Aspects of seed physiology which are influential in establishment problems of vegetable seeds.

Abstract

Introduction: Pre-emergence failures of pea seeds are most frequently reported in early spring, when the seeds encounter conditions of low temperature and high soil moisture in the field. These emergence failures occur even in seed lots that have germination capacities greater than the 80 percent declarable minimum. The absence of good correlations between laboratory germination tests and field emergence data, indicated that factors other than viability influenced the field emergence of pea seeds (Perry, 1970). The advent of precision sowing, and the stringent timing requirements of the food processing industry has focussed attention on the need for a better understanding of the factors that influence the field emergence of pea seeds so as to enable more accurate predictions of field emergence to be made. Seed peas are sold commercially as seed lots, a lot being a bulk of seed not exceeding 20 tons (Seeds Regulations 1961) which has usually been taken from one crop. Two attributes of a seed lot influence its emergence in the field the percentage of viable seeds in the seed lot and the vigour of the viable seeds (Heydecker, 1968). The first attribute can be and usually is determined by normal seed testing methods, but the vigour of seeds is more difficult to assess. Various methods have been used to measure vigour in seed peas (Isely, 1957; Heydecker, 1965). Assessments of the performance of seeds under stress conditions in the laboratory have been tried with some success. Clark and 2 Baldauf (1958) found that the percentage of seeds emerging from field soil in the laboratory at a low temperature correlated well with field emergence. Caldwell (1960) found a similar correlation using emergence in laboratory soil at high temperature and high moisture. More recently, Perry (1968) evaluated the vigour of seedlings in a modified germination test in which the seeds were germinated in a standardized sterile medium, and seedling growth was measured after several days. The relationship between seed exudation and infection by fungal pathogens, which is a common cause of emergence failures, has been reported on several occasions (Barton, 1957; Thomas, 1960; Flentje and Saksena, 1964). Matthews and Bradnock (1968) recognised that the relationship between exudation and emergence presented the possibility of a routine test for seed vigour, and they devised a laboratory test, based on the amounts of electrolytes seeds exuded into water. This test enabled the detection of seed lots of low planting value. A third type of vigour test for seeds, based on seed respiration rates during the early stages of imbibition, was shown by Woodstock (1966) to give a reliable indication of the field performance of seed lots of maize. The vigour tests so far mentioned can very usefully distinguish seed lots with a high potential field emergence from those with a low potential emergence (Heydecker, 1968). However, many seed lots do not lie clearly in either group since they show low emergence during adverse conditions but are capable of high field emergence under some sowing conditions (Perry, 1970). This lack of complete agreement between vigour tests and the field emergence cf seed lots emphasised the point made by Heydecker, (1968) that there are always two sides to the vigour problem, namely, the quality of the seed and the quality of the environment. The work of this thesis is concerned with the influence of soil factors on the field emergence of pea seeds and the interaction of these factors with seed quality.