No abstract

In the course of a recent address on the control of growth and reproduc tion in plants, Gregory (13) drew attention to the fact that over 90 per cent Of the nitrogen and phosphorus taken up by the developing cereal plant had been accumulated when the dry weight was only 25 per cent of the final value. This store of accumulated nutrient was the reserve on which all later growth and development depended, and its level determined the final yield. The nitrogen and phosphorus which was set free from the senescent leaves and tillers was reutilized for production of further leaves which, in turn, provided for the development of the inflorescence. Gregory also stressed the significance of the meristems with their potentially unlimited demand for nutrients, and their capacity to initiate "internal starvation" and senes cence. These observations serve very well to introduce the review which is to follow for, in examining the evidence for the redistribution of mineral ele ments during development, the reviewer has attempted to relate this to the wider problems of growth.From the outset it should be emphasized that we are primarily concerned with the demonstration and explanation of net losses of mineral elements from specific plant parts. To be relevant, therefore, papers for review had to cover two or more plant parts at several stages of growth. Relatively few of the many papers on the mineral composition of plants fulfil these require ments.It seems probable that some elements simultaneously enter and leave the same organ in the course of the normal metabolic flux, and may be regarded as being redistributed from organs which are still undergoing net intake. Such phenomena are not considered here, and it is for this reason that little reference is made to recent work with radioactive tracers. A borderline case is provided by Phillis & Mason (34) who found diurnal variations in the mineral content of the leaf of the cotton plant. The six elements studied showed increases by day and no change or decrease by night. Although the existence of dew losses complicated the picture, it does seem likely that phloem export was responsible for some losses even before maximum absolute contents had been attained. This evidence is a reminder that there can be no hard and fast line between redistribution resulting from normal metabolic activity within the organ considered and redistribution which is conditioned by senescent changes and by demands which arise externally to that organ.

Experiments were conducted in controlled environments to determine the effects of high temperatures on grain development and yield in wheat. Two Australian and three Indian cultivars of wheat were exposed, from a week after anthesis until maturity, to "day" temperatures of 25, 28, and 3l°C, and "night" temperatures of 9 and 12°C. There was a mean reduction in yield of 16%' for the 6° rise in day temperature, but the cultivars did not differ significantly in their response to these temperatures. There were no significant effects of night temperature on grain weight, but stem weight was less at 12°C. Senescence was hastened only slightly by high day temperature, and there were no differential effects between cultivars in this respect.In a subsidiary experiment one Indian and five Australian cultivars were subjected to three day-night temperature regimes (24/19°, 27/22°, and 30/25°C). Highly significant but complex interactions were established between temperature regime and cultivar. A growth analysis for the Australian cultivars Ridley and Diadem indicated that the developing grain of Ridley had a greater capacity for growth than that of Diadem from the earliest stage. This, together with the confirmation of grain size as a very stable characteristic for all the varieties, points to the developmental and synthetic activity of the grain as an important determinant of grain yield. The relevance of this study to the production of wheat in India is briefly discussed.

SummarySeedling growth of wheat in a constant environment is studied over a period of 21 days. Dry weights of leaves, leaf sheaths, stem, and roots are given for 11 occasions. The pattern of dry weight change is also presented in terms of the changing ratios of plant parts.Growth rates of leaf primordia are determined in terms of volume change based on the technique of serial reconstruction.For an ll-day shoot apex, a detailed account is given of cell-size distribution along the leaf primordia and within the apex itself. It is estimated that, prior to the onset of cell enlargement, the mean cell-generation times for the young leaf primordia range from 12 hr to 3 days.An integrated picture of the early growth of the primary shoot is attempted, mainly in terms of the concept of relative growth rate. The rates for leaves and roots are particularly high while seed reserves are available. There is a progressive change in dominance from leaf growth to stem growth. Early growth of each leaf primordium is exponential, but the exponent decreases with leaf number in a rather discontinuous manner. Following the exponential phase, the rates rise to maxima and then fall asymptotically to zero.It is suggested that intra-plant competition for energy substrates may play an important role in determining the pattern of development of the primary shoot of wheat.