Leaves are beginning to emerge on the lower branch of the aspen tree. Flowers are emerging on upper branches. Photo by Robert Dryja
By ROBERT DRYJA
Los Alamos
Springtime and plant growth go together, but how does a plant know when it is springtime?
The usual answer is that a plant senses increasing temperature or longer day light. But is it that simple? Let’s first consider “length of day”.
Today the sun rose at 6:20 in the morning and set at 7:46.
Yesterday it rose at 6:21 and set at 7:45. There was two minutes less of daylight.
Tomorrow it will rise at 6:18 and set at 7:47. There will be three more minutes of daylight.
It will be 57 more days until the summer solstice. The sun will rise 30 minutes earlier and set 37 minutes later when compared to today, April 25. This is a change of about half a minute for the morning and another half minute for evening for each day.
How does a plant sense such small changes in time? The length of day becomes more complicated when the weather is taken into consideration. A day appears shorter when it is cloudy, raining or snowing at sunrise or sunset. (Readers may enjoy going to the following website to explore changes in time: https:////www.solartopo.com/daylength.htm .)
A set of seeds are growing in a linear group called a catkin. Will leaves or more catkins emerge just above it? How does the aspen tree decide? Photo by Robert Dryja
The length of day also is the same twice a year as it cycles back and forth between the long days of summer and short days of winter. How does a plant distinguish between the spring and autumn equinoxes when the days are equal in length for daylight?
The variability in high and low temperatures from day to day also raises questions. For example, January had 28 days in which the maximum temperature was above freezing for at least part of the day.
Assumedly a plant would be ready to start growing when the temperature is above freezing and there is day light. Sap would not be frozen and capable of taking nutrients to branches. Indeed, gardeners occasionally see plants starting to grow when a “warm snap” occurs during a winter month. They also have noted the opposite when springtime flowers are frozen by a “cold snap” in a late spring month.
Length of day and temperature each by itself is not sufficient to guide the springtime growth of plants. Perhaps plants somehow track both length of day and temperature together. A short length of day, even if a day is warmer, indicates it is still winter. The combination of the two could guide a plant for when to start growing in the spring. There could be an occasional exception but the odds would be in a plant’s favor for when to start or stop growing.
But a plant does not have an obvious brain with a built-in clock or a thermometer. How does it do it?
An answer is being found at the molecular level inside of plant cells. A phytochrome molecule has been found that is attached to the DNA inside of plant cells. It activates/deactivates plant growth. During the day the molecule stimulates the plant cell DNA to support plant growth.
An aspen leaf has a distinctive shape. Support veins spread out across it. Hair-like extensions grow along its edge. What is the purpose of those hairs? Photo by Robert Dryja
However, the phytochrome molecule directs DNA to stop plant growth at night when the temperature decreases. Light during the day and temperature at night therefore alternate in starting or stopping plant growth via the phytochrome molecule.
Imagine if a plant also could count the number of days after a winter or summer solstice has occurred. Autumn could be arriving 90 days after the summer solstice and spring could be arriving 90 days after the winter solstice. Plants do not count days in this way, but they can sense the passing of a single day.
Cryptochrome molecules set the activity cycle over 24 hours for plants. This daily cycle is known as a circadian rhythm. A dark summer rainstorm therefore does not confuse a plant into acting as if the length of day is short and winter has arrived. A passing weather warm front in the night does make a plant start to photosynthesis as if daylight is arriving.
Three factors working at the molecular level therefore direct the growth of plants in the spring:
- The length of light during the day;
- The temperature decrease at night; and
- Tracking the actual passing of 24 hours.
But there is more to wonder about. How does a plant know where to grow new leaves on a branch? How does it direct the shape of growing leaves? What makes a flower grow rather than a leaf along a branch?
