A Borneo origin for the Chinese flowering cabbage Choi Sum (Brassica rapa var. parachinensis)

The origin of the Chinese flowering cabbage ('Choi Sum' in Cantonese)has been a bit of a mystery. It was long popular in China and Japan before it became known to the western world. Most authorities think it originated in Southeast Asia. It certainly grows well in the tropics where it is widely cultivated by ethnic Chinese farmers, but I have never seen it wild in the Malay Peninsula, and the seeds are usually imported (from China or Thailand?). The plant also does not 'escape' and grow by itself without human help. These observations made me doubt the theory of its Southeast Asian origin.

However I recently noticed a small form of this vegetable in native markets in Kuching. It is less than half the size of the form grown by Chinese farmers. I made enquiries and found that it is known as 'sawi dayak'. It is grown by broadcasting the seeds on newly burnt hill slopes on which the inland communities grow hill rice. The very small seeds are sold cheaply in small packets, each containing hundreds if not thousands of seeds, so the plants must be prolific seeders and perfectly adapted to the local environment. Hence I now believe that sawi dayak is the parental wild type of Choi Sum and its origin is indeed Southeast Asia, specifically Sarawak in Borneo.

The Chinese form of this vegetable needs daily watering and tending. The sawi dayak is watered by rain only. It is just the vegetable for the 'let-it-grow-naturally' organic farmer.

Physalis alkekengi (Chinese Lantern Plant) has very sweet fruits

I have just spent a week in Inner Mongolia province of China. As usual, I kept a lookout for interesting plants, but the plant diversity in Inner Mongolia is very low compared to Sichuan and Yunnan.

The most interesting find was the fruit of Physalis in a market stall. I think this must be Physalis alkekengi, the Chinese Lantern Plant. The fruit is an orange berry half the size of a cherry but completely covered by 5 expanded calyx lobes. There is a related weedy species in Malaysia, Physalis minima, locally known as letup-letup. Another species, Physalis peruviana, also known as the Cape Gooseberry, was introduced to Malaysia about ten years ago and is sometimes used in restaurants to decorate meat dishes. The fruits of all three species look very similar.

The fruits of P. minima and P. peruviana are both rather tasteless, but the fruits I found in China are as sweet as the sweetest grapes. It is also said to be a highly ornamental plant, with the calyces red in colour when fresh. P. alkekengi is perennial, surviving tbe cold winter by virtue of a resting underground stem. It may not thrive in the tropics. It would be interesting to hybridise it with P. minima to produce a sweet-fruited plant for the tropics.

The homing ability of a caterpillar

As a gardener, I love butterflies but detest caterpillars. Whenever I see them on my plants, I flick them off and that is the last I see of them. This is an account of a caterpillar that I flicked off three times and each time, it found its way back to the host plant.

It was a large caterpillar about 7 cm long, black in colour, with a pattern of gold ringed ‘eyes’ and white transverse bands made up of small closely spaced white spots). At the tail end, sticking up, was a spur. This was identified by an entomologist as the caterpillar of a hawkmoth.

I found two of these caterpillars feeding on a plant of Impatiens walleriana (busy lizzie) growing in mixture with other flowering plants on a raised flower bed in my garden. The bed is one brick high and is demarcated by a line of bricks. I flicked the caterpillars off with a stick and they landed about 30 cm away on the lawn . On the next day, one of them was back on the plant. The other was not seen again. At first I thought this was a caterpillar that I might have overlooked. I flicked this off and made sure there were no other caterpillars on the plant. An hour later, I noticed it was back on the plant.

Intriqued, I flicked it off for the third time and this time I watched as it recovered from the shock and crawled on the grass in the direction of the host plant. When it arrived at the dividing brick wall. It tried to climb the wall but gave up. Instead it crawled on the grass alongside the brick away from the host plant until it came to a gap between two bricks that was filled with soil and small weeds. It crawled up this gap to the top of the bed and headed straight for the host plant, past a number of other plants of other species. I was surprised by its ability to make a detour and still find its target.

I was going to give the caterpillar a rest before putting it though some further tests, but when I came back a few hours later, it had disappeared.

Can anyone put me in touch with other accounts of homing ability in caterpillars?

Acacia mangium growth in relation to light and nutrients

In a recent experiment, we grew Acacia mangium under five light intensities: 4, 7, 25, 50 and 100% side by side with four other rainforest species under conditions free of water stress. Acacia mangium outgrew all the other species as expected, not only under 100% light but also under 50% and 25%. It had the highest leaf turnover rate due to its short leaf life span, which is about 300 days at 4% light dropping to 100 days at 100% light. Its leaf life span drops at a rate of 40 days for each doubling of light intensity. Compared to the next fastest growing species, Shorea roxburghii, Acacia mangium grows 4 times as fast but takes up 15 times as much N, 10 times as much P and 16 times as much K.

Our paper has been published in the Journal of Tropical Forest Science 20 (3) 218-234 (2008). P.S. Tong and F.S.P. Ng: Effect of light intensity on growth, leaf production, leaf lifespan and leaf nutrient budgets of Acacia mangium, Cinnamomum iners, Dyera costulata, Eusideroxylon zwageri and Shorea roxburghii.

http://info.frim.gov.my/cfdocs/infocenter/booksonline/index.cfm?menu=jtfs

Leaf demography, phenology and light

In the humid tropics there is no obvious climatic stimulus to make a tree shed its leaves, but leaf-shedding occurs nevertheless. Some trees shed old leaves and produce new ones all through the year, good examples being the papaya, the oil palm and the coconut. The majority shed leaves all at one go and replace them simultaneously with new leaves. The switchover is usually so smooth that the trees never appear to be bare, so tropical humid forests appear evergreen. In most years there is little or no synchronization between trees, but once in a long while there is a baffling mass synchronization event.

In the 1930s, R.E. Holttum, then Director of the Botanic Gardens Singapore, had the idea that leaves have an inherent life span and are shed when their time is up. He monitored the leaf change cycles in many trees for 10 years or more and found that each species follows an approximate periodicity, with a variation of up to a couple of months. He could not explain the variation.

Recent studies have shown that leaves exposed to full sun have shorter life spans than leaves in shade. A just-published paper by Ms Tong Pei Sin (my student) and myself describes an experiment comparing 5 species (Acacia mangium, Shorea roxburghii, Dyera costulata, Eusideroxylon zwageri and Cinnamomum iners) under 5 light intensities (4, 7, 25, 50 and 100%). This study shows that leaf life span is shortened by a constant amount for every doubling of light intensity. For example, Acacia mangium leaves have a life span of about 300 days at 4% dropping to about 100 days at 100% losing about 40 days per doubling of light intensity. Shorea roxburghii leaves have a life span of about 550 days at 4% dropping to about 200 days at 100%, losing about 70 days for each doubling of light intensity.

Hence leaves do have a life span, but it is a service life span, not a calendar life span. We propose photosenescence of leaves as one of the drivers of phenology in the humid tropics.

Trees in nature, being subject to varying cloud cover, would vary in periodicity of leaf fall because no two periods would ever experience exactly the same combination of cloud cover conditions. A long period of cloudy weather would prolong the leaf life span of many trees and if this is followed by clear sunny days, many trees would trip over their tresholds and shed leaves simultaneously. This would be accompanied by production of new leaves and inflorescences in mass synchrony. The full paper is availble:

http://info.frim.gov.my/cfdocs/infocentre/Korporat/2003Publications/Links/JTFS%2020(3)/CONTENTS.html

or

http://info.frim.gov.my/cfdocs/infocenter/booksonline/index.cfm?menu=jtfs

Creating cannas for the ever-flowering tropical garden

The creation of new flowers is something most gardeners leave to professional plant breeders. Since there are very few professional plant breeders in the humid tropics, the rate of development of new varieties in the humid tropics is very slow. I would like to change this by getting more gardeners into plant breeding, starting with something really easy -- tropical cannas.

Cannas are good subjects because they come in many different floral colours including red, yellow, orange, pink, cream, near-white and mixed or mottled. The leaves are green, red, or striped. There are talls, mediums and dwarfs. Each flower lasts usually two days but a head of flowers may carry up to 20 flowers. An inflorescence usually bears 2-3 heads of flowers in succession and some bear up to 15heads. For every shoot in flower, another should be half-way and a third pushing up from the rhizome underground. Flowering can be prolonged for years.

Here are some other statistics.
Time from pollination to seed-ripening: about 20 days
Time for treated seeds to germinate: about 7 days
Time from germination to flowering: about 3 months
Time for doubling of plants by division of rhizomes: any time after 4th month

There are few plants that one can hybridize and evaluate within 6 months. This is a phenomenally rapid rate compared to tulips, curcumas and orchids.

As far as I know, all cannas, if fertile, will hybridize with each other. However, many garden forms are sterile. The fertile ones advertise themselves by producing fruits spontaneously and spradically. To start your breeding programme, make a collection of fertile varieties first.

To produce new hybrids, apply pollen from a fertile plant to the stigma of another fertile plant. This is best done in the afternoon, using freshly shed pollen.

Open a fully developed bud (tomorrow's flower). Strip off the sepals and petals until you are left with the two innermost members. One will be a stamen, somewhat like a small petal in appearance, which bears an anther on one side. The other will be a flattened pistil, at the tip of which is the stigma. The anther will already have split and deposited its pollen on to the side of the pistil. The pistil, with the pollen on its side, can then be used to dab the freshly shed pollen on to the stigmas of already opened flowers.

After about 20 days the seed will ripen. The seeds harden on drying and will keep for a year or more. To germinate, make a small cut in the hard seed coat with a wire cutter and plant about 1 cm deep in soil.

Springtime all the time--the ever-flowering garden

I gave a talk on this topic in Kuching a couple of days ago, and this is a brief summary.

The everflowering garden can only be created where it is warm and moist all the time, as in the humid tropics. The British had a go at it when they ruled what is now Malaysia and Singapore, but most of the flowering plants they used were annuals imported as seeds from Britain. In front of their bungalows, the British created lawns fringed by borders of flowering plants. The lawn tradition survives but the flowering border could not be sustained. By default, gardens in the humid tropics are evergreen--monotonously so! However, in the past 50 years, more and more tropical perennial ever-flowering plants have come into existence. I counted over 100species in my book Tropical Horticulture and Gardening. The ever-flowering garden is now well within reach and should be one of the aims of tropical gardening.

50 years ago, it was a problem to keep bougainvilleas in flower. Now there are ever-flowering bougainvilleas in a wide range of colours, thanks of plant breeding and selection. Other plants that have become ever-flowering are the drunken sailor Quisqualis indica and Kock's bauhinia Bauhinia kockiana. New forms of Hibiscus, all ever-flowering, have been bred in Hawaii and Australia. Ever-flowering heliconias have become common. Ever-flowering Canna were bred by the late Professor Holttum in Singapore but most of these have been lost; we have to start all over again.

I am particularly keen on cannas because they are easy to breed and select. I distributed hybrid seeds in Kuching and encouraged my audience to form an informal club for future breeding and dissemination of seeds. The breeding and selection of cannas will be my next blog.