The Physiology of Growth in the Wheat Plant I. Seedling Growth and the Pattern of Growth at the Shoot Apex

Abstract

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.

Physiology of Growth in the Wheat Plant IV. Effects of Day Length And Light Energy Level

“…9). Williams (1960) found that, apart from the seed-reserve effects already mentioned, the early growth of each leaf primordium was exponential, but that the exponent decreased with leaf number in a rather discontinuous manner. Figure 12 shows that the duration of the exponential phase varies from about a week in leaves 3 and 4, to as much as 5 weeks in leaves 12 and 13 of the short-day treatments.…”

“…They exhibit high initial rates of increase while the seedling is dependent on seed -reserves, but there is a rapid transition to lower steady rates based on photosynthesis alone (cf. Williams 1960). The rates decline further after about day 30, but the experiments did not continue long enough to judge ifthey would settle down to another steady rate such as that suggested for field-grown wheat by Williams (1964, fig.…”

“…These median axes were very uniform and, in general, only one was eventually used for the estimation of volumes. The volume integration procedures are fully described by Williams (1960), and his regression equation (loc. cit.…”

“…Williams (1960) described the early growth of the primary shoot and, in particular, established the pattern of growth of successive leaf primordia on its apex. Williams and Rijven (1965) extended this description to the changes in DNA, RNA, proteins, and cell wall materials of the fourth leaf from an early stage to full maturity.…”

Distribution of Assimilate During Stem Elongation in Wheat

“…There is some rather circumstantial evidence that supports this contention; that is, the differentiation of vascular links that connect an expanding leaf at the apex with the rest of the plant coincides with the relative growth rate of that leaf reaching a maximum. This may be interpreted to mean that the establishment of a more efficient supply of substrate partially releases the limitations on the growth of the primordium (Williams 1960;Patrick 1972b). Similar observations have been made in tobacco, whereby the halt in the decline in the relative growth rate of an expanding leaf is associated with the appearance of sieve tubes in the lamina (Hannam 1968).…”