Production : Growth and development
In the previous section we ended with the daily crop growth rates
Net assimilation is the result of the gross assimilation minus the respiration
Net assimilation is more or less equal to crop growth, growth relates to increase in dry matter
But how is this dry matter distributed over the different plant organs, roots, leaves, stems and storage organ?
This is determined by the development of a crop, as we will see, development is genetically determined and modified by external factors
When the supply of assimilates increases evenly over the season, all plant organs benefit
When the genetics favour leaves above storage, more assimilates are directed to the leaves and less to the storage organs
Although the development is different, the net amount of assimilation used for growth remains the same
It then depends on the composition of the organs how much dry matter is actually produced
Remember that per unit of assimilates more carbohydrates can be formed than fats
So far we have looked at a growth rate of a hypothetical crop, we assumed that it could intercept all light and grow at a maximum rate during the whole growing season
The part of the year a crop can really grow depends on the climate, on factors such as temperature and light, and on crop genetics
During the growing season a crop passes through different phenological stages, indicated in the bottom figure
The vegetative stage is from emergence, development stage 0, to flowering, development stage 1
The reproductive stage is from flowering to ripening, development stage 2, as indicated on the x-axis
The middle figure gives the cumulative growth of the crop in kg DM per ha and the top figure represents crop growth rate
Crop development and crop growth will be linked between in these figures
First the seeds germinate, roots are formed and penetrate the soil, then leaves emerge and expand
The area of the leaves determines the fraction of the light that can be intercepted, just after emergence this is very low, as only small leaves are present
As soon as the root system has developed to ensure uptake of water and nutrients, more dry matter will go to the leaves and a small part to the stem
It takes quite some time before full ground cover or a closed canopy is reached and until that moment daily growth rate will increase exponentially and most dry matter is allocated to the leaves
At canopy closure the maximum growth rate is achieved, all light is intercepted and used for photosynthesis
A fast closure of the canopy can increase total seasonal production, as the crop catches more light, the crop is still in the vegetative stage
At some point in the development plants make a transition from the vegetative to the reproductive stage
From this moment on part of the dry matter is distributed to the storage organs
This can for instance be grains or tubers, sometimes this stage is initiated by flowering or anthesis, such as in grain crops
Other crops do not need flowering to form storage organs, such a potatoes and carrots
The investment in storage organs is a survival strategy for unfavourable growing conditions
Subsequently fruit setting starts, followed by fruit filling, the leaves lower in the canopy hardly contribute to assimilation anymore
Old leaves are aging and die, you see the leaf biomass decreases a little bit
Senescence occurs at suboptimal temperatures or under stress, for instance due to drought or nutrient shortage
The green chloroplasts disintegrate and nutrients, such as nitrogen can be reallocated to the storage organs, for instance in grains
The yellow leaves intercept light, but cannot contribute to assimilation anymore
Finally all leaves die, think of a maize field, with ripe cobs, all vegetative plant parts are dead
This substantially reduces the respiration losses, the storage organs mature and can be harvested
Adding daily growth rates over the growing season gives the total dry matter production per ha
The growth period lasts from emergence to harvest and is determined by the development rate
In the figure you can see that maximum growth rate is only realised during part of the growing season
A high development rate leads to a shorter growth period and generally to a lower overall production
In this example the annual production is 20 000 kg of dry matter per ha
The timing of the growth period is also important for total production. Low development rates, for instance due to a low temperature, may hamper fruit filling or maturation
This can lead to a lower production or a lower quality of the product