POLLINATION AND REPRODUCTIVE BEHAVIOUR OF CROP PLANTS
C Kameswara Rao
Foundation for Biotechnology Awareness and Education, Bangalore, India
Biotic pollen vectors such as honey bees and bumble bees and some others have an important role in sustainable agriculture, but that has been exaggerated, romanticized and emotionalized by expansive claims by the environmentalists. One such statement was attributed to Albert Einstein who reportedly said that ‘If there are no bees on the globe, then man would only have four years of life left. No more bees, no more pollination, no more plants, no more animals, no more man’. Without biotic pollen vectors certainly there would be problems with a few agricultural or some horticultural crops, but that would not be the end of the world.
The pollination behavior of the cereal and millet crop plants shows that most of them are highly self-pollinated or wind pollinated. Biotic vectors do not visit these species. In some other crop species biotic vectors that visit the flowers only take what they want such as nectar and/or pollen, and not necessarily pollinate. However, biotic vectors are important pollinators of a considerable number of species of fruit and vegetable crops and several wild species.
Relative antiquity of flowering plants and bees
The relative antiquity of biotic vectors and crops shows that they are not overly dependent upon each other.
The bulk of archaeological evidence indicates that cereals and pulses were in cultivation for about 6,000 to 7,000 years and the cucurbits, the oldest vegetable group, have been around for 5,000 years.
Molecular evidence based on chloroplast DNA sequences, supported by analyses of the nuclear genes encoding ribosomal RNA subunits, fixes the upper bound for the onset of flowering plants at 340 million years before present (mybp). The dicot-monocot divergence, the major event in flowering plant evolution, has occurred at around 200 mybp. Lineages of well defined dicots (the group of vegetables, pulses, tuber crops, etc.) are dated around 170 mybp and the divergence of grass groups (rice, wheat, barley, maize, sugarcane, etc.) was dated around 100 mybp.
The fossil of Trigona prisca, a stingless honey bee, reported from New Jersey amber was dated around about 85 mybp. This is a fairly advanced species, closely similar to modern neotropical species. This is the oldest fossil bee known but with no evidence of any morphological evolution for the past 75 mybp. Since the fossil is a worker, social organization had arisen by its time.
A phylogenetic analysis involving nucleotide sequences of five genes plus 101 morphological characters suggested an African origin for bees, which is not true for most of the crop plant species.
Comparative evidence indicates that honey bees appeared much later than the groups that gave crop plant species, though long before agriculture has originated about 10,000 years ago. While bees and ancestors of crop plants shared the same environment for millions of years, along with thousands of other animal and plant species, their association often considered as co-evolution, is not so in the same intimate sense as that of pests and pathogens and their hosts. Besides, crop plant species arose very rapidly during past 5,000 years through conscious human selection, the pace of which is no match to the much slower evolution of the bees and other pollinators by natural selection. Considerable evidence indicates that there was no appreciable evolutionary progression in several groups of insects during the past 50 mybp or more. Seasonality of flowering in crop plants, and largely shifting and nomadic cultivation till about the turn of the Christian Era (the plough came in around 100 CE), did not promote pollinator-crop dependence. Throughout their evolutionary history bees continuously discovered suitable wild plants, including the ancestors of cultivated plants, for nectar and pollen, like the herbivores, plant pests and pathogens. At the same time, both wild and cultivated plants had alternative means for pollination, just as they had before bees and other pollen vectors came onto the scene.
Pollination in Cereal Crops
The cereal and millet food crops of rice, wheat, corn, barley, oats, sorghum, pearl millet and finger millet all belong to the grass family. They offer no incentives to biotic pollination vectors such as nectar, nutritionally rich pollen or have attractants like fragrances or bright colors.
The pollen of grasses are small (around 30 to 40 μm in diameter, more often smaller) smooth, dry and powdery, features ideally suited to be airborne. They do not stick together, or to the body parts of the vectors. They cannot be easily compacted either in the pollen baskets or the hives without additives.
It is difficult to distinguish pollen of one grass species from another under a light microscope as they have very few surface features to characterize them. However, under a scanning electron microscope one may find features that help in distinguishing pollen of different species, and in some cases even pollen of different varieties.
The grass pollen have only a meager but physiologically functional pollen kit. Grass pollen are very sensitive to temperature, sunlight and humidity. At below 260 C and very high humidity anthers do not dehisce and at more than 320 C pollen viability suffers.
Grass pollen are trinucleate at the time of dispersal and so have notoriously short periods of germinability and viability. The pollen of many grasses are difficult to germinate in the lab.
The grass crops are all largely self-pollinated. Cross-pollination, to whatever extent that may occur, is by wind borne pollen, as biotic vectors do not normally visit grass inflorescences.
In the predominantly self-pollinated rice crop, the pollen are viable for less than 10 min, the stigma is receptive for about an hour and the florets close in less than 2 h.
Corn has unisexual inflorescences, the tassels (male) and cobs (female). Corn pollen are among the largest (about 100 μm in diameter), rich in starch and are heavy. The pollen settle more readily than windborne and the viability is less than 2 h.
When the pollen from the tassels of a corn plant reach the cobs below on the same plant, it amounts to self-pollination in genetic terms.
In cotton the anthers and stigmas are seated deeply in the bell shaped flowers. Cotton pollen are about 50 μm in diameter, highly hydrated, heavy, spiny and sticky. The floral structure and pollen features do not facilitate air lifting of pollen.
Over 80 per cent self-pollination occurs in cotton. Several vectors visit the flowers but this does not ensure cross pollination. On dissection immediately after they fed on cotton flowers none of 32 honey bees contained even a single pollen grain in their honey stomachs. The bees drew the nectar but did not take pollen.
Pollination in Potato, Tomato and Aubergine
Potato (Solanum tuberosum), tomato (Solanum lycopersicum, Lycopersicum esculentum) and aubergine (egg plant, brinjal, Solanum melongena) are similar in floral structure and pollination biology.
The anthers of these species are hollow tubes that open by small apical pores, unlike in most other plant species where the anthers open dehiscing longitudinally to fully expose the pollen to the air and pollinators.
Solanum pollen are sticky and do not travel long distances, even if they become airborne. When species of Solanum were introduced outside their native South American regions the original pollinators were not taken along to their new homes. Hence, even varieties of these species are not normally cross-pollinated in nature. Insects visit Solanum flowers but they can collect nectar without touching the anthers or stigmas, as the petals open flat out like the spokes of a wheel.
As potato is vegetatively propagated by the ‘eyes’ of the tuber (the farmer’s seed), pollination is not a matter of concern. The breeders who are interested in the true fruits and seeds of potato hand pollinate the flowers.
In the wild state, tomatoes required cross-pollination. Domesticated cultivars of tomato have been selected to maximize self-fertilization. Experimental studies with tomatoes have shown that pollinators such as bumble bees are ‘buzz pollinators’ which actually ‘sonicate’ the anthers causing the pollen to move out of the tubular anthers. Gushes of wind, even artificial wind or cultured bumble bees can provide sufficient motion to produce commercially viable crops.
In cultivated aubergine the extent of self-pollination is over 90 per cent.
Pollination in Cultivated Brassicas
Most of the cultivated members of the mustard family (the Brassicaceae), such as Canola, oil rape, oil mustards, condiment and leafy vegetable mustards and turnip all belong to the genus Brassica. Cabbage, cauliflower, knolkohl, broccoli, Brussels’ sprouts and related group of vegetables are different varieties of Brassica oleracea. Radish (Raphanus sativus) also belongs to the same family. Farmers had no difficulty in maintaining them distinct without loss of their identity, even when they are highly interfertile like the varieties of Brassica oleracea.
Wild species of Brassica and many other members of the family Brassicaceae have genetically determined self-incompatibility factors that prevent true self-fertilization. However, during centuries of domestication and cultivation this has changed considerably to the extent that cross-fertilization is usually less than 30 per cent.
Brassica flowers are honey flowers, visited by bees, which may pollinate the flowers. Pollen may also be airborne, much depending upon the temperature, rain or humidity, flowering stage and other related factors.
Pollination in Legume crops
The flowers of the legume crops such as pea, chickpea, soybean, and several others are intricate structures evolved to promote cross-pollination, but in practice they are self-pollinated, most often even before the flower opens and in the groundnut the flowers may not even open.
February 22, 2008