A study of the growth and development of yarrow (Achillea millefolium L.)

Bourdôt, Graeme W.
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The response of yarrow (Achillea millefolium L.) seedlings to reduced light, interference from barley (Hordeum vulgare) and some aspects of regeneration from rhizomes were the subject of investigations from 1976 until 1980. Seedlings grown under four intensities of photosynthetically active radiation (100, 46.8, 23.7 and 6.4% of full summer daylight) were harvested on six occasions and the changes with time in the logarithms of leaf area, leaf, stem, root and total dry weights per plant were described by polynomial regression equations. Relative growth (RGR), net assimilation rate (NAR), leaf area ratio (LAR), specific leaf area (SLA) and leaf weight ratio (LWR) were derived directly from the growth curves. SLA and LWR increased with increased shading causing LAR to rise, while NAR declined. Response curves of RGR on light intensity, derived from linear regressions of LAR and NAR on the logarithm of relative light intensity predicted maximum RGR to occur at light intensities which decreased with time. This was a consequence of ontogenetic changes in LAR, and changes in NAR apparently related to self shading. Linear regressions of LAR and NAR at a constant total plant dry weight of 1.62 g showed that the increase in LAR almost completely compensated for the reduction in NAR down to approximately 40% full daylight, and maximum RGR was predicted to occur at 59% full daylight. The light compensation point was estimated to be 3.6% full daylight. Yarrow populations established from 25 and 50 10 cm rhizome fragments m⁻² were grown alone and with barley at 194 or 359 plants m⁻². The barley populations were also grown alone. Growth analysis employing the regression technique showed the RGR of yarrow was reduced by barley from before jointing (Feekes Scale, Stage 6) as a consequence of reduced NAR. The NAR of yarrow was significantly reduced in the continued presence of barely, which by the time of the final barely harvest resulted in 91 and 94% reduction in the accumulated yarrow dry matter at 194 and 359 barely plants m⁻² respectively. The proportion of total dry matter allocated to seed and rhizome was also reduced by barley but the barley was unaffected by the yarrow. During the autumn and early winter, after removal of the barley, the suppressed yarrow had a higher RGR than the unsuppressed population, owing to higher LAR and NAR. Rhizome growth was vigorous during both autumn and winter in all yarrow populations, but the RGR of rhizome dry matter was higher in the suppressed yarrow during the autumn. This resulted in a progressive reduction in the difference in rhizome dry matter between suppressed and unsuppressed populations. Several aspects of the development and regenerative potential of rhizomes were investigated. In the first experiment, plants were established from seed and rhizome fragments and harvested on several occasions. Plants from both propagules formed rhizomes on which approximately 97% of auxiliary buds remained dormant, as long as the plants were undisturbed. Buds on rhizomes attached to the parent plant formed rhizome branches when the apex was damaged, had emerged from the soil, or in situations where internodes were congested. In the second experiment, rhizome fragments of 4, 8 and 16 cm in length were planted in soil at depths of 0, 2.5, 5.0, 10.0, 20.0 and 30.0 cm. All fragments on the soil surface died without forming shoots owing to desiccation whilst 100% mortality at 20 and 30 cm was probably the result of flooding. Within the 2.5 to 10.0 cm range, an increasing percentage of fragments survived (produced an aerial shoot(s)) as burial depth was reduced and fragment length increased. Within this depth range, the percentage of buds which had become active on undecayed fragments declined with increased length and burial depth. In the third experiment, single-node rhizome pieces were excised from rhizomes retrieved from field populations over a one year period, and incubated at 25°C for 10 days in darkness. More than 90% of buds formed vertical shoots throughout the year, indicating there was no period of innate dormancy in isolated buds. The effect of time of planting on the pattern of early regenerative development was assessed in the fourth experiment, in which 10 cm rhizome fragments were planted at 5 cm depth in soil on two occasions (in November and April). The developmental pattern was the same regardless of month of planting and new rhizomes were initiated at nodes on the vertical subterranean shoots when 5 to 6 aerial leaves had developed. The planted rhizome fragments declined in dry weight and a minimum weight occurred at about the time when rhizome initiation began.
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