased in NP 720 after day 28 and in Pb C 281 after the 32nd day. In sowing II the grain width was similar in the two varieties at day 32, after which it decreased. The maximum width was larger in sowing II tha 1. in

sowing III in both varieties, and the final width was significantly larger in sowing II than in sowing III and in NP 720 than Pb C 281 in sowing II (Table III).

Table III. Final length, width, and volume of 20 basal grains from the ten middle spikelets

Sowing

I

II III

Mean L.S.D. at 5% P

Length in em Width in em Volume in ee

T\P Pb C Mean NP Pb C Mean NP Pb C Mean

720 281 720 281 720 281

13·0 13·0 12·7

12·9

Variety Sowing Variety X

Sowing

14·6 13·8 0·804 0·888 0·846 14·4 13·7 7·26 7·07 7·16 0·755 0·836 0·795 14·2 13·5 6·90 6·86 6·88 0·673 0·773 0·723

14·4 0·085 0·104

0·147

7·08 0·072 0·072

0·101

6·96 0·744 0·832 0·012 0·015

N.S.

17

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Fw. 2. A-E. Changes in length, wictb, volume, fresh weight,and moisture content of 20 basal grams.

of Pb C 281 in sowing Ill had attained the maximal volume. The grain of NP 720 decreased in volume by this time in sowing III. The volume decreased in sowing II by day 36, whereas in sowing I it was still increasing. In sowing I the maximum volume was reached by day 44, it being higher than that in sowings II and III. In sowing I reduction in grain volume occurred after day 44. The maximal values of grain volume varied with sowing as I> II> III, and with variety as Pb C 281 > NP 720 and the same order prevailed at maturity (Table III).

Fresh weight and moisture content of basal grains.-Varietal difference in the fresh weight of the basal grains was established in sowings II and III between 9 to 15 days from anthesis, the weight being higher in Pb C 281 (Fig. 2D). The final weights were higher in sowing II than in sowing III. In sowing I, observations on fresh weight commenced only on the 28th day from anthesis. On this day the fresh weights in sowing I were lower than in sowings II and III. They, however, continued to increase for a much longer period in sowing I and eventually were higher than in sowings II and III.

The water content of the basal grains was consistently higher in variety Pb C 281 (Fig. 2E). It increased in sowings II and III up to 20 days from anthesis and decreased steadily there-after. The fresh weights increased, on the other hand, up to 36 days in sowing II and up to 28 days in sowing III. In sowing I, the fresh weight increased up to 44 days, whereas the water content increased up to 36 days only.

The water content as percentage of dry weight, however, decreased steadily from the ninth day in sowings II and III. It was slightly higher in Pb C 281 (Fig. 3A).

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DAYS FROM ANT/1£515

Fm. 3. A--D. Changes in percentage moisture content, dry weight, and

absolute and percentage pentosan content of basal grains.

Table IV. Final dry weight of basal grains of ten middle spikelets in gm

NP 720

Pb C 281

Mean

L.S.D. at 5% P

Sowing I Sowing II

1·060 1·178

0·945 1·070

1·119 1·007

Variety 0·022

Sowing 0 · 02 7

Sowing III

0·879 1·016

0·947

Mean

0·961 1·088

Dry weight of upper grains.-The grain weight was slightly higher in NP 720 than in Pb C 281 up to 28 days in sowings II and III. In sowing II the dry weight increased up to the 36th day from an thesis, and in sowing III only up to 28 days in NP 720 and 32 days in Pb C 281 (Fig. 4B). There was little difference between the final grain weights of the two sowings and between the varieties in sowing II; in sowing III, Pb C 281 had higher grain weight than NP 720. The final 1,000-grain weight of these grains was higher in sowing JI than in sowing III and in Pb C 281 than in NP 720 (Table V).

Table V. Final dry weight and 1,000-grain weight of upper grains

of

ten middle spikelets

Grain wt (gm)

NP 720 Pb C 281 Mean

Sowing II

Sowing III

0·733 0·689

Mean 0·711

L.S.D. at 5% P Variety

Sowing Variety X

Sowing

0·748 0·841

0·781 0·050

N.S.

0·071

0·740 0·751

1000-grain wt (gm)

NP 720 Pb C 281

40·5 34·8

37·6

47·4

44·1

45·7 0·784 0·784 1·372

Mean

43·9 39·4

Grain weight of the ten middle and of the rest of the spikelets of the ear.-In sowing II the grain weights of the ten middle spikelets were similar in the two varieties on the 28th day from anthesis (Fig. 4C), but Pb C 281 attained a higher value on the 32nd day. In sowing III the grain weights were similar in the two varieties up to the 20th day and were higher in Pb C 281 there-after. In both sowings grain weight did not increase further after 32 days from anthesis.

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SOWING ill

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9 20 30 40 44

9 20 30 40 44

20 30 40 48 9 20 30 40 44

DAYS FROM ANTHES/5

Fw. 4. A-D. Increases in dry weight of different categories of grains.

was slightly higher in NP 720 than in Pb C 281. The subsequent values were more or less similar in the two varieties. In sowing II grain weight of Pb C 281 exceeded that of NP 720 only after day 28.

Pentosan content of basal grains.-The percentage pentosan content of the basal grains increased up to the 20th day in all the three sowings, then decreased gradually to the same, or slightly higher, level at final harvest as at the beginning. On the whole, the percentage pentosan content was slightly lower in Pb C 281 (Fig. 3D). There was little effect of time of sowing on the final percentage pentosan content. The absolute pentosan content per grain increased up to 36 days in sowings II and III and up to 44 days in sowing I, and it decreased slightly there-after. The grain of Pb C 281 had, on the whole, a slightly higher pentosan content than that of NP 720. The absolute final pentosan content for the three sowings varied in the order sowing !>sowing Il>sowing III.

Lipid content of basal grains.-The lipid content of the basal grains was estimated on the 15th and 20th day from anthesis and in the final sample (Table VI). The amount as well as the percentage content of lipids was generally higher in variety Pb C 281. The amount increased and the percentage content decreased with time. According to Jennings and Morton (1963b), the increase in amount of lipid probably reflects an increase in the amount of intracellular membranes.

Table VI. Absolute (ftg) and percentage* lipid content of basal grain from ten middle spikelets

---~-- - -

-Sowing I Sowing II Sowing III

Days from - - -

-an thesis NP 720 Pb C 281 NP 720 Pb C 281 NP 720 Pb C 281

15 835 854 848 720 881 650

11· 2 11·5 6·2 4·6 5·8 3·9

20 542 637 894 1163 926 1041

3·8 4·6 3·6 4·1 3·5 3·5

44 788 1211

1·8 2·3

48 1008 1258

2·0 2·2

60 901 1343

1·7 2·3

- - - --- - -

-*Lower figures refer to percentages

III, and by day 36, in sowing I. The ear remained green up to 36 days from anthesis in sowing I, but-by day 44 it became yellow. In sowing II the ear yellowed completely by day 32 and 36 in NP 720 and Pb C 281 respectively. In sowing III the yellowing was complete by day 32.

Size of the endosperm cells.-The endosperm of NP 720 was slightly larger than that of Pb C 281 along the plane TV, but it was more or less of equal size in the two varieties along the plane TH. In the plane LH the endosperm of Pb C 281 was substantially longer than that of NP 720 (Table VII). Th~ num-ber of cells was slightly more in NP 720 than in Pb C 281 along the planes TV and LH, whereas along the plane TH the cell number was similar in both (Table VIII). The results indicate

Table VII. Dimensions of the endoJjmm of basal grain in mm - - -

-Plane of measurement

TH (width)

TV (girth)

LH (length)

Days from anthesis

20

15 15

-~p 720

1·64

1· 71

5·52

Sowing III

Pb C 281

1·72

1·56

6·27

Table VIII. Number and size of the endosperm cells of basal grain

Plane of measurement

TH (width)

TV (girth)

LH (length)

Days from anthcsis

20 15 15

Sowing III

KP 720 Pb C 281

Cell No. Cell size(!-') Cell No. Cell size(!-')

18·4 89·1 19·1 90·1

27·3 62·6 25·4 61·4

98 ·:i 56·7 95·2 65·9

-that (i) Pb C 281 has a larger volume of endosperm than NP 720; (ii) the endosperm cell number is perhaps slightly less in Pb C 281 than in NP 720; and that (iii) the endosperm cell of Pb C 281 has a larger volume than that of NP 720.

percentage DNA content of the two varieties (Table IX). It decreased steadily from the 5th to 11th day from an thesis, remained constant or increased slightly during the next three days and then declined sharply between 14 to 20 days.

Table IX. DNA content of basal grains*

Days from an thesis

5 8 II 14

20

DNA content/grain (!'-g)

NP 720

12·8

14·3 16·0 24·4 24·4

Pb C 281

12·2 14·7

18·4

26·4 25·4

*Each value is an average of three replicates

DISCUSSION

DNA content as per cent of dry weight

NP 720

0·312 0·265 0·224 0·235 0·125

Pb C 281

0·305 0·260 0·230 0·234 0·130

It was seen that the two varieties had similar grain weight per ear up to 28/36 days from anthesis and more dry matter was added to the grains ofPb C 281 between 28 to 32 days in sowings II and Ill and between 36 to 44 days in sowing I (Table X). The basal grains of Pb C 281 had higher weight whereas the upper grains (of the ten middle spikelets) and the other grains (of the remaining spikelets) had lower weight than those of NP 720 (up to 28 days). It is evident that the photosynthate was diverted in relatively greater proportion to the middle spikelets than to the remaining spikelets, and to the basal than to the upper grains of a spikelet. This differential partition of assimilate did not depend entirely upon differences in the number of grains. The observations of Bonnett (1936) and Abolina (1959) are relevant in this connexion. According to Bonnett ( 1936) the heaviest grain weight is in the middle spikelet and the weight decreases towards the base and apex of the spike; this sequence apparently follows the order of differentiation within the spike, which (Jegins first in the middle region and in succession in other spikelets basipetally and acropetally. Abolina ( 1959) did not succeed in increasing the size of the grain in the upper and lower spikelets to the level of the grains in the central spikelets by removing the latter.

•

..

•

Table X. Trends in increase in grain weight in gm - ---·-Sowing I Sowing II Sowing III ---Days from anthesis 36 28 28

0 ~

NP 720 Pb C 281 NP 720 Pb C 281 NP 720 Pb C 281

~ z

Category

of

grain

t:! t:rl

Basal 0·845(20) 0·900(28) 0·830(20) 0·902(20) 0·834(20) 0 ·952 (20)

<: t:rl

Upper

-0·606(19) 0·558(15) 0·740(22) 0·715(18) t"' Rest -0·940(27) 0·860(23) 1·036(31) 0·973(25)

0 't)

Increase in grain wtfear 2·416(68) 2·640(63) a:: 2. 331 (57) 2·376(66) 2·320(58) 2·610(73)

t:rl z

After

day

36

28

28

>-3

z

Category of grain ~ Basal 0·215(20) 0·278(20) 0·115(20) 0·168(20) 0·045(20) 0 ·064(20) ::t: Upper -0 ·127 (18) 0·190(16) -0·051(20) 0·099(19)

t:rl

>

20th day in sowing I. Its volume was also larger in Pb C 281 on the 15th day from anthesis in the sowings II and III. During the first fortnight from anthesis, production of photosynthate is more than what is utilized in grain development, as indicated by increase in sugar content of the stem ( cf. As ana and Saini, 1962). It is, therefore, surmised that the grains of Pb C 281 had a greater growth potentiality, which was expressed sooner in the basal grains and which was not entirely determined by the ratc- of photosynthetic activity.

The DNA content per grain increased from the 5th to 14th day from anthesis after which it remained constant for another six days (Table IX). Jennings and Morton (1963b) suggested that the constancy of the DNA content of the wheat grain after 19 days from anthesis indicated cessation of cell division. We conclude that in our varieties cell division in the endosperm ceased within about a fortnight from anthesis. The pentosan content increased four-fold betwt'cn 9 to 20 days and the rate of increase slowed down thereafter; as percentage of dry weight it also increased up to the 20th day and thereafter decreased slightly. Jennings and Morton (1963a) have suggested that the increase in pentosans is very probably due to increased synthesis of new cell walls associated with thc- enlargement of endosperm cell. Although the size of the endosperm cells was relatively larger, the pentosan content as percentage of dry weight was lower in Pb C 281 than in NP 720, whereas the amount per grain was more or less similar in the two varieties. The lipid content on the other hand was slightly higher in Pb C 281. The rates of synthesis of these two constituents thus differ in these varieties.

in the endosperm of our two varieties differed in any way. It is, however, possible that the larger volume of the endosperm cell of Pb C 281, attained at the end of the first phase of growth, provided more space (or sites) for the formation of starch granules. The weight of grains per ear, however, did not increase more in Pb C 281 during the first 28 days, probably because the supply of photosynthate was not larger than in NP 720. The question naturally arises as to why the other grains of Pb C 281 did not draw upon the stem sugars before 28 days if they had a higher growth potentiality. Did their endosperm cells or their starch granules develop more slowly? We have no information on this point at present.

We may now consider how the grains of Pb C 281 increased in weight at a greater rate than those of NP 720 after 28/36 days. The data on the yellowing of leaves, stem, and ear indicated that the ear alone was partially functional photosynthetically during this stage, and its photosynthesis alone could have contributed to grain development. It must be assumed that the photosynthetic activity of the ear was larger and/or its respiratory activity was lower in Pb C 281 than in NP 720. Carr and Wardlaw (1965) found that on the 15th day from anthesis, photosynthesis was more than respiration in the ear of Dural, while the reverse was the case in the other two cultivars, Sabre and Bencubbin. Thorne (1965) and Stoy (1965), however, could not correlate the differences between grain yields of their wheat varieties with differences in photosynthetic activity of the ear. Asana and Joseph ( 1964) found that Pb C 281 produced higher grain yield than NP 720 even when the whole shoot was kept in the dark during the last week of maturity. The question whether photo-synthesis of the ear of Pb C 281 made greater contribution to the grain weight, during the late stage of maturity, remains therefore open. It is noteworthy that the percentage water content of the basal grains of Pb C 281 was consistently higher than that of NP 720 for some time after 28 days in sowings II and III and after 36 days in sowing I (Fig. 3A). If this were any indication of the moisture status of the ear, it might well be that the uptake of water somehow diminished earlier in NP 720 and thus slowed down the synthetic activity in its grains.

mmtmum temperatures during the period which varied as 28· 7/9·8, 26·9/8·5, and 23·2/6·2° C. Temperature had a more pronounced effect upon the development of the upper grains of the middle spikelets and on those of the other spikelets. After 28 days the increase in the weight of grains varied with sowing as Ill<II<I, the corresponding mean temperatures during the period being 30·2(13 ·4, 32 · 7jl4. 5, and 29 ·6/10 ·8" C. The final grain weight per ear varied with sowing as I> II> III and the corresponding mean tt'mperatures during the entire period were 26·5/8·9, 29·2/11·0, and 30·1(11·6" C.

The maximum length of the basal grain was attained by day 20 in sowings II and III and by day 28 in sowing I and varied with sowing as I>II> Ill. The corresponding volumes and fresh weights also varied in the same order. As the mean temperatures during 20(28 days corresponding to the above order of sowings were 22 · 2/6 · l, 25 · 5(7 · 7, and 27 · 3(8 · 1 o C, it appears

that a smaller volume developed with increase in temperature. Since the endosperm volumes, at the end of the initial phase of growth, varied with sowing as I>ll>lll, their capacity for synthesis and/or storage of starch presumably varied likewise. It may be noted that after 28 days increase in dry matter of the grain as well as its percentage water content varied with sowing as I>ll>Ill. The relation of volumes on the 20th day in sowings II and III and on the 28th day in sowing I to increase in weight of grains/ear and of basal grains after 28(36 days, shown in Fig. 5, is suggestive in this connexion.

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Fro. 5. Relation between grain volume at the maximal grain ~ength. stage. and

increase in grain weight after 28/36 day~ from anthesis. A, increase m gram we1ght/

In conclusion, one may visualize three ways in which varietal difference in grain yield may be brought about. The first obvious possibility is high photosynthetic activity of leaves, peduncle, and/or ear as was suggested by Watson, Thorne, and French (1963). Experimental proof for this inference is still lacking (cf. Thorne, 1965). The other situation may involve difference in the growth potentiality of grains and of water content, besides that in photosynthetic activity, as illustrated by varieties, Pb C 281 and NP 720. A third possibility may be slower sene-scence and therefore, longer continuation of photosynthetic activity in a high yielding variety ( cf. Stoy, 1965).

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