Autoflowers, Their Time to Flower and How To Maximise Yields in Small Containers - by Zer0Tolerance

Howdy Meph Heads!

Welcome to the first of our new series of blog posts guest written by members of our Tester team. In this series, you'll hear from a diverse group of growers all with different prospectives and methods but with the same end goal; growing something they're proud of. Whether you like to keep it simple or get down to the nitty-gritty and comb over the finer details, hopefully, you'll find the voice here that matches your vibe. 

Our first in the series is written by Zer0Tolerance, who tackles some of the science behind the real triggers behind why autoflowers flower and how he manages to pull 4oz consistently from tiny container grows!

If you want to see more of what Zer0Tolerance has achieved, along with his stunning photography skills, you can find him on Instagram @zer0tolerance0

Tester's Perspective

BLUF

The ratio between Florigen (FT) and Anti-Florigen (TFL1) controls whether the plant continues vegetative growth or transitions to reproductive growth. This can be modified with root expansion, nutrient and water availability, root zone temperature (RZT) control, Nitrogen (N) availability, and Phosphorus (P) reduction, 

Purpose

Examine and describe the correlation between various growth parameters to maximize vegetative growth in day neutral plants prior to reproductive growth.

Background

It's a common misconception, and thrown around often, that autoflowers begin flowering “when the taproot hits the bottom of the pot.” However, what causes an autoflower to transition from vegetative growth to reproductive growth is far more complex.

Disclaimer - I'm just a self-taught basement botanist. Most of my information comes from theory to practice and studies from sources like nih.gov. I tried to minimize all the fancy talk, but some of it is needed to explain why each part matters. 

Glossary of terms/Assumptions

Florigen (FT) - Systemic plant protein signal produced in leaves that travels through the phloem to the shoot apex, triggering reproductive growth.

Primary Anti-Florigen (TFL1/CEN) - Floral inhibitors that counteract florigen to maintain vegetative growth. 

Phosphatidylethanolamine-binding protein (PEBP) - the gene family of Florigen and Anti-florigen like genes.

LEAFY gene (LFY) - Regulator gene that converts meristems to flower production, versus leaf production.

Cytokinins - class of plant growth hormones that promote cell division, differentiation, and shoot development. 

Callose - carbohydrate that physically plugs the plasmodesmata (PD)

Introduction

FT and TFL1 compete to determine whether the plant shoot tip continues vegetative growth or shifts to reproductive growth (flowering). Both are in the PEBP protein family and share the same binding partners (FD protein) at the shoot apical meristem (SAM). Initially, TFL1 occupies FD sites in the genome, creating vegetative growth. As FT increases, it begins displacing TFL1 on the FD sites. When FT binds to FD, it forms the Florigen Activation Complex, which turns on flowering genes like LFY.  Conversely, when TFL1 binds to the same FD, it forms the Florigen Repression Complex, leaving the flowering genes turned off. 

This competition happens directly in the plant DNA (chromatin). TFL1 occupies thousands of FD target sites in the genome to suppress reproductive development. As levels of FT increase, the FT molecules physically displace TFL1 from those FD sites and a shift occurs on the LFY, triggering flower growth. If there is a high FT/TFL1 ratio, the stem terminates and flowers begin. If there is a high TFL1/FT ratio, the plant keeps growing and shooting branches. The goal is to maintain a high TFL/FT ratio, minimizing stress triggers, and extend vegetative growth, repressing reproductive growth.

Using the methods discussed in this article, this 2oz autoflower flourished and remained healthy through the entire grow. Final yield was approximately 20g. Max DLI 36, temps 65-70F, continuous drip feed 1.5-3EC.

Discussion

Photoperiod plants have a molecular brake that stops the production of flowering until the day/night ratio is met and the brake is released (such as protein from circadian clock genes like PRR37). Autoflowers have no such brake and depend on the FT/TFL1 ratio to determine vegetative or flowering growth. Ruderalis is a product of its environmental conditions. Faster lifecycles allowed its reproduction cycle to be completed before the end of the season. 

Roots are the primary synthesis site for cytokinins. As roots expand, they deliver more cytokinins to the shoot. High cytokinin levels are correlated with the expression of TFL1 and repression of FT. Extended root growth shifts the plants hormonal balance toward TFL1, favoring vegetative growth. This causes a high cytokinin to auxin ratio, stabilizing the anti-florigen repression complex at the shoot apex. 

The plant's primary purpose is to drive its own reproduction. Stress responses, such as resource limitation, often signal FT production. Restrictions to root growth trigger a spike in abscisic acid and a reduction in cytokinins. This signals the plant to flower, ensuring seed production before end of life. Likewise, if roots continue to find resources, it delays the metabolic shift toward FT synthesis, keeping the FT/TFL1 ratio low. 

The best small container method I've found has a mesh bottom (nursery bag material as shown) and a small gap between the fabric and solid container. This ensures proper drainage, air to roots, and continued root expansion via air prune. In this photo, observe the bottom of the rootball. The roots are not compacted, but free to grow and self prune.

Additionally, extensive root systems enhance nitrogen (N) uptake. High N levels are known to upregulate anti-florigenic genes. High N also drives an increase in leaf surface area, diluting the amount of FT reaching the meristem. While FT travels from leaf to apex, a signal also travels from the roots to apex signaling to maintain vegetative growth. The larger the root complex, the larger the signal that's sent, maintaining dominance in the florigen repression complex.

Root zone temperature (RZT) also plays a role due to the way cell membranes physically control the rate at which the different molecules can transfer between them. At lower temperatures (<60°F), the FT protein binds more easily to phosphatidylglycerol (PG), physically keeping the FT in the leaves longer before transport to the shoot apex. At higher temperatures, the FT protein is released in soluble form, allowing its mobilization and trigger flowering. The speed that the FT signal is sent is directly tied to the gate permeability between cells. Permeability can be significantly reduced as cold temperatures trigger the production of callose, delaying florigen delivery to the SAM. Higher temperatures can cause a signal to be sent to the shoot tip to counteract the incoming FT so that reproductive growth doesn't start when temperatures don't support pollen viability, 

Temperature controls the speed of the signal, while nutrients (specifically Nitrogen and Phosphorus) control the volume and receptivity of said signal. Nitrogen is the primary driver for TFL1. A combination of high N and high RZT maximizes vegetative growth. The warm roots more readily uptake nutrients and high N upregulate TFL1, which physically competes with incoming FT at the shoot apex. When N is restricted, this triggers a stress response that downregulates TFL1. This causes an increased sensitivity to FT, allowing even lower amounts of FT to trigger flowering. 

Phosphorus is a FT amplifier and is essential for cell division and ATP production that's required for flowering. Higher RZT increased solubility and P uptake significantly. Although P doesn't make more FT, it makes the SAM more sensitive to it. If the RZT is too low, P uptake slows way down so even if the leaves are producing a lot of FT, the shoot tips lack the P to complete the transition and flower stalls. 

If roots sense a change in moisture or nutrient availability, they can send variation potentials to the shoot. This alters the membrane polarity of cells along the phloem, making the phloem more permeable, increasing the transport of FT to the shoot. In day-neutral plants, the electrical signals from the roots can be stronger and more frequent, bypassing the need for a specific light trigger to begin FT synthesis. 

Strategy

• Maintain high frequency/ low volume/low EC fertigations, signaling abundance. This creates high cytokinin production, high N uptake, and keeps TFL1 high. 
• Maintain low P (Phosphorus) concentration until flowering has commenced. 
• Maintain optimal temperatures (65-72°F)
• Maintain container bottom free of obstructions (mesh) to ensure minimal root restrictions.
• Maintain root saturation and PREVENT heavy drybacks during the vegetative cycle.

Conclusion

Over the last two years, I practiced these principles to maximize efficiency in my small container autoflower grows. These plants are more efficient than we give them credit for. I would have never thought that I could get over 4oz yield from a 1/4gal container or upward of an ounce from a 2oz container (with an autoflower). But, by following these principles, that became a reality and I think there's even more room to improve. 

Sources

Regulation of Flowering Time by Environmental Factors in Plants

The Divergence of Flowering Time Modulated by FT/TFL1 Is Independent to Their Interaction and Binding Activities