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Garden with Insight v1.0 Help: Plant next day functions: decide if should move to next stage of growth

The pattern of growth throughout life typical to a plant species is called its life history. This simulation models plant growth in six possible stages: seed, vegetative period, vernalization, floral induction, floral initiation, and reproductive allocation. We will go through the meanings of these stages and the transitions between them in turn. Note that the entire reproductive model is not in EPIC and was added.

In the seed stage, the plant (actually an embryo) is dormant. Seeds can survive for various lengths of time depending on the species and conditions. In this simulation, if a seed does not germinate during the first year it is in the soil, it may or may not survive until the next year. The probability of an ungerminated seed surviving in the soil until the next year is a plant parameter. Also, if the mulch on the soil patch is above an arbitrary amount (10 metric tons per hectare), seeds in their second year cannot sprout. Reseeded plants are not really ever seeds (since it is magic), so these limits don't apply to them.The process by which a seed becomes a seedling is covered in more detail in the section on germination.

Vegetative period
During the vegetative period, the plant puts no resources into reproductive growth and is not responsive to any environmental triggers of reproduction. The vegetative period is delimited by the heat unit index, and the HUI at the end of the vegetative phase is a plant parameter. The value for most plants is between 0.1 and 0.2.

Floral induction
During the floral induction period, the plant is still growing vegetatively but is responsive to environmental cues that trigger (or induce) flowering. You could say that during this period the plant is listening to its environment to find out when it should flower. The most common triggers of flowering are photoperiod (day length) and temperature. We simulate both triggers here by using accumulation of photothermal units (of both light and temperature) towards floral induction. The plant has parameters for the optimal, minimum and maximum temperatures for floral induction; an S curve of how the hours of darkness affect flowering; and the number of photothermal units required for floral induction to be complete. On a day with the optimal temperature for floral induction and the best photoperiod, one floral induction unit is accumulated. Deviations from the best accumulation on any day are based on a sine curve around the optimal temperature and on the S curve of hours of darkness. The thermal and photoperiod units accumulated in any day are multiplied to get the combined photothermal units.

Plants fall into three general groups based on their response to photoperiod: 1) Short-day (long-night) plants require long dark periods, usually more than twelve hours, to flower. These plants will have an ascending S curve. 2) Long-day (short-night) plants require short dark periods, usually less than twelve hours, to flower. These plants will have a descending S curve. 3) Day- neutral plants do not respond to photoperiod. These plants will have a flat S curve. Most plants have at least a little response to temperature whether or not they respond to photoperiod.

Floral initiation
When the required photothermal units for floral induction are accumulated, the plant moves into the floral initiation phase. Floral initiation is the stage when the plant has decided to flower and is making the biochemical changes necessary to create inflorescences, flowers and fruits. The length of the floral initiation period depends largely on temperature in many plants. We model it in nearly the same way as floral induction but only with temperature -- as thermal units. The same temperatures are used for floral induction as for floral initiation. This is not necessarily because the same temperatures are required, but because we don't have data for floral initiation temperatures. On a day in which the optimal temperature for floral initiation exists the plant accumulates one thermal unit. When the required number of floral initiation units are accumulated (a parameter), floral initiation is complete.

Reproductive allocation
The reproductive allocation period is when the plant has decided to flower, has initiated flowering, and is actively moving resources into floral structures as they become available from photosynthesis. The allocation of new biomass to reproductive structures is covered in the section on biomass allocation.

In many biennial plants, which flower in their second year of growth, cold temperatures are used as a cue that the required time has passed and conditions are now favorable for flowering. Some plants can be made to flower early or late by applying or denying these cold temperature cues. Cueing on cold temperatures is called vernalization (vernal means spring, and these plants often flower in the spring). In this simulation we model vernalization in the same way we model floral initiation, by accumulating thermal days based on an optimal, minimum and maximum temperature. Of course the best temperatures for vernalization are very low, usually in the range of zero to five degrees Centigrade.

Plants can either require vernalization obligately, in which case they will never flower unless they experience the required cold temperatures, or they can use vernalization quantitatively as a cue but still flower eventually if the vernalization signal is never received. Few plants require vernalization obligately.

In this simulation, if a plant requires vernalization obligately and the required period of cold temperature has not been experienced by the time the plant is one year old, it enters a stage called "failed vernalization" and flowering never occurs. If the plant does not absolutely require vernalization, after one year the accumulated cold temperatures are compared to the vernalization requirement (a parameter). The floral induction requirement is increased by the ratio of required to achieved vernalization units. This means that if a plant acquires all its vernalization units it begins floral induction normally, but if it acquires (say) half of its vernalization requirement, its floral induction will take twice as long.

calculation of germination, heat unit index, biomass allocation to plant parts, accumulation of vernalization thermal days
More on the flowering and fruiting submodel
More on the biomass partitioning submodel
Model contents

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Updated: May 4, 1998. Questions/comments on site to
Copyright © 1998 Paul D. Fernhout & Cynthia F. Kurtz.