The Daylength Decoder
How Lloyd Evans Unlocked the Secrets of Flowering
For centuries, how plants "decide" to flower was one of botany's greatest mysteries. Enter Lloyd Thomas Evans, a plant physiologist whose meticulous work uncovered the dual hormonal control system that governs this critical transition.
Discover the StoryThe Invisible Switch
Imagine if a single long summer evening could trigger an entire field of wheat to flower, determining the success or failure of a harvest. For centuries, how plants "decide" to flower was one of botany's greatest mysteries. Scientists knew that daylength was crucial, but the inner workings of this delicate timing mechanism remained a black box.
Enter Lloyd Thomas Evans, a plant physiologist whose meticulous work would uncover the dual hormonal control system that governs this critical transition. His journey, spanning from the rivers of New Zealand to a revolutionary laboratory in Canberra, not only decoded a fundamental process of life but also provided the scientific tools to help feed a growing global population 2 4 .
A Life Rooted in Nature
Childhood in Wanganui
Born in 1927 in Wanganui, New Zealand, meaning 'the long river' 3 .
Inspirational Reading
A book by Sir John Boyd Orr on world food situation changed his career path 3 .
1927
Born in Wanganui, New Zealand. His childhood was steeped in the natural world, with the Wanganui River as his backyard 3 .
Leaving School
Encountered a book by Sir John Boyd Orr revealing that one-third of the global population was grossly underfed. This steered him away from conventional forestry and toward a career in agriculture 3 .
University
Excelled at the University of New Zealand's Canterbury College, earning top marks that could have secured him a place in medicine, but he remained undeterred in his new mission 3 .
"His passion for botany was further fueled by mountaineering trips through New Zealand's varied subalpine plant communities, observing firsthand how plants adapt and survive." 3
The Flowering Puzzle: A Scientific Mystery
Before Evans' work, plant scientists were locked in a debate over the mechanism of flowering. It was clear that leaves perceived daylength and sent a signal to the shoot apex, where flowers form. But what was the nature of this signal? The field was divided between two competing hypotheses 2 4 :
Russian physiologist Mikhail Chailakhyan proposed that in long days, leaves produce a universal flowering hormone, which he called "florigen," that travels to the apex and switches it on 2 .
The scientific community needed a clear, experimental verdict. Evans found the perfect partner for this investigation: an unassuming grass known as Lolium temulentum, or darnel ryegrass 2 . This plant, the "tares" of the Bible, was a wonderful experimental subject because it would flower reliably in response to a single long day 2 8 . This precision allowed Evans to dissect the flowering process with unprecedented accuracy.
An Ingenious Experiment: Isolating the Signal
To crack the code, Evans designed an elegantly simple yet labor-intensive experiment. He grew plants of Lolium temulentum in short-day conditions (with darkness beginning at 4 p.m.) for five to six weeks, keeping them in a vegetative state. Then, he would subject just one leaf on a plant to a long day (with light until midnight), while carefully wrapping all the other leaves in aluminium foil to keep them in a simulated short-day environment 2 4 .
The key was the timing of his next steps. By cutting off either the single long-day leaf or the shielded short-day leaves at different times after the long-day treatment, he could observe the effects on flowering. The results were definitive 2 4 :
- When he removed the single long-day leaf shortly after the long day, flowering was reduced.
- Conversely, when he removed the short-day leaves, flowering was enhanced.
The conclusion was clear: both processes were operating simultaneously. The long-day leaf was producing a positive, "go" signal, while the short-day leaves were producing a negative, "stop" signal. Flowering was the net outcome of this push-and-pull interaction 2 4 . This discovery settled the long-standing debate and established a more complex, and accurate, model of plant development.
Evans' Experimental Logic and Results
| Condition of Leaves | Experimental Manipulation | Flowering Outcome | Interpretation |
|---|---|---|---|
| One leaf in long day; others in short days | None | Flowering occurs | Long-day signal overcomes short-day signal |
| One leaf in long day; others in short days | Remove long-day leaf early | Reduced or no flowering | Positive "go" signal was removed |
| One leaf in long day; others in short days | Remove short-day leaves | Enhanced flowering | Negative "stop" signal was removed |
Research Reagents and Solutions in Evans' Flowering Studies
| Tool | Type | Function in the Experiment |
|---|---|---|
| Lolium temulentum | Model Organism | A grass that flowers in response to a single long day, providing a precise biological system for study. |
| Photoperiod Control | Environmental Cue | Manipulating light duration (long vs. short days) to trigger the flowering response in leaves. |
| Gibberellins | Plant Hormones | A group of over 150 related compounds; Evans worked to identify which specific gibberellins could trigger flowering. |
| Aluminium Foil | Physical Barrier | Used to meticulously wrap leaves, isolating them from light to create localized short-day conditions. |
The Hunt for the Flowering Messenger
Identifying the positive "go" messenger became Evans's next great quest. A crucial clue came in 1956 when his colleague Anton Lang showed that a newly available plant hormone, gibberellic acid, could induce flowering in some plants 2 . Evans immediately tested it on his Lolium plants and was thrilled to find that it made them flower in short days 2 .
However, the story was not so simple. There are over 150 different gibberellins in plants, and only a select few are effective in promoting flowering 2 4 . In a decades-long collaboration with chemist Lew Mander, Evans worked to define the exact structure of the active hormone. They discovered that the placement of a single hydroxyl group on the complex gibberellin molecule was critical: on carbon 2, it blocked flowering; on carbon 3, it promoted it 2 4 . This painstaking work moved the field from the concept of a mythical "florigen" to the precise biochemistry of specific, active gibberellins.
Evans' Findings on Gibberellin Structure and Function
| Structural Feature of Gibberellin | Effect on Flowering in Lolium | Scientific Implication |
|---|---|---|
| Hydroxyl group on Carbon 2 | No flowering | Tiny changes in hormone structure can completely alter biological function. |
| Hydroxyl group on Carbon 3 (and not on C2) | Very good flowering | The plant's flowering receptor is highly specific for certain gibberellin forms. |
| GA1 & GA4 (common forms) | Promote stem elongation, but not effective for flowering | Different plant processes (e.g., elongation vs. flowering) are triggered by different gibberellins. |
The Phytotron: A Cathedral for Plant Science
Evans's research required an unprecedented level of control over the plant environment. He was recruited to CSIRO in Canberra by Sir Otto Frankel precisely to help design and build an Australian version of the phytotron he had worked in at Caltech 2 4 . This facility, named CERES, was a revolutionary "cathedral for plant science" 5 .
The phytotron was not merely a set of greenhouses. It was a controlled environment research facility where temperature, humidity, light intensity, spectral quality, and daylength could be precisely regulated and reproduced 2 5 .
This allowed Evans and his colleagues to create everything from a simulated tropical summer to a brisk autumn day, all within the same building. By holding all environmental variables constant except one, scientists could finally isolate the specific effect of, say, a single hour of extra light. The phytotron was the indispensable tool that made the rigorous, repeatable experiments of modern plant physiology possible, and Evans was its chief architect and champion in Australia 2 4 6 .
From Laboratory to Global Harvest
Evans's curiosity extended beyond flowering. He dedicated a major part of his career to understanding the physiological basis of crop yield, asking the fundamental question: what limits how much grain a plant can produce? 4 While many scientists focused solely on photosynthesis (the supply of sugars), Evans argued that the "demand" from the developing grain was equally important 4 .
In elegant experiments with wheat, he demonstrated that if you could increase the "sink" capacity of the ear—for instance, by manually inducing more florets to set grain—the plant would mobilize more resources to fill them, thereby increasing yield 4 . This work highlighted that future yield improvements would come from optimizing the entire system of supply, transport, and demand, not just one part alone.
Global Food Security
His deep interest in global food security led him to influential roles with international research institutes.
His deep interest in global food security led him to influential roles with international research institutes like the International Rice Research Institute and the International Wheat and Maize Improvement Center, where his science directly impacted efforts to feed the world 2 4 .
International Rice Research Institute
Applied his understanding of flowering physiology to improve rice cultivation and yields.
International Wheat and Maize Improvement Center
Contributed to breeding programs that developed higher-yielding wheat varieties.
CSIRO Division of Plant Industry
Served as Chief from 1971-1978, guiding Australia's premier plant research organization.
A Legacy of Curiosity and Impact
Lloyd Evans's career was marked by distinguished leadership, including serving as Chief of the CSIRO Division of Plant Industry (1971-1978) and as President of the Australian Academy of Science (1978-1982) 1 7 . His numerous honours, including being elected a Fellow of the Royal Society and becoming an Officer of the Order of Australia, speak to the high esteem in which he was held 1 7 8 .
However, his true legacy lies in the fundamental knowledge he created. By patiently unraveling the dual controls of flowering and rigorously linking basic science to agricultural yield, he transformed how we understand the inner life of plants. He showed that a simple grass could reveal profound biological truths and that a controlled environment like the phytotron could unlock the complexities of nature. His work stands as a powerful testament to a lifetime of curiosity, and his answers to the question "how do plants know when to flower?" continue to resonate in laboratories and fields around the world.