ENVIRONMENT WITHIN THE CROP

Crop responses will largely be dependent on crop canopy environment. Halligan ( 1 986) showed that the maximum daily temperature measured within the canopy of a pea c rop can be 3-5°C higher than that measured above the crop canopy. Similarly, Perry and Harrison ( 1 973) demonstrated that peas within the pods experienced temperatures 4-5°C higher than the above c a n o p y . The d e v e l o p i n g pea see d s there fo r e , experience tem peratures which are se veral degrees hi gher than the ambient air temperature.

The effects of seed position on garden pea seed quality has not been well studied. Studies with soybeans have shown that seed quality was improved when seeds were obtained from the top of the plants. Although reasons for enhanced quality of top seeds are not certain Adam et al. ( 1989) suggested three possible reasons: ( I ) The soybean plant enters senescence at the same time that it begins to mobilise significant reserves o� dry matter, especially nitrogen (Sinclair and De Wit, 1976). During this period, competition for

these nu trients from the roots and nodules is greatly reduced as they become physiologically inactive. Thus, the newly developing seed s become the major sink o f the assimilates and produced greater seed weight (Adam et al., 1 989). (2) Com pared to top leaves the bottom leaves of a soybean canopy have reduced photosynthesis levels due to shading which

results in a reduced supply of photosynthate to the bottom seeds (Schow et

al., 1 978) potentially resulting in smaller seeds and lower quality (Adam et

al., 1 989). (3) Seeds in pods originating from early flowers mature longer in the field than seeds in pods originating from late flowers (Dunphy et al. , 1 979) and are potentially exposed to more environmental stress factors such as hi gher temperature and humidity which increase seed deterioration (Adam et al., 1989).

2. 1 0 . S EED DETER I O RA T I O N DU R I N G S E ED M A TU RA TION A ND HA RVEST

Following maturation the seeds (in "dry fruits") continue to dry down until they reach harvest maturity, i.e. the moisture content at which they can be effectively threshed with mechanical harvesters (TeKrony et al., 1 9 80; G ane, 1 9 8 5 ) .

Climatic conditions during this post maturation pre-harvest period have a great influence on the quality of the seed harvested (Delouche, 1 980; TeKrony et al., 1 980). Adverse weather conditions during the pre-harvest period cause moderate to severe seed quality problems in many areas every year which is commonly termed weathering (Delouche, 1 974).

Weathering is a major problem in seed production. The severity of weathering and the limitations imposed on seed quality by weathering generally increase from cool to warm areas. The worst situation is in the humid subtropics and tropics. The quality of seed produced is generally low and deterioration continues at a rapid rate during storage because of high temperatures and humidities (Delouche, 1 980). These situations and problems might also occur in garden peas, but no thorough study has yet been done.

In soybeans, seeds are usually produced in areas where the crop is grown. Delayed harvesting of soybean seed caused by inclement weather results in a reduction in viability and an increase in mechanical damage during harvest (Green et al., 1 966). However, in a study on the effect of seed position, planting and harvesting dates on soybean seed quality, Adam et al., (1989) found that early planting (May 1 ) and early harvest (September 15) also decreased seed quality. Seed deterioration can occur following either early or late h arvest if the environment is not favourable for the production of quality seed.

Variation in the e nvironment of seeds at about the tim e of completion of maturation and harvesting can result in differences in their potential to perform, either in the field or in storage (Bewley and Black, 1986). Seeds exposed to pre­ harvest weathering, particularly in warm and humid conditions exhibit reduced germination, reduced field emergence, increase leachate conductivity, reduced respiratory oxygen uptake and deterioration of membranes (Woodstock et al.,

1985). Weathered cottonseeds release more potassium, manganese, magnesium

and calcium during imbibition than unweathered cottonseeds and the release of potassium and calcium was significantly correlated with measurements of seed quality (Woodstock et al., 1 985). Under the electron microscope, weathering deterioration of mem branes is manife sted by pale cytoplasm and lack of structurally distinguishable ribosomes.

Mechanical damage inflicted during harvesting can severely reduce the viability of some seeds, especially large seeded legumes. Injured or deeply bruised areas may serve as centres for infection and result in accelerated deterioration. Injuries close to vital parts of the embryonic axis or near the point of attachment of cotyledons to the axis usually bring about the most rapid losses of viability. It is therefore of the utmost importance to harvest and thresh seeds very carefully, making sure the drum speed is slow and seeds are harvested at the right time (Matthews, 1 973; Gane et al., 1 984; Gane, 1 985). In the U.K., one method of minimising damage is to harvest seed at a moisture content of around 25%, at which higher vigour seeds and high quality seeds are produced (Gane et al.,

1984). High temperatures during drying or drying too quickly or excessively, can

2.1 1. CONCLUSION

Seed development in garden peas is well understood, and this understanding of the growth and development of the crop can serve as a guide in the production of high quality seed. For example correct harvesting at the right time can assist in m inimising the problem of poor quality and low vigour seeds. However, garden peas are grown under contrasting environments and a result obtained in one area might not be appropriate in other areas. Further, most seeds used in development studies are grown under controlled condition which at times does not reflect crops grown under field conditions.

Garden pea seed production is affected by factors such as environmental influences associated with crop growth, soil conditions, soil fertility, plant population, row spacing, and time of sowing. These factors have been thoroughly studied and research results have served as the basis for recommendations for crop management for commercial scale seed production. However, differences in seed quality characters resulting from various crop management strategies exist. Production practice effects on seed quality have not generally been taken into consi deratio n in m ost of the previous studies, and are therefore a m ajor component of this study.

There are few studies on the effects of climatic factors associated with the growth of the mother plant in garden peas. Temperature within the canopy is higher than above the canopy and tends to increase the incidence of hollow heart. Moisture is very important in the growth of peas especially during the onset of flowering, because the roots have ceased to grow, making the plant more vulnerable to water shortage. However, excessive water has deleterious effects on pea seeds during harvest. The environment within the canopy can influence the qual i ty of harvested seeds. Seeds exposed to adverse weather conditions during m aturation may have low quality because of seed deterioration resulting from weathering. Time and method of harvest influence production of high quality seeds. Seeds can be damaged during harvesting and processing, which lowers their qUality. Such damage can be obvious (i.e. cracked testa) or more subtle (Le. loss of membrane integrity).

Environmental effects associated with the mother plant in garden peas have not been well studied. The influence of the environment within the canopy under field conditions was therefore investigated in this study, and the influence of time and method of harvest on seed quality in garden peas was also explored.

CHAPTER THREE MATERIALS AND METHODS

3.1 . EXPERIMENTAL FIELD

The experiments were established on the campus plots at the rear of the Seed Technology Centre, Massey University, Palmerston North, New Zealand (400S, 1 75°E). The 1 988- 1989 field experiments were sited on plot No. 8 and the 1 989- 1 990 experiments on plot No. 1 1 . The soil classification is the same as described in Section One.