The physiological factors limiting yield are those that determine how efficiently crops convert the limited resources of carbon dioxide & light into carbohydrates & how much of these carbohydrates moves into storage organs that form useful part of the crop.
2. Cereal grains are the major food of mankind.In
many of the developed countries of the world,
cereals provide two third of dietary calories.
Rice in Asia, maize in South America, sorghum
in Africa& wheat in middle east are recognized
as the staple food on whose yield, famine or
feast depends.
A high grain yield of any crop can be achieved
only with a proper combination of variety,
environment and agronomic practices.
3. Analysis of cause and effect relationship in crop grain
yield is extremely complex(Yoshida 1972) .
Improved agronomy has contributed greatly to the recent
increase in cereal yield , and has better control of disease
& pests whether by genetic or chemical means.
Plant breeding has played major role in two ways, by
selection of disease & pest resistance varieties & by
selecting cultivars with greater yield potential which can
response to higher inputs.
4.
5. The physiological factors have been studied
experimentally & theoretically using mathematical
models.
The useful part of the major crops such as cereals,
potatoes, sugar crops are composed almost entirely of
carbohydrates & water.
The physiological factors limiting yield are those that
determine how efficiently crops convert the limited
resources of carbon dioxide & light into carbohydrates &
how much of these carbohydrates moves into storage
organs that form useful part of the crop.
6. Leaf area is frequently correlated with growth &
yield.
The relevant measure of leaf area in relation to
the final yield of total dry matter of most crops is
the integral of leaf area index over the whole
growth period.
LAD( leaf area duration) will increase if LAI
increases faster at the beginning of growth
period,or if LAI declines more slowly at the end
of the growth period .
7. Increasing leaf area early in the growth would be
best except that cereals with longer survival of
leaves should be available.
Yield is likely to increase with maximum LAI
above 3-5 for sugarbeet and potato or above 10
for cereals.
Higher grain yields are possible only when
maximum leaf area index and net assimilation
rate occurs by flowering stage.
8. When LAI is large , crops with upright leaves
should photosynthesise faster per unit of leaf area
than crops with horizontally disposed leaves.
Species without photorespiration grow and
photosynthesise faster at higher temperature and
light intensities than those with photorespiration.
Increasing yield potential by increasing
photosynthesis efficiency & sink size are the most
popular approaches.
9. However large the total dry weight of the crop,
agricultural yield will depend on the distribution
of dry matter.
Harvest index is was positively correlated with
grain yield , but negatively with vegetative
growth.
A high degree of variability was found in the
harvest index of winter wheat & other cereals.
Stem contributed the greatest amount of dry
matter.
10. DENSER CROPS:
The higher LAI values would reduce net
photosynthesis by the crop due to respiration by
lower shaded leaves exceeding their
photosynthesis.
Both dark respiration and net photosynthesis
reach a plateau as LAI increases, so does crop
growth rate.
11. FASTER GROWTH & DEVELOPMENT:
Greater use of fertilizers & better control of diseases and
weeds allows crops to grow more rapidly.
Faster establishment & vegetative growth allow the
young crop to intercept more radiation but this
advantage is lost once the canopy has closed.
Rapid development in the reproductive phase, b/t ear
initiation and an thesis, has drastic effect on yield
components in wheat.
12. Human existence & civilization have always
depended on an adequate supply of food.
The challenges are to ensure that “farming will
function efficiently & thus be a profitable
profession’’. Obviously the challenge ahead
involves improvement in the production
efficiency.
13. This may contribute to improvements in crop
productivity by the following 2 factors:-
1.Increasing total biomass production: Yield was
positively and highly correlated with total biomass,
harvest index and grain straw ratio and was concluded
that the yield was directly dependent upon the total
biomass production.
2.Favouring assimilate transfer to the harvestable
portion of the crop.
14. ENVIRONMENTAL EFFECTS ON ASSIMILATE
PARTITIONING:
The realization of potential capacity of the crop to
partition assimilates efficiently to harvest organs is
determined by the environment .
Management programmes based on an understanding of
assimilate partioning responses' to various environmental
factors can lead to improvements in crop productivity.
They are as follows-
1- Effect of light.
2- Effects of water stress.
15. The science of chemistry & chemicals have been an
essential element in achieving the remarkable increases
in agricultural productivity.
Crop growth regulators:
a- Growth promoter( auxins, gibberellins, cytokinin).
b- Growth inhibitor(MH, ABA, ethylene) .
Nitrogen: The input of N is essential to achieve the
current level of crop productivity.
Water: A shortage or an excess of water is the most
limiting factor in the crop production.
16. Each crop has its own growth and yield characteristics
& hence its own physiological limitations.
In varieties having low drought tolerance , grain weight
was more dependent upon the availability of the post-
anthesis dry matter per grain.
Spikelet sterility is a major constraint is rice production
as it influences grain yield by determining the number
of filled grains per unit area.
17. Pulses are characterized by low productivity and high
instability. Therefore , it is necessary to understand the
basis of yield in these crops to evolve suitable strategy
for their improvement.
Sinha (1977) enumerated several possible factors which
may be influencing sink realization due to flower bud or
fruit drop:
1- Limitation of photosynthesis.
2- Reduced plant intensity in plant canopies.
3- Canopy temperature.
4- Humidity in crop canopies.
5- Soil and water factor.
18. To achieve next breakthrough in productivity like the one
of semi dwarf types over the traditional indigenous types,
there are very limited possibilities in the near future.
The breeders will have to concerned with marginal
increases in productivity through systematic & judicious
combination of components of yield.
Further avenues may be opened by fundamental
researches in the field of genetics & physiology of crop
plant.
19. In cereals the borderline for a further increase in
the harvest index may be reached in near future.
Water use efficiency should be improved by
decreasing mesophyll resistance and by
achieving a better understanding of control of
mechanism of stomata.
