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Increase Of Cold Hardiness Of Echium Pininana Through Natural Selection

International Symposium on Selection and Breeding of Woody Ornamentals, ISHS Acta Horticulturae Angers, France, 1 December 1992 Muriel Robinson 2009

Abstract

E. pininana from the Canary Islands is a highly decorative sub-shrub that grows well in mild coastal areas of Ireland . It is valued for its single spike of blue, purple or rose flowers which can grow from over 1 m to a height of 6 m between February and early June.  The plant flowers in its second or third year and, being monocarpic, dies after flowering.

E. pininana is cross pollinated and produces seeds prolifically (in excess of 200,000/plant), which germinate readily and dense carpets of Echium seedlings are commonplace. Plants developing from these seedlings survive most winters but, in some years, a proportion is killed by cold; on two occasions during the last 17 years all plants were killed by low temperatures of -6 C in January. After these severe winters new seedlings develop from the seed bank in the soil.

It seems possible that natural selection from cold stress will, over time, produce a more cold hardy strain of E. pininana,, better adapted to conditions in Ireland . This hypothesis is being tested.

Introduction

E. pininana is one of the great Borages which grows on the stony hill sides and open forests of Laurus azorica in the Canary Islands . There it is an endangered species through loss of habitat to agriculture. However, it is naturalised in some mild coastal areas of Ireland , where it is highly regarded as a striking and beautiful ornamental plant.

It is valued for many qualities. It grows rapidly and, in the vegetative state, makes a round dome of large textured leaves with good architectural qualities. In the second or third year from germination the plant will start the flowering process. Usually robust plants that have grown well and are around 1.5 m tall in the early winter will flower during the following year. This does not always occur and plants may remain vegetative for a further year. Occasionally small plants will send up a weak flowering shoot.

When E. pininana flowers normally, it produces a tall slender spire; this may, on occasions, be 7 m or more in height and is often taller than in its native habitat.  Plants of E. pininana usually produce a single massive spike of light blue, rose or purple flowers on scorpioid cymes. Although individual flowers do not last long, the plants remain in bloom from late April to October because of the large numbers of flowers that are produced successionally during the season. Being monocarpic, E. pininana, dies after flowering.

The greatest disadvantage of E. pininana, is that it is not completely hardy in Ireland and, even in mild areas, is liable to be killed in an exceptionally severe winter. It is also very difficult to grow at all away from milder coastal districts. 

E. pininana is cross pollinated and produces seeds prolifically, which germinate readily. This echium forms hybrids easily with some other species, such as E. wildprettii. (Anonymous 1992). Within stands of E. pininana, there is considerable variation among plants, both in the colour of the flowers (blue to purple) and in the maximum height of the plant when fully grown (usually 2m to over 6m). Because of genetic variation within the population of plants in Ireland , it may be possible to produce a hardier strain through natural selection.

Materials and methods

Seed of E. pininana, collected at the Logan Botanic Gardens, Wigtownshire , Scotland by Colonel David Price, Kilmokea, Co. Wexford in 1975 was sown under glass at Kinsealy Research Centre, Dublin in a peat compost.  Three pot-grown seedlings were planted in the open at Earlscliffe, Baily, Co. Dublin (Latitude 53.4 N) in September 1975.

The soil at Earlscliffe, derived from Cambrian shale and quartzite,is a clay to silty clay loam, well drained with approximately 12% coarse sand, 30% fine sand, 47% silt and 28% clay in the 0 - 100 mm layer. The natural pH of the soil is between 5 and 5.5 and the organic matter content around 4.5% in the top 75 mm.

The three echium plants overwintered successfully, flowered in 1977 and produced many self-sown seedlings in 1978. As a result of natural seeding, E. pininana is now well established throughout all parts of the garden at Earlscliffe, in other gardens in the locality and on waste ground on the Howth peninsula.

Between 1970 and 1987, meteorological data was collected at Danesfort, Baily, Co. Dublin about 400 m due north of Earlscliffe.  Lowest air minimum temperatures were also recorded at Earlscliffe, throughout the test period.  Records were also kept of years of total or substantial elimination of the Echium population as a result of low winter temperatures.

Growth increments of two 2-year-old plants, similar in size and growth habit, and both 1.4 m high on December 1991, were measured periodically up until June 14, 1992 .

Results

Meteorological data show that the lowest annual minimum temperature occurred mainly in the period January to March and varied from -1.7 to -6.0 C. The lowest temperature (-6.0 C) was recorded on the night of 31 December 1978/1 January 1979 and again on 13/14 January 1987. January was usually the coldest month each year with an average temperature of +5.6 C.

Highest maximum temperature occurred usually in July or August and ranged from 22 to 27.5 C.  Average annual rainfall was approximately 650 mm spread over all months.

Of the two plants where growth increments were recorded, one plant flowered in 1992 and the other remained vegetative. The growth pattern of both plants is shown in Figure I. The vegetative plant increased slightly in height between December and early March. From then on, stem growth thickened considerably, but plant height remained unchanged.

In contrast, the plant that flowered in 1992 increased rapidly in height from early February onwards and grew particularly rapidly (around 50 mm/day) between late April and mid May. The rate of upward growth slowed markedly in mid June.

E 2 to 3 scorpioid cymes each with up to 19 branchlets and each bearing 20 to 30 flowers over the 6 month flowering season. Each flower produces up to 4 seeds and so a medium-sized plant may carry well over 200,000 seeds. During the late autumn and early winter the seeds are scattered widely by wind.

After the first plants flowered, dense stands of Echium seedlings were commonplace in open situations, over 100 seedlings per 100 mm square often being recorded. In such dense stands only the most vigorous of the seedlings survived, the others being killed by inter-plant competition.

Self-sown seedlings of E. pininana were generally hardy and were unchecked by most winters. However, on the two occasions during the last 17 years (1978/9 and 1986/7) when temperatures fell to -6 C, all seedlings and one and two year old plants were killed. These low temperatures had, however, little or no effect on dormant seeds in the soil for after both of these severe winters there was a strong flush of new seedlings in the spring from the seed bank in the soil.

In some other years, a proportion only of the seedlings were killed; for example, in 1985/86 approximately 1/3 of the seedlings were killed, 1/3 had their growing points damaged and 1/3 survived uninjured.  Plants which survived with damaged growing points subsequently produced two or more flowering shoots.  After these moderately severe winters, the seed bank in the soil was augmented with seeds from the surviving, presumably hardier, plants.

Discussion

Figure 1 shows the remarkable speed of growth of E. pininana, in its flowering year. Some plants can grow from a height of between 1 and 2 m to over 6 m between February and mid June. Growth increments of 40 to 50 mm/day are not uncommon during this period. Growth slows down rapidly in mid June, presumably when the plant begins to devote more of its resources to seed production.

Plant loss from low temperatures during the winter is well known in many fruit, vegetable and ornamental crops. Breeding programmes, involving both conventional breeding methods and cell hybridization techniques are in progress in many countries aimed at preventing or reducing damage from low temperatures. These programmes are concerned mainly with crops of economic importance and much less work has been done on ornamental crops because financial returns from such work would generally be low.

In the past, natural selection from cold stress on seedling populations of fruit and vegetable crops, played an important role in crop improvement. For example, in Canada tree fruit varieties such as McIntosh apple and Lambert sweet cherry were products of natural selection and remain among the most important cultivars produced in Canada because of their superior hardiness (Quamme, 1987).  Allowing the local climate to select out improved plants from large populations is a very inexpensive method of crop improvement.

Because of the large numbers of seeds of E. pininana, involved in this study, it seems possible that natural selection from cold stress will, over time, produce a more cold hardy strain better adapted to conditions in Ireland .

To test this hypothesis, seedlings from the populations at Earlscliffe are being distributed to colder locations in Ireland , such as Trinity College , Dublin and the National Botanic Gardens , Dublin , where previous plantings of E. pininana, have not survived.

An attempt will also be made to import further samples of seed from the Logan Botanic Gardens and the Canary Islands so that the hardiness of the 'Earlscliffe' strain can be compared with plants raised from the imported seed.

References

  • Anonymous, 1992. Echium. The new Royal Horticultural Society Dictionary of Gardening. McMillan Press Ltd. London and Basingstoke . Vol 2, D-K, 140.
  • Quamme, H.A., 1987. Low temperature stress in Canadian horticultural production - an overview. Canadian J. Plant Sci. 67:1135-1149.

Figure 1 - Growth increments of Echium pininana

Flowering and non-flowering plants, 1992  

[Copy of original article can be found here.]

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