Journal Number 97
November 2005

NOTES etc

Does winter chilling hasten flowering in Nematoceras? asked Bruce Irwin.

"In a letter to our editor, I mentioned the possibility that winter chilling may encourage early flowering in the Nematoceras rivularis complex. Ian thought that a novel concept and asked for a short article. I must emphasise that my impressions on the subject are just that - impressions.

"Until recently it was thought that the name Corybas (Nematoceras) rivularis applied to a single taxon. If this were so, it would be difficult to accept that C. rivularis flowered in early September at Waiouru, but not until early November near Kaitaia. Nowadays it is generally accepted that several taxa, whether species, subspecies or varieties make up an aggregate under the name N. rivularis. This being so, the anomalous flowering times might seem to be explained, but let us consider what we know about the two species from the localities mentioned.

"Distributions of the various taxa within the N. rivularis agg. are far from complete, but N. rivularis s.s. appears to be confined to the far north, except for a disjunct colony at New Plymouth. Plants in the far north flower in November, later than any other taxon in the group. Significantly the New Plymouth colony flowers much earlier, in September & October. Those facts are surprising.

"Also surprising is that N. longipetala, the earliest form to flower, does so in much colder climates, as early as July in Westland and at Pukeiti near New Plymouth. Even at Waiouru, mature flowers have been recorded as early as 4 September. Clearly some Nematoceras, despite their fragile appearance, can withstand severe frosts. Why then does N. rivularis flower so late and N. longipetala so early? Is it because the far north experiences a more severe climate than Waiouru? I borrow a famous phrase: 'Not bloody likely'.

"Whether or not chilling encourages early flowering may become clear when the various taxa in the N. rivularis agg. are better understood and their flowering times recorded more precisely".

Editor's Comment

Well, isn't that a fascinating concept? I looked on the net, and came up with a few possible pointers about winter chilling.

For instance, fruit trees grow rapidly during the spring and first half of the summer. Later in the season, the growth rate declines, and in the autumn stops, as length of day and temperatures decrease and the trees drop their leaves. In a reaction to length of day and temperature, the trees produce growth inhibitors (eg, endogenous gibberellins and abscisic acid). In the winter the trees are dormant because of the growth inhibitors.  

Dormancy is broken when sufficient cold temperature breaks down the growth inhibitors.
This is called vernalisation, "chilling," or "winter chill." A cumulative number of hours of chilling (temperature below 7 deg C), is required to break dormancy, and the number is specific to each species. Once the number of hours of chilling has been achieved, active growth can resume in the spring when the trees are exposed to warm enough temperatures for natural growth processes to begin.

Temperate trees grow best where the winters are warm enough that plant tissue is not killed from extreme low temperatures, but not so warm that buds receive inadequate chilling to break dormancy. Flower and shoot buds of deciduous fruit trees and olives will grow normally in the spring only after exposure to sufficient winter cold. After winters with inadequate chilling, the plants leaf out late in the season, blossoming is prolonged, buds may deteriorate and drop, and few flowers are produced.

In Te Puke a year of high winter chilling will generally mean more kiwifruit flowers, an earlier flowering period once spring temperatures arrive, and often a more compacted flowering period.

Phaius tankervilleae is a large terrestrial orchid, native over a wide range from China to Australia, and plants from the northern regions can withstand temperatures down to 40°F (5°C), possibly lower. In fact, although in general the plants like warm to intermediate temperatures, they need a period of winter chilling to bloom satisfactorily.

Joseph Arditti (in "Fundamentals of orchid biology") lists orchids whose flowering appears to be related to photoperiodism (relative light and dark periods), and thermoperiodism (alternation of, or specific temperatures). Low temperature dependent plants include a number of Cattleya, Dendrobium, Cymbidium, Sarcochilus and Thrixspermum species.

For some orchids, the chilling requirements have been found to be quite specific: for instance Paphiopedilum insigne needed night temperatures of 13 deg C  for 2-3 weeks.

Of course we know nothing of the chill requirements of our native orchids, but if they behave
in similar fashion to other plants, including other orchids, Bruce may well be right: chilling of Nematoceras may destroy growth inhibitors that may be present in the tubers during dormancy, and may thus induce early flowering; the danger of early flowering is of course frost damage, and I have seen Nematoceras longipetala whose tepal ends were withered by frost - Ed.