Stem Elongation Phase Research Articles

Effects of two photoperiod treatments on leaf photosynthesis throughout ontogeny in 'Fergus' barley

Leaf photosynthesis was measured throughout ontogeny for barley plants (Hordeum distichum L. emend. Lam. cv. Fergus) grown under 12- and 24-h photoperiods. Plants were grown in silica and irrigated with one-half strength of a modified Hoagland solution at 700 μE m−2 s−1. The 12-h photoperiod was extended to 24 h with low-intensity incandescent lamps (10 μE m−2 s−1). The ontogenetic pattern of change in the leaf carbon dioxide exchange rate (CER) differed between photoperiodic treatments. Under the 12-h photoperiod CER increased from leaf 1 to leaf 5 and declined to a minimum for leaf 9; whereas CER under the 24-h photoperiod continued to increase with ascending leaf position. Both stomatal and residual, which was six times larger, resistance components were responsible for the changes in CER. The pattern of change in CER with individual leaf age differed between stages of plant development but was similar at a given stage of development between photoperiodic treatments. During the vegetative phase, CER decreased slowly for 5 days from full expansion and rapidly thereafter, while during the stem elongation phase CER decreased rapidly soon after full expansion. During the grain development phase in the 24-h treatment, two plateaus were observed in CER of the flag leaf. It is clear that photoperiodic treatments affect leaf photosynthesis not only directly but also indirectly through changes in the developmental pattern.

Gibberellins and the photoperiodic control of stem elongation in the long-day plant Agrostemma githago L.

Agrostemma githago is a long-day rosette plant in which transfer from short days (SD) to long days (LD) results in rapid stem elongation, following a lag phase of 7-8 d. Application of gibberellin A20 (GA20) stimulated stem elongation in plants under SD, while 2-isopropyl-4-dimethylamino-5-methylphenyl-1-piperidine-carboxylate methyl chloride (AMO-1618, an inhibitor of GA biosynthesis) inhibited stem elongation in plants exposed to LD. This inhibition of stem elongation by AMO-1618 was overcome by simultaneous application of GA20, indicating that GAs play a role in the photoperiodic control of stem elongation in this species. Endogenous GA-like substances were analyzed using reverse-phase high-performance liquid chromatography and the d-5 corn (Zea mays L.) assay. Three zones with GA-like activity were detected and designated, in order of decreasing polarity, as A, B, and C. A transient, 10-fold increase in the activity of zone B occurred after 8-10 LD, coincident with the transition from lag phase to the phase of rapid stem elongation. After 16 LD the activity in this zone had returned to a level similar to that under SD, even though the plants were elongating rapidly by this time. However, when AMO-1618 was applied to plants after 11 LD, there was a rapid reduction in the rate of stem elongation, indicating that continued GA biosynthesis was necessary following the transient increase in activity of zone B, if stem elongation was to continue under LD. It was concluded that control of stem elongation in A. githago involves more than a simple qualitative or quantitative change in the levels of endogenous GAs, and that photoperiodic induction alters both the sensitivity to GAs and the rate of turnover of endogenous GAs.

The relationship between pre-anthesis development and seed yield of spring rape (Brassica napus L.)

Experiments were carried out to determine the influence of flowering time and the pattern of pre-anthesis development on the seed yield of spring rape. The responses of six cultivars to variation in sowing date at a site 100 km east of Perth indicated that the duration of the vegetative phase and the time of the season at which seed development commenced were both important determinants of seed yield. Seed yields decreased with a reduction in the duration of vegetative development and an extension in the time between 1 October and first flowering. These data suggested that an optimal flowering time for this and similar environments would, assuming a mid-June sowing, be one that allowed for at least 100 days of pre-anthesis development and ensured that seed development commenced no later than 1 October. In a controlled environment experiment, different patterns of pre-anthesis development induced in the cultivar Target by exposure to different photoperiodic regimes were associated with substantial variation in seed yield. This variation in seed yield was primarily associated with differences in the duration of the stem elongation phase: an increase in duration resulted in the production of greater numbers of flowers (and pods) per plant and ultimately greater numbers of seeds per plant. As seed size was largely unaffected by variation in photoperiod during pre-anthesis development, extension of the stem elongation phase was accompanied by significant increases in seed yield. Substantial genotypic variation in the duration of the stem elongation phase was observed among a group of spring rape cultivars and their F1 hybrids grown in a controlled environment comparable with that prevailing in the field during this phase of development. Longer periods of stem elongation were accompanied by increased pod production and higher seed yields. Evidence from this and earlier studies indicated that there was sufficient genetic variation in pre-anthesis development to facilitate yield improvement through selection based on developmental characters. The utilization of controlled environment facilities in conducting such a selection program is discussed.

GIBBERELLIN RELATIONSHIPS IN THE ‘ALASKA’ PEA (PISUM SATIVUM)

'Alaska’ peas (Pisum sativum L.) grown under a 16‐hr photoperiod at 20 ± 1 C and an 8‐hr dark period at 16 ± 1 C in their ontogeny exhibit two periods of sensitivity to applied gibberellin (GA), namely, prior to and subsequent to but not during the linear phase of stem elongation. This paper describes experiments conducted primarily with seedlings. Growth‐saturating doses of GA, applied to dry seeds before planting (10−3 m) and to the shoot tips of 3‐day‐old seedlings (10 μg), evoked growth rates equal to the growth rate of etiolated seedlings. Sensitivity of seedlings to applied GA decreased with age through the first 2 to 3 weeks of development; by the time seedlings were about 14 days of age and had four elongating internodes they no longer responded to applied GA. As endogenous growth rate diminished late in ontogeny, the plants again became sensitive to applied GA. Growth response was used as a criterion for determining apparent translocation of applied GA. ‘Alaska’ pea seedlings appeared to transport GA, both acropetally from the cotyledons and basipetally from the shoot tip, to all internodes with remaining extension potential. Excision of both cotyledons at any time during the first 9 days of development caused a significant reduction of growth rate, and applied GA did not restore normal growth rate. No evidence was found that the cotyledons supply endogenous GA to the shoot axis in normal seedling development. It is suggested that the normal growth rate of light‐grown ‘Alaska’ peas is correlated with the rate of synthesis of GA and that GA is rate‐limiting for stem elongation during early seedling development and during the period of decreasing growth rate and onset of apex senescence.