Developmental analysis of populations in the cereals and herbage grasses I. Methods and techniques
Authors: Cooper J. P. 
Published: March 2009

1. This work presents an analysis of the ecological differences between populations in terms of one important developmental system, the timing of inflorescence development on the shoot apex, and its corollary, the partition of energy between seed production and continued vegetative growth.

The object of the first paper is to establish rapid and accurate greenhouse methods for testing response to low temperature and photoperiod, the most important environmental factors influencing inflorescence development.

2. The timing of inflorescence development was studied by the dissection of the shoot apex in several lines of cereals under various temperatures and photoperiods. The rate of leaf appearance is linear on any shoot and is unaffected by the initiation of spikelet buds on that shoot. The elongation of the shoot apex is exponential, being gradual during vegetative growth but increasing at spikelet initiation. The elongation of internodes begins only at the late spikelet bud stage, and progresses stepwise, one internode beginning to elongate as the preceding one ceases.

The general pattern of inflorescence development is similar in all material, but differences occur between lines in the time at which the spikelets are initiated and in the rate of subsequent elongation. These pilot results make it possible to choose the most suitable measures of reproductive development and the most effective environmental treatments to distinguish between populations. The leaf number before heading on the main shoot is used as a quantitative measure of inflorescence development. It records the physiologic age at which spikelet initiation occurs, and is only slightly affected by temperature fluctuations between 10 and 20° C.

3. In the tests for response to vernalization, the germinating seeds are exposed to low temperatures (0–5° C.) for varying lengths of time, and then planted in long day or continuous light. The temperature immediately after planting is kept at 10–15° C. (50–60° F.) to avoid either further vernalization or devernalization. The photoperiod is optimum, and the rapidity of heading is a measure of response to low temperature.

4. The usual method of testing for response to photoperiod under a range of constant photoperiods has the disadvantage that in short days the plants may head slowly or not at all, thus increasing the demand for time and labour. A more rapid turnover can be obtained by exposing the seedlings to differential photoperiods from germination onwards and transferring at a series of dates to long day or continuous light. The leaf number at transfer and the leaf number before heading are recorded, and from these the inductive effects of the initial photoperiods can be calculated. All seedlings must be fully vernalized, i.e. competent to respond to photoperiod, before the tests begin.

The following applications of the methods are suggested:

(i) The agronomic classification of populations and the prediction of their behaviour under specified local climate and farming practice.

(ii) The rapid selection and progeny testing of parental material in plant breeding work.

(iii) The assessment of the potential genetic variation within populations and the study of the adaptive changes occurring under selection.