MATERIALS AND METHODS :

7 .2.1 Experimental site and field procedure:

The experiment was c arried out at the P asture and Crop Research Unit, Massey University, u sing a two-year-old "Grasslands Pitau" white clover crop growing on Tokomaru silt loam. Table 6.1 shows the prevailing temperature, precipitation, and sunshine hours. Before the start of the expe1iment, the crop had been grazed by sheep whenever the herbage reached a height of 1 0-20 cm. The last preparatory grazing was completed on 26 August 1989. The experimental area received a spring application of 250kglhectare of 30% potassic superphosphate on 9 October 1988.

The trial was laid out in a randomized complete block design and all treatments replicated 6 times. The area per replicate was 2m2 (Fig.7 . 1 ). All the plots were defoli ated on 3 0 O ctober 1 9 8 9 using the herbicide Paraquat at the rate of 3 litres/hectare. Four experimental treatments were given:

A O p en canopy: The plots were manually defoliated for a second time on 1 5

December to leave residual herbage of about 3-4 cm high.

B O p en canopy with p re-ferti l i zation shade (Pre. f.S h ade) : The plots were

manually defoliated for a second time as in treatment A. Overcast weather conditions were simulated by artificially shading plants from 1 6 December 1 989 to 15 January 1990 with neutral shade cloth as shown in Plate 6. 1 These plants received only 45 % of incoming radiation. After removing the shade, about 1 6 flower heads in which the corollas of the oldest florets were just showing white were tagged per replicate.

CP3

DPl

FIGURE 7. 1 : Trial layout. Field Experiment 1989·90 1 • 6 = Replicates 1 to 6 (Blocks).

C = Open canopy.

P = Pre-fertilization shade.

GP = Open-Post fertilization shade.

D = Dense canopy.

DP = Dense Post-fertilization shade.

Path.

c 4 DP4 04

C Dense canopy: The plots received no second defoliation treatment.

D Open canopy post-fertilization shade (Post.f.Shade) : The plots were manually

defoliated for a second time as in treatment A. On 1 5 January 1 990 about 1 6 randomly selected flower heads per replicate were tagged just before anthesis. After these had been pollinated (as j udged by the degree of reflexion of their florets ) , they were shaded with shade cloth as shown in Plate 6. 1 until 1 5 February so that they received only 45% of incoming radiation.

E Dense canopy post-fertilization shade: As in treatment C, the plots received no

second defoliation treatment. On 15 January, about 1 6 randomly selected flower heads p er replicate were tagged just before anthesis. After these had been pollinated, they were shaded with shade cloth until 1 5 February so that they received only 45% of incoming radiation.

7.2.2 Measurements

On 1 5 January, about 1 6 flower heads per replicate were tagged in all treatments. Ten randomly selected flower heads were harvested from open canopy plots to count the ovule number per floret. On 1 5 February, the tagged flower heads were harvested and the following measurements were made:

1 Number of florets per inflorescence : 9 6 inflorescences were sampled per

treatment.

2. Number of seeds per pod: 1 200 pods per treatment were sampled both from the apical (600) and the basal (600) florets of 96 inflorescences. To count the number of seeds per pod, the pods were placed on Polaroid photographic paper and X-rayed using a Faxitron Hewlett-Packard X-ray machine (25 KVA, 1 min exposure time).

3 . Seed weight: Using the bulked samples of seed for each treatment, four weight counts of 100 seeds each were made. The 1000-seed weight was standardized to 1 0 % moisture content.

Treatments were a) open canopy : pl ots were defoliated twice b) open canopy with pre-fertilization shade : plots were defoliated twice and plants artificially shaded before fertilization (45% of incoming radiation) ; c) dense canopy : plots were defoliated once; d) open canopy wi th post fertilization shade : plots were defoliated t wi ce and plants artificially shaded after fertilization (45% of incoming radiation) ; e) dense canopy wi th post fertili zat i on shade : plots were defoliated once and plants artificially shaded after fertilization . Val ues followed by the same letter between treatments (in same col umn) are not significantly different at 5% level . += 1 0 % moist ure

conten t . Treatment Open canopy Open/pre . f . Shade Dense canopy Open/pos t . f . Shade Dense/pos t . f . Shade Number of fully developed fl orets ( ± SE) 5 7 . 8 ± 1 . 1 7a 4 6 . 0 ± 2 . 65b 4 4 . 5 ± 2 . 1 9b 52 . 8 ± 2 . 3 7a 4 8 . 8 ± 1 . 23b

Seed Number per Floret

api cal basal mean

2 . 67a 3 . 74a 3 . 21 ±

2 . 3 6a 3 . 2 0b 2 . 78 ±

2 . 1 0b 3 . 1 5b 2 . 63 ±

2 . 2 7b 3 . 05b 2 . 66 ±

2 . 29a 3 . 39b 2 . 85 .!:

Seed number 1 0 0 0 Seed Weigh t +

per head (mg) ( ±SE) 0 . 12a 1 8 6 ± 6 . 1 6a 0 . 635 ± 0 . 026a 0 . 1 4b 128 ± 8 . 72b 0 . 59 7 ± 0 . 0 0 8a 0 . 1 1b 1 1 6 ± 3 . 1 1b 0 . 599 ± 0 . 030a 0 . 09b 1 4 0 ± 1 1 . 2 7b 0 . 5 7 7 ± 0 . 0 1 9a 0 . 1 4b 1 4 0 ± 5 . 43b 0 . 608 ± 0 . 012a I-' I-' w

TABLE 7 . 2 : Percen tage reduction in vari ous seed yi eld component s .

Val ues are cal cu l a ted for each t reatment wi th respect t o the con trol t reatmen t (open can opy) . See capti on of Tabl e 7 . 1 for further deta i l s .

Treatmen t Open/pre . f . Shade Dense canopy Open/pos t:. . f . Shade Dense/p ost . f . Shade Fl oret Number 2 0 . 4 23 . 0 8 . 6 1 6 . 6 Seed Number per fl oret 1 3 . 4 1 8 . 0 1 7 . 1 1 1 . 2

Seed Number Seed per head Wei gh t

per head (mg) 31 . 1 35 . 3 3 7 . 3 41 . 2 24 . 8 31 . 6 24 . 5 2 7 . 3 1 1 4

7.3 RESULTS

7.3.1 Number of florets per head

The results obtained show that the flower heads developed in an u nshaded open canopy had the highest number of florets per head (Table 7. 1 ). In dense canopies and in plants which were artificially shaded before pollination, there was a reduction in floret number by 23%. The post-fertilization shading of dense canopy plants also led to a statistically significant ( 1 6%) reduction in the number of fully developed florets. Flower heads which developed in an open canopy and which then were shaded after fertilization had a slightly reduced floret number per head but the reduction was not statistically significant (Table 7 . 1).

7.3.2 Number of ovules per floret

The average ovule number per floret was 5 .54.

7.3.3 Number of seeds

The effect of treatments on the number of seeds set per floret and flower head are shown in Table 7 . 1 . The number of seeds per pod was highest in the open canopy and lowest in the dense canopy. In the dense canopy the number of seeds per floret was reduced by 1 8% in comparison with the open canopy. Pre-fertilization shading of open canopy plants reduced the seed number by 1 3.4%, and post-fertilization shading reduced it by 1 7 . 1 % . Post-fertilization shading of dense canopy plants reduced seed number by 1 1 .2% (Table 7.2). There was a tendency for the apical florets on a head to possess fewer seeds than those developing lower down (basal) (Table 7. 1 ). Although no count was made of the total number of seeds per inflorescence, this was estimated by multiplying the average seed number per floret by the average floret number per inflorescence. The results of such calculation s (Table 7 . 1 ) show that the total number of seeds per head was highest in the open canopy and most heavily reduced in the dense canopy (by 37% ). Artificially shading the open canopy plants before or after pollination also resulted in a reduction of the number of seeds per flower head by 3 1 and 2 5 % respectively. The post-fertilization shadin g o f dense canopy plants also reduced the seed number by 25% (Table 7.2).

1 1 6

7.3.4 Seed weight

Table 7. 1 shows that treatments given in thi s experiment had no significant effect on the 1 000-seed weight.

7.4 DISCUSSION

The results obtained in the present experiment show that the flower heads developed in a dense canopy produced 37% fewer seed s per head than those formed in an open canopy (Table 7 .2). The reduction in seed number was conn·ibuted to by an increase in the number of florets aborting in a dense canopy. The flower heads developing under shade before pollination either by canopy shade or by artificially shading the plants (simulated overcast weather) produced between 20-23% fewer florets per inflorescence than those developed in an open canopy. Flower heads which developed in an open canopy and those which were shaded after fertilization had a slightly reduced (8.6%) floret number. The above results suggest that low light levels during the early stages of inflorescence development probably increased the number of florets aborting, possibly as a result of limited availability of photosynthate.

T h e redu ction i n seed set per fl oret ob served due to treatments given in this investigation (Table 7 .1) could be the result of (a) inadequate photosynthate leading to post-fertilisation competition either between ovules in a floret or between florets in a flower h ead or (b) an increase i n ov u l e steri lity as observed in the earlier field experiment. In the present experimen t, only 5 8 % ovules set seed in open canopies, 50% in pre-fertilization shaded open canopies, and 47 % in dense canopies. The close correl atio n between the p ercentage of app aren tl y normal embryo sac s and the percentage of ovules setting seeds in the earlier field and glasshouse experiments with "Grasslands H u i a " clones strongly s u g g e s t s that the reduction i n seed set i n "Grasslands Pitau" was also brought about by a n increase i n ovule sterility.

The results obtained in present and in earlier field experiment clearly show that when plants were artificially shaded before or after pollination there was a 24-3 1 % reduction in seed n umber per head. This suggests that overcast weather during early stages of inflorescence development or during seed maturation could lead to about 30% reduction in seed yield per flower head, p o s sibly due to sh ortage of available photosynthate.

The most striking observation was that post-fertilization shading of dense canopy plants produced 1 6% more seeds per head than those flower heads which developed in a dense canopy. This result was quite unexpected and is difficult to explain. One possibility is that artificially shading the plants with shade cloth changed the micro­ climatic conditions of the plants, perhaps by increasing moisture retention. Clifford

( 1 98 6) found that, in clover, the yield per inflorescence declined by 34% in later flowering of an unirrigated crop as moisture stress increased. However, in contrast, later flowering of an irrigated crop gave the highest yield per inflorescence. He found that the increase in the yield per inflorescence was contributed to by 4% increase in seed weight and by a 27% reduction in ovule abortion. The weather data (Table 6. 1 ) also clearly show that in February 1990 the average temperatures were high with low rainfall, and the plants developed in artificial shade might have retained more seeds as moisture stress was reduced. Because it was a dry and hot summer, precautions were taken to harvest the tagged flower heads before pods dehisced and shed seed.

Defoliation prior to flowering has been reported to have beneficial effects on flower production and other components of seed yield of white clover. According to Zaleski ( 1 970), flower head initiation is enhanced as a result of more light reaching the stolon level. In the writer' s opinion, enhancement of flower initiation as a result of more light reaching stolon level is unlikely because observations by Thomas ( 1987) suggest that the photoperiodic stimulus is perceived by the leaves. He also reported that there is no evidence that stolons were also perceptive. The increased number of flower heads observed by Zaleski and other workers could be the result of reduction in abortion of very young inflorescences. Results of the present investigation have showed (Chapter 5 ) that shading i ndividual flower heads led to 5 - 3 5 % abortion of developing inflorescences. This suggests that the major advantage of the practice of defoliation at the time of closing for seed production might be the enhancement of floret fertility; and that decreased seed yield in duller, wetter summers is probably, at least in part, attributable to increased ovule sterility and floret abortion in the dense canopies formed under these conditions. This must be a major reason for delaying "closure" for seed production when weather conditions favour rapid and lush growth.

SECTION B

Influenc e of low light on the growth and sink activity of young flower

INTRODUCTION

S eed yield of white clover tends to be correlated with climatic conditions, being higher in warmer, sunnier, drier summers than in cooler, duller, wetter ones, and it is often suggested that this is the result of lower activity of bees as pollinators in cooler, duller conditions (Van Bogaert, 1977; Romero, 1 985).

However factors other than bee activity might be important in duller summers. Low light intensities, for instance, are known to affect flower head development adversely. Thomas (1961 , 1987) and Zaleski (1 964) found that when plants were grown in warm short photoperiods or low light intensities many of their flower heads aborted either completely or partially. Partial abortion reduced the number of functional florets per head. The light intensities found by these authors to be most effective in causing abortion ( 1 000-3000 lux) are below the intensity of natural radiation falling on the foliage of white clover growing in the field, even in the dullest summers. But light intensities beneath the foliage canopy of a white clover seed crop are often as low as this even at midday when incoming radiation is most intense (Brougham, 1958 and Appendix 5 give data on the effect of a clover c anopy on PAR photon flux beneath it). Flower heads of white clover emerge from the stolon apex in the axil of the youngest leaf. At this stage all their florets have been initiated, the oldest being about one quarter of final size and the youngest much smaller, and they are borne on very short peduncles (Thomas, 198 1). Over the next few days the flower heads continue to grow and are gradually raised above the stolon by elongation of their peduncles. In a dense canopy this post-emergence growth takes place for several days in heavy shade before the flower heads are raised above the foliage.

Developing inflorescences of white clover contain chlorophyll and under laboratory conditions in situ photosynthesis can contribute as much as 70% of their c arbon requirement (Pasumarty, 1 987). In field conditions the contribution of developing inflorescences to their own carbon economy i s probably less because they are generally heavily shaded. In other species (eg. soybean), however, there is evidence that shading of organs reduces their ability to draw assimilates from source leaves (Heindl and Brun, 1983). It is thus possible that shading may also affect sink activity of white clover inflorescences.

The present investigation was undertaken to determine the extent to which the growth and sink activity of young flower heads and peduncles is influenced by the shaded conditions that exist within dense white clover canopies.

CHAPTER 8

GENERAL MATERIALS AND METHODS