Portulaca oleracea: Early Years

פורסם: April 25th, 2010 | עודכן: 17/01/15

My acquaintance with [Portulaca oleracea] started in my early childhood when my brother and I accompanied our parents on a trip to Hadera to visit Ezra and Zipora, our uncle and aunt. In a photograph from that time, which may be in the album but is definitely imprinted in my memory, Ezra showed us [Portulaca oleracea] plants. At the edge of one of his carp-fish ponds he explained to us how the leaves of [“Portulaca oleracea” P. oleracea] may be eaten. We helped him in collecting a considerable number of the fresh plants to bring to Aunt Zipora’s kitchen. During the same week or on another occasion we were invited to a party at the home of Ezra and Zipora; one of the first dishes (at what I remember as a kind of a cocktail party) was a white borsht based on [“Portulaca oleracea” P. oleracea].

The other thing I remember from that party was that one of the VIPs was David Ben-Gurion. Another surprising meeting with the plant in my youth was after my 13th birthday. My grandparents Hanna and Arieh gave me a precious present for my Bar Mitzvah – the Analytical Flora of Palestine by Eig, Zohary & Feinbrun (1948). Getting to know the plant through the book, I learnt that the authors did not find it in the Judean Mts. However, we had it growing in our garden in Bet Hakerem. I added the information to my own copy and learnt that the book, detailed as it is, does not fully reflect all the information available in the field. The next meetings I had with the plant were mainly as a farmer or a gardener dealing with this weed in the field or in the garden ([useful_em_plants_k1 Fig. 11.1.14]). In my first years at the university I did not deal with plants affected by human activity. [“Portulaca oleracea” P. oleracea] does not have a natural habitat here and it may inhabit a site only after human disturbance has created an appropriate habitat for it.

I was sent to Nicaragua by the agricultural department of TAHAL (an Israeli company dealing with developing water resources). The aim of my visit was to study the feasibility of using plant indicators in detecting problematic soils in tropical countries. TAHAL’s expert advisor, my uncle Ezra Danin, read an article I wrote about plant indicators for soil salinity in the Dead Sea area. There, I reported having detected 5-7 plant communities along a 70 cm section (yes, centimeters). Each community developed on a different soil type but had a characteristic salinity value. I was told: “Try to assist us in the soil surveys in tropical countries.” In Nicaragua I went to the Atlantic Coast area for a few days. There were rain-forests with highly diverse vegetation. It was rather difficult to find any similarity to the plant-soil relationships I was acquainted with from Israel. I had to give up the dream of discovering plant indicators for the moist tropical soils.

I returned to Managua, the capital city, and did not go again to the tropical forest. Amnon Horev worked as an agricultural advisor to the locals in Sebaco, about an hour’s drive from Managua. I told him about my disappointment and my inability to discover plant indicators for the tropical wet land. He told me that from his experience in dry zones of western Nicaragua, not far from the Pacific Ocean, there are problems in raising tomatoes and cotton in sodium-rich soils. He gave me an aerial photograph which showed marks where samples of alkaline soils were taken for chemical analysis. In these places the natural vegetation was hardly developed, but I found there Portulaca oleracea which I knew as a weed from my childhood. Sites with no other annuals, shrubs and trees in the tropical savannah (Fig. 11.2.1) of Sebaco valley were populated by [“Portulaca oleracea” P. oleracea] plants that become red during their drying process. A few days after the first rain shower I photographed plenty of seedlings in these Portulaca patches (Fig. 11.2.2). The high temperature in that area facilitated immediate germination.

Fig. 11.2.1: A tropical savannah in Sebaco Valley, Nicaragua; a patch of soil rich in sodium (Na) dominated by Portulaca oleracea which becomes red while drying.

Fig. 11.2.2: Germination of Portulaca oleracea in a patch of soil rich in sodium after the first winter rains.

I planned to carry out a small research project in Berkeley, California where I continued the sabbatical year from my university. I intended to study the relationship between sodium in the soil and sodium in the leaves and stems of the plants. I collected plants in several places as well as seeds. All the seeds from Nicaragua were put into one empty glass jar. I dried out the plants with the aid of the kitchenette oven in our hotel room in Managua. Later, I collected more seeds, plants, and soil samples at additional sites in California. Berkeley weather is much colder than the weather in Nicaragua and I found [“Portulaca oleracea” P. oleracea] plants near the air outlet of heating ventilators on the campus (Fig. 11.2.3), which induced early germination in that area. The plants’ ash and the soils were analyzed by experts and I was happy to find a close relationship between sodium in the soil and in the plant growing in it (in the “scientific atmosphere” of Berkeley we talked of the “significant linear correlation” as in Fig. 11.2.4). I wanted to continue my line of research there and carry out quantitative experiments on sodium in the soil and in plants, in order to make [“Portulaca oleracea” P. oleracea] a tool for sodium analysis without digging holes in the soil.

$p BOOK882b Fig. 11.2.3: Portulaca oleracea growing in Berkeley near the outlet of a ventilator surrounding them.

Fig. 11.2.4: Linear regression between the amount of sodium in the soil and the amount in the P. oleracea body.

Since Berkely is located near the ocean, I was unable to utilize the opportunity of my visit, because of the air-borne sea salts. I then asked Dr. Herbert Baker, from the Department of Botany: “What can I do with the large seeds of Portulaca oleracea that I collected in Berkeley and Palm Springs, California, versus small seeds of the same plant that I have from Nicaragua? He instantly opened a book that contained reports of the results of chromosome counting all over the world. There were plenty of cases with 54 chromosomes in the vegetative cells, several instances of 36 chromosomes (tetraploid) and one case of 18 chromosomes (diploid). The last one was in the “heart” of the Saharan Desert near Timbuktu. Whenever I appeared before an American audience and mentioned that place, a wave of laughter was heard in the hall. For Americans, Timbuktu is regarded as a legendary far away place – rather like “the end of the world.” Following the advice of Irene Baker, who ran her husband’s laboratory, I immediately germinated seeds in order to get root-tips, which are used for chromosomes counting. I was very happy to count 36 chromosomes in plants that germinated from Nicaragua-collected seeds. I began to consider writing a short article on the discovery of tetraploid [“Portulaca oleracea” P. oleracea] in the Western World; the previous report on a tetraploid was from India or Bangladesh. Irene insisted that she wanted to see another root with prominent chromosomes. In the second preparation she made, after treating with colchicine and adding dye, she said – “I am sorry, but despite your 36 chromosomes, I see here only 18.” A check up on the seeds collected in Nicaragua with a dissecting microscope revealed bluish seeds covered with wax, and black seeds with no wax (Fig. 11.2.5). The epidermal seeds differed as well and I started to investigate seeds from various locations and tried to classify them. I continued to investigate the chromosome number of populations sampled in the field. A significant relationship was found between the chromosome number and the seed size. The seed diameter of those with 18 or 36 chromosomes is below 0.7 mm. Those with seeds larger than 0.8 mm have 54 chromosomes (Fig. 11.2.6). I started to raise plants derived from populations sampled in the field (Figs. 11.2.7, 11.2.8) and carefully checked in my laboratory through the dissecting-scope. Results began to accumulate fast. During that sabbatical year I checked about 1000 herbarium specimens that had been collected in all the continents, and I had them on loan as was customary when doing taxonomical research. We (the Bakers and I) came to the conclusion that the [“Portulaca oleracea” P. oleracea] complex may be divided into nine taxa below the species level, based on epidermal cell morphology and seed diameter. Using these parameters, nine differentiate species may be separated genetically (by having different chromosome numbers) and by recognizable morphological characteristics. Since the plants could not be differentiated using leaves, stems, flowers etc., I regarded them as subspecies. I did not want my botanist colleagues to be angry with me for forcing them to use a dissecting microscope before providing even a simple name. I had an interesting meeting on this issue with one of the greatest evolutionists in the USA – Dr. Ledyard Stebbins. I had already met him at an international conference in Manchester in 1971. He was the PhD tutor of Prof. Daniel Zohary, was visiting Israel and was prepared to hear my Portulaca story. I gave him a private seminar in one of the large halls of Berkeley – only he and I; I talked – he fell asleep and I continued talking. At the end of my lecture he woke up and asked me serious and difficult questions concerning the whole lecture including those parts I thought he had “slept” through. At a certain point in our conversation I asked his advice: should I recognize the taxa at a species or subspecies level. He suddenly became very angry saying that taxonomy did not interest him whatsoever, and how dare I take up his valuable time with such a question. One of the greatest botanists on earth, Dr. Peter Raven, deals with biology and taxonomy of plants, and in 1978 I asked him about the level I should give the independent units recognized in our study. He answered in 1978 and restated his opinion in 2003: that I should regard them as species. In 2003 I accepted his opinion (as will be discussed later).

Fig. 11.2.5: A seed of Portulaca nicaraguensis: a. Close-up of an epidermal cell where wax covers most of the cell surface, b. An entire seed with wax-covered epidermal cells, c. Cells of a rare form without wax cover, d. Close-up of a seed with prominent wax cover.

Fig. 11.2.6: Linear regression between seed diameter in P. oleracea and the number of chromosomes in its somatic cells.

Fig. 11.2.7: A pot in a greenhouse raising P. oleracea from seeds collected in the field in Nicaragua.

Fig. 11.2.8: Pots with various forms as they grow in a greenhouse in Berkeley.

A very old question exists in scientific literature asking if [“Portulaca oleracea” P. oleracea] came to America from Europe (as claimed by Alfonse De Candole in 1888 “as the plant is so common in Europe”), or was it the other way around?! When De Candole’s book was published, the American botanist Asa Gray quoted the captain’s log Columbus wrote when he reached the island Hispaniola: I recognize no plant here except for Verdolagas (=P. oleracea in Spanish). Thus it was clear that at the end of the 19th century the plant was present on both sides of the Atlantic Ocean before the Spaniards arrived in the “New World” in 1492. My findings showed that most of our nine subspecies existed in the Americas, including the most common taxon of Europe in mountainous areas of Mexico. We had to offer a way of natural seed transportation. In order to check up on “marine transportation” we conducted the following experiment: we carefully laid plenty of [“Portulaca oleracea” P. oleracea] seeds on the surface tension-crust of sterilized sea water. A week later we removed some ten seeds to a Petri Dish with distilled water and paper. All the seeds germinated within one day if they had enough light. We repeated this activity in the following weeks and months, and the seeds laid in the beginning were sufficient for half a year. We thus succeeded in proving that the plant may withstand many months of floating on sea water without harm to their viability. I then found an article declaring that seeds from the Gulf of Mexico arrived at the beaches of Ireland via the Gulf Stream, a year later, transported with the aid of tree trunks. I read an article written by Knoerzer and his colleagues who discovered [“Portulaca oleracea” P. oleracea] seeds in an archaeological site in Germany from the 1st to the 4th century. This statement came full circle more or less proving how seeds could travel such a distance without human assistance. In 2010 we are now trying to get hold of Knoerzer’s seeds to determine to which micro-species they belong.

Fig. 11.2.9: A dried plant of P. oleracea in Hortus cliffortianus (= Cliffort’s Garden).

Fig. 11.2.10: Portulaca trituberculata. The seeds of the plants in Fig. 11.2.9 look like this photograph.

During the preparation of the manuscript of my article with the Bakers we had to typify and state which specimen Linnaeus held while describing [“Portulaca oleracea” P. oleracea]. The plant was described in 1753 in his book Species Plantarum. I was visiting London for that purpose, and after seeing in the few lines in his book the words “Hort. Cliff.”, I learnt that it referred to the book-album “Hortus Cliffortianus” deposited in the Natural History Museum in London (also known as the British Museum). I studied the dried plant on the page shown here with great excitement (Fig. 11.2.9). It is as if placed in the decorated vase with the Latin description of it on the right, and the hand-written names. I understood that these names were written by Linnaeus himself. For me, a botanist just starting out, it was an exciting moment. I checked the seeds of the plant in Fig. 11.2.9 and they were of the plant known today as [“Portulaca trituberculata” P. trituberculata] (Figs. 11.2.10, 11.2.11). The second herbarium that was available to Linnaeus, is today situated in the cellar of a building in Piccadilly Street, London. I checked the plant there and the seeds were identical to what we called in our article ([http://flora.huji.ac.il/browse.asp?action=showfile&fileid=15111 Danin, Baker and Baker 1978]) Portulaca oleracea* subsp. *stellata (Fig. 11.2.12). For many years I saw in that article the pinnacle of my studies with [Portulaca oleracea].

Fig. 11.2.11: Close-up of a Portulaca trituberculata seed.

Fig. 11.2.12: Portulaca oleracea (in the narrow sense of the name). The seeds of the plant used by Carolus Linnnaeus to describe it, look like this.

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