Plant Physiology and Crop Improvement

In this article we will discuss about the contribution of plant physiology towards crop improvement.

Food is an essential requirement, and the demand for food shall keep on increasing with the increase in population. Till recently the increase in the rate of production of food matched or even exceeded the population growth rate.

The classical breeding programmes have contributed enormously to the improvement of various crops and subsequently molecular genetics which today constitutes the basis of genetic engineering research has added new direction to crop improvement. There is growing concern about future prospects for yield enhancement.

Consequently, crop improvement research has to be a holistic effort wherein all disciplines must contribute towards achieving goals. The role of crop physiologist is to integrate information, synthesize new levels of knowledge, and develop systems for problem solving, all the while interfacing with plant breeder, biochemist, and researchers in other areas of science especially molecular biology, appears immense.

In a recent discourse eminent scientist Dr. M.S. Swaminathan has highlighted the significant developments in food crops including maize, wheat, rice and potato where revolution was accomplished in some parts of India using good agronomic management including irrigation besides using best of the genotypes.

This implied the realization of the full genetic potential of a variety. Thus hybrid corn with hybrid vigour has built in physiological mechanisms—a discussion done by Drs. S.K. Sinha and Renu Khanna-Chopra (IAR1, New Delhi).

In rice hybridization followed by selection and mutation resulted in an increased rice yield concomitant with a shift from tall to dwarf varieties. The dwarfing of the variety is attributed to gibberellic acid mechanism.

Further change in the wheat and rice archetype could be accomplished through the use of plant growth regulators or through molecular biology techniques enabling the plant to utilize soil nutrients and water more effectively, without lodging. With good soil fertility and irrigation management, we can enhance the yield several fold.

In rice several aspects of plant morphology, plant architecture, root system were altered and at the same time plants which are insensitive to photoperiod were selected. Thus, the semi-dwarf varieties could be grown all through the year and has shifted from a season-bound to a period-fixed variety.

Thus, it is possible to have 100-day variety or 120-day variety by manipulating both photo-insensitivity and thermo-sensitivity. In Asia this has resulted in Rice Revolution and even in India rice production has increased considerably. Same increases have been reported in China and many other countries.

Ever since the discovery of the technique of anther culture developed by Profs. Sipra Guha and S.C. Maheshwari in India, and Chinese scientists having exploited the technique effectively in rice, it has speeded up segregating generations towards greater homozygosity through doubling of androgenic haploids.

More recently apomixis has become very important tool and in maize apomixis has been transferred from teosinte and allied genera. The advantage is that if you have a very high-yielding hybrid e.g. in rice, and if it has apomictic factor, the farmers can preserve their own seeds and there is no need to buy fresh seeds.

Consequently added attempts have been made to combine apomixis with heterosis in crop improvement programmes. In vegetatively propagated plants like potato it is possible to fix hybrid vigour but in rice and wheat which are self-pollinated crops, apomixis has a great role in benefiting the small farmers.

They can avail of yield advantages through heterosis and help maximize factors like resistance to both biotic and abiotic stresses. One of the areas of further study is pertinent to hybrid vigour where combinations could be worked out i.e. what are the plant functions that are responsible for the vigour in a given crop? Is it possible to break the different components involved in yield and study as to where the gain is coming from ? In other words we seek answers whether the gains are coming from morphology or physiology related attributes.

There are several promising true-line hybrids available in rice which could be used to exploit from heterosis viewpoint by adding apomixis trait. Dr. M. S. Swaminathan made several efforts to develop hybrid rice varieties by crossing indica x japonica varieties which responded well to irrigation, water and good soil fertility, as proposed to him by Ramiah.

In wheat development of dwarf varieties brought about Wheat Revolution in India and resulted in quantum jump from 6 million tonnes to 72 million tonnes over the past five decades. Several factors contributed towards this jump which included input-output pricing, assured marketing to the farmers and announcement of floor price before sowing so that the farmer is sure of the price that he shall fetch.

This has developed into a whole setup of package of technology, package of services and package of public policies. In fact all these factors contributed to the Green Revolution. Several factors e.g. seed production, credit availability for marginal farmers to buy inputs and assured markets are essential to translate technology into Green Revolution.

Concerted efforts are being made at IARI (New Delhi), Ludhiana, Pant Nagar and Nati Wheat Directorate at Karnal on the development of hybrid wheat. New production technology to optimize seed rate, nutritional uptake, water usage etc., will cut cultivation cost to meet hybrid seed production in wheat.

In Punjab as high as 3.3 t/ha yield is being obtained in wheat though the potential is to secure as high as 7-8 t/ha. High application of fertilizers to high-yielding varieties poses serious problem to ground water pollution. This is one of the aspects which plant physiologists need to study in detail.

It is very much desired to work out a balanced nutrition schedule. The breeder is primarily concerned with the release of high-yielding variety and plant physiologist is required to work out the schedule of balanced nutrients to cause high uptake of nutrients and their utilization and simultaneously ensuring that environmental problems are avoided.

These include nitrate and ground water pollution or any other ecological constraint. It is important to determine the stage of application and quantity of nutrients to secure maximum yield.

With the advent of chromosome mapping, first with Human and then several organisms including Arabidopsis, rice genome mapping is nearly completed and contribution of Japanese scientists is appreciable. It is aptly argued that sequencing of the genome is not really an invention but a discovery. There is growing concern and desire to analyse more and more plant genomes.

Over the years the Science of Proteomics where analyses of the entire set of proteins of an organisms is carried out is turning out to be a bigger challenge than genomics.

Being on the topic of rice, employing recombinant DNA technology, remarkable progress has been made towards production of rice strains for increased yield, improved nutritional quality (through introduction of provitamin A synthesizing genes; enhanced iron content, etc.) and improved resistance to insects, viral, bacterial and fungal pathogens. Below figure shows several agronomic traits which have been engineered in rice using transgenic technology.

Several abiotic stresses e.g. sub-and supraoptimal temperatures, excess salt levels, reduced water availability leading to drought stress, excess water resulting in flooding stress and oxidative stress caused by the combination of high light intensity with other stresses adversely affect almost all major field-grown crops.

Rice is especially sensitive to excess salts, reduced or excess water supply and sub-optimal temperatures. Some stresses like drought and submergence stresses affect rice cultivation more than the biotic stresses.

Studies have shown that genes which transform rice can lead to superior tolerance against abiotic stresses are:

i. The response of plants to abiotic stresses is multigenic in nature

ii. The biochemical/physiological reactions/processes associated with tolerance to abiotic stresses are yet to be precisely identified, and

iii. There is scanty information on genes which would have a positive effect in imparting tolerance to abiotic stress. To date some important generalizations can be made and these are:

iv. Selection of transformed cells can be exercised with several genes including those providing resistance against kanamycin, hygromycin, bialophos, glufosinate, etc.

v. Reporter genes tested and proved useful for rice transformation work include P-glucosidase, luciferase, etc.

vi. Transgene expression has been tested using a wide range of different promoters including 35S promoter, Al DH promoter, actinol promoter, LH CP II promoter, ubiquitin I promoter, etc.

A general protocol to accomplish rice transformation involves the following steps (Fig. 29-2):

(i) Raising embryogenic calli from mature seed embryos.

(ii) Co-cultivation of embryogenic calli with the competent A. tumefaciens cells harbouring the gene of interest

(iii) Selection of the transformed calli from the non-transformed calli by repeated subcultures on hygromycin containing media

(iv) Regeneration of shoots from the putatively transformed calli and induction of rooting

(v) Transfer the tissue-culture grown seedlings from test tubes to pots through proper hardening treatments

(vi) Optimizing conditions for raising the transformed plants to maturity up to seed setting and

(vii) Analysis of putatively transformed plants by PCR or by Southern blotting to verify foreign gene transfer.

Two directions have been followed for raising abiotic stress resistant rice’s:

i. Raising transgenic using genes which have already been characterized and shown to work in other systems and

ii. Isolation and characterization of novel genes from rice for increasing the present level of abiotic stress tolerance.

Successes have been obtained as follows:

i. Overexpression of hval gene (producing LEA protein)

ii. Overexpressing cod A gene (encoding choline oxidase)

iii. Overexpressing cor 47 (encoding cold-regulated protein of 47 KDA) gene

iv. Overexpressing O1-pyrolline 5-carboxylase synthase gene.

The future programme in research on several aspects and some are discussed below:

i. M. S. Swaminathan research centre is using mangroves as a source of tolerance to sea water salinization. Mangroves are shown to discharge large amount of nutrients. One of the researcher at the foundation has reported that glycine-betaine synthesis was largely responsible for its sea water tolerance. This gene was transferred to tabacco, mustard and then rice.

The transgenics were efficient to accumulate completive solutes in diverse plant species at the cellular level i.e. glycine- betaine level increased which contributes to salt-water tolerance. GM glycine-bataine accumulating plants were able to tolerate 300 mM of sodium chloride, which is much higher than prevailing salinity levels in cultivated rice soils.

ii. The physiology/biochemistry component should involve in sharing the responsibility of providing/ and testing various reactions/processes for their role in stress tolerance using suitable plant material provided by the breeders.

This group should also develop methods for checking stress tolerance with appropriate tissue amounts, in large scale field experiment. It is possible that this activity might require specific tanks for subjecting plant to stress in a field experiment. This is most basic requirement for the work in consideration.

Through hybridization and transgenics we have succeeded in achieving some yield and further yield level could require enhancement on phytomass or biomass production. We have exploited 50% of the harvest index, the major increase will only be possible when the total biomass per ha will be accomplished. Even an increase by 15% of HI will yield 12 t/ha of rice. One way is to introduce gene(s) from alajta to australiens.

One rough estimate indicates that India shall have to import 20 mt food in 2020 and China shall have to import 216 mt as we are not attending to problems like water conservation, prevent misutilization of agricultural and for non-farm purposes.

It may be stated that in rice we are number one and in wheat we are number two; in pulses we are number one in peanut either for area or in production but in our productivity India’s number 32, in rice 51, maize 105 and pulses 118. As a plant physiologist it is our duty to seek answers for the gaps between production and productivity.

There is immediate need to improve productivity/ha or yield/ha. In most states e.g., Haryana majority of the farmers have small hoardings and the total produce may not be sufficient to market unless they have high yields. Plant physiologists and geneticists must, analyse the reasons for the yield gap and how to improve productivity.

In rice, Tamil Nadu, Punjab, and Haryana have an average yield of over 3 t/ha. Most of the Southern Asian countries have 4 t/ha or even more; China has 5 t/ha and Japan 6 t/ha. In India UP has 1.5 t/ha and same is true of Assam. We have to seek answers for the reasons for low yields. Bihar has enough water, Assam has high rainfall, these are green belts but without green revolution.

Reknowned food scientist, Dr. M. S. Swaminathan (1982) in his paper entitled ‘Biotechnology Research and Third World Agriculture’ has listed ten physiological factors for reducing the gap between actual and potential yields, with special reference to rice, though same is applicable to other Crop species.

These are: fast initial growth; low rates of maintenance respiration; total elimination of photorespiration; increased photosynthesis; increased sink size; greater assimilate partitioning; enhanced harvest index (HI); slow senescence during grain filling period; maintenance of healthy root system and short and stiff culm stature.

Further Dr. Swaminathan has emphasized the role of plant growth regulators (PGR) in increasing assimilate partitioning and maintenance of healthy roots system.

Plant Growth Regulators (PGRs) have seen a substantial upsurge as regards production and utilization in the last decade or so and have emerged as significant chemicals which could be profitably exploited to overcome the physiological constraints leading to increased crop production.

The performance of these chemicals is unprecedented and some of them are shown to circumvent many of the barriers which otherwise are imposed by environment or genetics. It may be stated that no area in plant sciences is filled with excitement, intrigue, fascination and fulfilment than this ‘biochemionization’ of the plants.

Among the various groups of agrochemicals the PGRs are youngest class of products following fertilizers, insecticides and herbicides and they form a small proportion of the total sales of the agrochemicals market world over. The global sale of the agrochemicals at present is about 20 billion US $ per annum and contribution made by plant growth regulators accounted for only 3-5% of the sales.

Different classes of Growth regulators, both natural and synthetic, their mode of synthesis and action have already been discussed in one the earlier chapters. Table 29-1 summarizes the chemical name, common name, commercial formula and action of individual PGR on various fruit crops and vegetables.

Empirical approach based on grain yield is rather difficult to breed genotypes which could withstand water stress environments since soil moisture varies annually. Thus physiological and biological criteria could be used to raise water resistant or drought tolerant plants. The search for new physiological traits is continuing and epicuticular wax is one such example.

Glaucousness is the Waxy covering which imparts adaptation to drought. Thus, several workers have used epicuticular wax (ECW) as an important factor for controlling cuticular transpiration (CT) and hence an effective method of screening varieties which are drought tolerant in different crop species.

The role of genes in controlling salinity stress and hence several parameters are suggested to adopt selection strategy for salt affected areas : identity salt tolerant germplasm e.g. mangroves; compare germplasm responses to identify a morpho-physiologic trait (s); calculate salt tolerance indices; estimate the levels of glycine-betaine or proline, etc.

Thus, it is desirable to compute salt tolerance in a holistic way and examine the behaviour and response of the genotypes under field conditions. Some attempts have also been made and the selection pressure employed at the pollen level assuming that genes expressed in the sporophyte are also expressed at the gametophyte level.

Another area which deserves special attention is understanding heat shock proteins and their distribution in the tissues experiencing heat shock or are subjected to high temperature stress. Several mechanisms have been proposed which affect salt tolerance and these include genotypic differences in salt tolerance of crop plants, regulation of salt transport to preclude salt accumulation in mature leaves.

Another mechanism concerns with salt exclusion e.g., wheat employs compartmentation and free transport of ions to the flag leaf. In wheat salt tolerance traits are contributed by the D genome and therefore, Aegilops squarrosa the donor of D genome appears to have much to contribute.

Similarly Agropyron elongatum is shown to contribute to salt tolerance in wheat. In plant genes responsible for production of osmoprotectants (osm genes) have been isolated and in bacteria it is possible to raise osmo-tolerant bacteria from osm-sensitive ones.

Micro-propagation has opened new avenues in agriculture industry where several fruit plants, timber trees, vegetables, medicinal plants are micro-propagated under controlled lab conditioned, hardened and then transplanted under field conditions. These are raised from meristems, apical buds, axillary buds or even single cells either through callus or directly from the explant.

In several instances production of micro-tubers as in potato, sugarcane, strawberry micro-progated plants have already reached the farmer’s fields and have added profits to his earnings. The technique has the advantage to raise the plants which are pure, disease free and elite.

In cardamom it has been possible to plant several hundred ha of land with tissue-cultured plants in states of Karnataka, Kerala, Tamil Nadu and has resulted in 40% increase in yield as compared with seedling derived plants.

Several Tissue Culture Pilot plant facilities have been established by the Deptt of Biotechnology, Govt., of India and subsequently they will be converted into micro-propagation parks to help in the production of millions of tissue cultured plants of poplar, teak, desert teak, eucalyptus in various states of India. Protocols for tissue culture of tea, coffee, cocoa, citrus, ginger, strawberry, potato, tomato, etc. have been developed.

Hardening facilities in Rajasthan, Orissa and other States shall produce more than several million tissue-cultured plants catering to the requirements of the States, especially the timber and horticultural plants. Currently, there are more than 70 registered tissue culture production units now, with 35 being fully operational and catering to domestic and export requirements.

Technology development and demonstration of a range of bio-fertilizers with more than 7000 experimental trials is in vogue, as many as 6000 farmers have been trained and more than 250 training programmes have been successfully carried out by DBT.

In paddy increase in yield by the application of blue green algae as bio-fertilizer has ranged from 13-17%. In pulses and oil seeds increase in yield had been accomplished through the application of Rhizobim. Tamil Nadu is leading in the use of BGA fertilizers.

Some of the plants are used are bio-pesticides and two pilot plants in Madurai and Coimbatore have been producing these pesticides and thousands of ha of land are using these bio-pesticides. Cloning of and sequencing of at least six genes related to storage proteins, disease and pest resistant has been achieved in India through the financing and co-operation of DBT, Govt. of India. Transgenic mustard, tobacco, chickpea and bt cotton are already being evaluated in the fields.

DNA finger printing technique is being used to decipher the quality of Bansmati rice against adulteration, evaluation of various fruit tree cultivars as in mango, ber, pear, grapes citrus, etc. Synthetic seeds are being raised in various plants like spinach, lettuce and even some expensive trees like santalum, etc.

Here quiescent embryos are enclosed in protective covering that contains an antibiotic, PGR, certain food reserves required for initial growth of the embryo. These encapsuled embryos are referred to as synthetic seeds and are ideal for storage, export, quick propagation and generally free from pests.

Plant physiologists of today visualize plants as small, less expensive and cost effective factories to churn different chemicals including antibiotics, flavours, colours, specialty chemicals including proteins, secondary metabolites, etc. under controlled conditions and free from the effects of environments, attack of pests and climatic variations. The chemicals thus produced shall be uniform qualitatively.

A wide variety of foreign products can be obtained from transgenic plants, not only proteins, but also all kinds of modified compounds through the expression of the appropriate enzymes or even catalytic antibodies. The production of biomass is inexpensive; the plant cell machinery can provide specific processing unavailable from microbial systems.

The isolated products are free from bacterial contaminants, the plant seeds can be used as a low-cost storage, and finally plants use sunlight .as their source of energy. These facts make us believe that the production of foreign compounds in plants is becoming a competitive alternative system to traditional methods.

The strategies for genetic engineering developed during the last decade or so have proved successful. Major breakthroughs for the engineering of metabolic pathways have been accomplished. Now the fine-tunning and product development is needed as well as a success of the new crops and products in the market is desired.

Present limits of engineering pathways will be overcome by a major contribution of gene technology itself. Transgenics will provide a tool for in vivo analysis of pathways leading to a far better understanding of a particular pathway and additionally the interaction of pathways.

Thus, by introducing genes from maize i.e. PEPc to rice has enhanced photosynthesis by several folds. The complexity of metabolic pathways would be gradually better understood and it should be possible to design desired alterations in end products in crops. Nonfood crops should easily acceptable, having altered traits of desire and need.

Genetic engineering appears to be the only technique that is expected to result in plants for production of renewable resources in sufficient quality and at low price. It may be stated that the demand of 200 million metric tons of mineral oil cannot simply be met by natural products. We need to exploit several different activities and biotechnology is one such discipline.

Role of biotechnology for the production of secondary metabolites has received a major impetus by the positive response of cells and tissues, roots, and shoots cultured in vitro. In some instances the level of production has been high, many times more than the source material. In future added attention shall be required for the production of spectrum of products and also to raise production levels.

Future studies will concern with the function of vacuoles, catabolic processes, and the response of plants to the varied environments. This knowledge will boost the capacity of cells for product accumulation.

Through genetic manipulation of production in plants, efforts could be shifted to the production of secondary metabolites in in vitro systems, and also alter the compounds. In all, plant biology especially plant physiology will come out ahead.

There will be return form bioreactors to the field. Green revolution is an high-input agricultural system and has left several undesirable effects in Punjab, Haryana and Western UP. Further high price of inputs has increased the price of the produce and poor farmers with small holdings find it unaffordable.

Consequently, there is a shift towards eco-friendly agriculture or organic farming which involves two major steps: use of organic matter instead of inorganic fertilizers; use of biological and herbal pesticides instead of more dangerous chemical pesticides.

Plant physiologists have major role to play in sustainability along with genetic engineering. These aspects include: eco-friendly agriculture; conservation of indigenous plant biodiversity; encouraging traditional system of agriculture; encouraging green technologies.

The exploitation of Plant Growth Regulators for increased productivity has been discussed already. In fact PGRs are being used for disease resistance (salicylic acid), suppress grass growth (inhibitors), suppression (fruit ripening delay) or enhancement of ethylene (quick ripening), induce flowering (as in hybrid rice) through GAs, regulate transpiration (ABA, antitranspirants).

Several research imperatives deserve deep studies by the plant physiologists and some of these are listed below:

i. Improvement in productivity of most crops has taken place by modifying physiological processes through genetic manipulation. Possibly while selecting for yield there has been unconscious selection for some physiological processes. Should it be possible to analyse such traits in a large segregating population, then it would be an advantage in crop improvement programmes.

As early as 1994 Evans called plant physiology a “retrospective science” and hence made plant physiologists hesitant to be ‘prospective analysts’ for crop improvement. Arabidopsis which can be easily managed in the lab can be used as a model plant for studying genetic variability and genetics of processes which are relevant to crop improvement. In fact chromosomes of this sp have been sequenced.

Thus the identification of the significant physiological, morphological and architectural components giving rise to high yield, good nutritional quality is environmental adaptation. The quantum jumps in yield accomplished in dwarf wheat and rice varieties have given rise to new conceptual ideotypes.

Thus understanding of and identification of significant physiologic and architectural components should help in evolving new strategies for increased crop yields. Such an information could be correlated with genetic traits.

ii. The efficient, inexpensive and novel techniques for screening genetically variable populations for biochemical-physiological yield components should be devised

iii. Breeding procedures should be evolved to ever-come physiological and genetical constraints to recombining components into a single or common genotype

iv. There is a need to add mission-oriented plant physiologist to the existing teams of plant pathologists, entomologists and breeders

v. The internal and external controlling timing, intensity and duration of flowering have a direct effect on grain yield. Genes affecting maturity, and the photoperiod influencing flowering should be identified. The effects on yield should be determined, and the mechanisms affecting yield should be investigated

vi. The factors affecting root architecture and development and their relationship to root functions in nutrient and water uptake is poorly understood. We need to know more on the effect of soil environment on root growth. Similarly relationship between varieties, stem growth and productivity both in dwarf and tall varieties should be deeply understood.

vii. More emphasis should be laid on stress factors e.g., high and low temperature, chemical regulation of growth and development in some cereals offers great scope for improvement of yield. The vagaries of climate, the soil types, organic proteins provide an ever-increasing altered background of stress conditions in rice and wheat and also cotton.

The impact of use of various weedicides on plants should be evaluated in regard to their effect on physiological traits and yield parameters. Understanding of physiologic and genetic bases of response to extreme soil traits will help breeders to evolve stress resistant varieties. Further information regarding genetic- physiologic factors controlling responses to population density would greatly enhance efficiency of selecting for high yields.

viii. At least in some crops e.g. wheat it has been demonstrated that the supply of assimilates is not the controlling factor for storage. Several workers have suggested hormonal regulation of sink capacity of grains. Same could be said of maize where large number of top grains remains unfilled.

Such an approach will explain the factor (s) causing differences in the grain size and weight of different spikelets (as in wheat) or grains situated on the top of the ear (as in maize).

ix. Exploitation of hormonal applications to overcome germination constraints and enhancing seedling vigour due to various stresses e.g., chilling, drought etc.

x. In a recent review Drs S.K. Sinha and Renu Kanna-Chopra (1999) have succinctly discussed crop improvement: interaction of physiology and genetics. The yield of crop is built through several processes and functions. The process of yield formation involves germination, growth, differentiation, development and senescence.

Each of these processes involve several physiologic and biochemical steps and reactions, and hence several genes must have been involved. Some workers claim of identifying quantitative trait loci (QTLs) for yield. Thus QTLs for heterosis have also been identified, but such loci influence physiological and morphologic events leading to yield has to be understood.

Presumably different organs of the plant act in sysnchrony leading to yield and yet maintain their individuality in terms of variation. Thus a plant may exhibit fast growth but may not have many spikelets to cause high yields. On the contrary a plant species may have large number of spikelets but poor leaf population to support their growth and hence there is poor yield. Thus we must subdivide a trait into components.

So, if we are looking for the genetics of dry matter production and wishing to breed for it, following components have to be borne in mind:

Dry matter (total assimilates-respiratory loss), could apply to any part of the plant (leaf, stem, panickle, grain, etc.). Total assimilates also has two components (photosynthetic surface and rate of photosynthesis) and these are related with each other.

Rate of photosynthesis is linked with photosynthetic surface of leaf, stem, spikelet and grain. The assimilate partitioning is equal to assimilation x K where K is the partitioning to non-photosynthetic organs like roots etc.

As the plant grows the value of K would alter as in groundnut. Figure 29-3 shows precise relationship between various components of photosynthesis. The genetic basis of some of these components is known but for other components genetics is hardly known. We need to re-examine the role of Rubisco in total assimilate. It may be added that enhanced rate of photosynthesis may not imply enhanced yield.

More than the rate of photosynthesis, it is significant to have high photosynthate tanslocation to the sinks. One study has indicated that in wheat chromosome 7D is responsible for the low photosynthesis of flag leaves under high irradiance.

Some correlations have been made as in maize between leaf area at anthesis and grain yield but genetics of leaf area is not clear and hence manipulation of leaf area in the post anthesis period is impossible. In wheat yield improvement is correlated with increase in stomatal conductance. Possibly the latter contributes towards maintaining cool canopies and high rate of photosynthesis (Fig. 29-4, 5).

Carbon assimilation is complemented by the uptake and use of other nutrients e.g. nitrogen, sulphur, P,K, Ca, Mg, iron, Zn, Cu, Mn, boron, etc. for the accumulation of dry matter. Genetic basis of variability in uptake of these nutrients by some genotypes has to be detailed. Thus, decline in the yield of wheat and paddy in Punjab and Haryana is attributed to deficiency of several micro-nutrients.

It is highly essential to evaluate crop species for tolerance to micronutrients deficiencies. Sunflower and triticale are found to be more efficient to micro-nutrient deficiencies. There is a dire need to evaluate germplasm for more efficient genotypes for a particular element, either for its deficiency or its toxicity.

Initiation of flowering, and its profuseness are related to yield components and yield. But in some corps e.g. potato it is not desirable vernalization and hormonal factors and hence its genetics appears to be complicated. However, it is possible to regulate the duration of crop for increasing productivity per unit of time. This permits hybridization, adaptation to environmental factors such as water and temperature.

Genetic information on flowering should be used to develop varieties or hybrids of specific duration. Several mutants for flowering have been reported in Arabidopsis for characterization of genes for the various events in flowering including those controlled by photoperiod, gibberellic acid and other factors. It will be profitable for the crop physiologists and plant breeders to take advantage of this information.

The breeding of an ideotype involves combination of several functional relationships and for details one may refer to a discussion by Sinha (1990). Emerging from his conclusions it is clear that it is essential that a phenotype and morphological ideotype is required for functioning. Modelling techniques are available and could set the limits of productivity in a given environment but also help in designing an ideotype.

In summary, it may be mentioned that a greater relationship among the disciplines of genetics, molecular biology, biochemistry and physiology is essential for future advances in crop improvement.