Physiological parameters like water use efficiency, nutrient use efficiency, assimilate partitioning, harvest index, and respiratory efficiency can impact crop yield. Water use efficiency measures how efficiently a crop converts water into biomass or grain. Nutrient use efficiency measures how efficiently a plant transforms nutrients acquired into economic yield. Assimilate partitioning and harvest index influence how photosynthates are distributed among plant parts or converted into economic yields. Respiratory efficiency impacts crop yields by reducing biomass losses from respiration. Improving these physiological parameters can enhance crop water productivity and yield potential.

1. Physiological
Parameters
affecting crop yield

2. Water use efficiency
• Water use efficiency (WUE) is the measure of a cropping
system’s capacity to convert water into plant biomass or grain.
• Crop water use efficiency: the efficiency with which an
individual crop converts water transpired (or used) to grain.
• Crop WUE (kg/ha/mm) = grain yield (kg/ha)
crop water supply (mm) – soil evaporation
• WUE can affect the yield potential of the crop.
• WUE is a measure of crop’s ability to extract stored moisture
from the soil
• This is a physiological property as WUE is also a measure of
plants ability to convert biomass into grain i.e. Assimilate
Partitioning

3. Nutrient Use Efficiency
• Physiologically it is defined as the yield increase in relation to
the increase in crop uptake of the nutrient in above-ground
parts of the plant.
• It is the ability of the plant to transform nutrients acquired
from the source applied into economic yield
• It is the ratio of total change in yield upon application of
nutrient and the total amount of nutrient added
• Partial factor productivity (PFP) is a simple production
efficiency expression which is a long term indicator of overall
cropping systems ability to transform the biomass into a more
economic form i.e. grain
• PFP can be calculated for large areas and comes under
fertilizer policy issues as well

4. Assimilate Partitioning
• It refers to distribution of assimilates i.e. photosynthetic
products to various plants parts or more technically the sinks
• There may be many sinks in a plant and many a times an organ
may behave as a source and as a sink at various times of plant
growth
• Biological and economic yield of "crops" can be influenced by
interactions of source and sink
• Sink demand for assimilate (i.e., sink strength) influences the
photosynthetic rate of source leaves. Sink strength is a
product of sink size and sink activity
• Highest yield will be obtained when there is a balance
between source and sink

5. Harvest Index (HI)
• Harvest index is the proportion of the aboveground dry matter
at physiological maturity that is allocated to the economic
product (e.g., grain in maize or wheat).
• It is the ratio of Economic yield to the Biological yield
• Maximum harvest index varies among crop species (e.g., 55%
for maize hybrids vs. 20% for canola varieties), among
genotypes within a crop species (e.g., 55% for North American
maize hybrids, 40% for tropical maize cultivars, and 1050% for
maize inbred lines), and among ‘environments’ in which it is
measured (e.g., harvest index is usually negatively affected by
biotic and abiotic stresses)
• Provides an indication of a crop’s ability to convert plant
biomass into grain yield.

6. Harvest Index (HI) Contd..
Fig-HI of some important crops
• Improving harvest index has been
critical to advancing the yield
potential of some important
cereal crops
• Harvest index in grain crops is the
result of partitioning of dry
matter to the grain during the
grain filling period, either from
net canopy photosynthesis during
the grain filling period or from
remobilization of pre anthesis dry
matter accumulation

7. Respiratory Efficiency
Fig-
• A significant amount of the CO2
fixed by photosynthesis is
respired to produce the energy
needed for production of new
organs and maintenance of old
ones. This is often termed a
“cost”.
• Costs associated with growth and
maintenance of crops can be
represented as biomass
equivalents
• One of the physiological
interventions is to reduce the
respiratory losses i.e. to increase
the efficiency of respiration for
better yield

8. Aerobic Rice
• Aerobic rice is a production system where rice is grown in
well-drained, non-puddled, and non-saturated soils
• A fundamentally different approach is to grow rice like an
upland crop, such as wheat, on non flooded aerobic soils,
thereby eliminating continuous seepage and percolation. This
also reduces evaporation
• Aerobic Rice cultivars have
1. Vigorous seedlings
2. Rapid biomass development
3. Deep roots
4. Erect leaves
• They have a high HI even under moderate stress

9. C4 Rice
• This is the most ambitious
project of IIRI in association
with Bill and Melinda Gates
foundation
• It basically aims at
transforming the convention
C3 system in rice to a
Physiologically more
successful C4 from
• Identification of most of the
genes and machinery
development is over and now
they are heading towards
transformations