Nature’s Timekeeping

By: Kit Smith,

University of California Cooperative Extension/El Dorado County Master Gardener

Publication Date: December 10, 2004

Do plants grow because of the sun? Do plants, like humans, have biological time clocks? How does a plant know to come out of dormancy? Why do flowers open their petals at a certain time? There are reasons for what we see in our gardens each season, day, and hour. If you’ve ever wondered about some of the science behind plant growth, this article is for you.

Plants generally react to one or more signals from their environment, such as day length, latitude, altitude, temperature and chill requirement (whether the plant received the required number of winter chilling hours below 45 degrees). There is a strong correlation between flowering and factors like temperature and day length. Plants calculate day length by measuring the interval between the onset of darkness and the beginning of the following light period.

Genes are also responsible for switching on and off the plant’s rhythm. Two genes, the TOC1 and the ZTL, control a plant’s circadian (or biological clock) by interacting with each other to keep the plant in tune with the current seasonal clock. This regulation is based on the plant’s latitude, which dictates day length. The TOC1 gene has a 20 hour time clock and the ZTL gene a 27 hour clock, balancing the circadian clock to run at the right speed. The genes use the alternating light and dark over a 24 hour period to set and maintain their rhythm. The plant’s circadian clock, its internal time, is reset at sunrise and sunset each day, detecting the presence and quantity of light to link the plant to astronomical time. Based on yesterday’s day length, a plant is ready to absorb sunlight and start photosynthesis as soon as sunlight is available today.

Environmental change initiators are external cues to a plant to undertake certain internal processes. A plant goes dormant to protect itself from extreme environmental conditions. Cues from the environment can prepare a plant for the upcoming adverse seasons or for dormancy break. For winter-blooming plants, dormancy is broken by cool autumn rains, which increase the soil moisture and lower air temperature. For summer-blooming plants, dormancy is broken by increasing day length, higher temperatures and decreased rain. Some plants also produce growth inhibitor hormones to prevent them from growing, thereby going dormant.

Lower night temperatures and shorter days trigger the onset of dormancy in spring-blooming plants. These cause a decrease in auxin, a growth hormone. The plants experience leaf senescence, during which chlorophyll breaks down cell walls and green leaves show their autumn colors. Ultimately, the leaf petiole weakens, causing the leaves to fall off. When plants are truly dormant, they will not grow, even if provided with optimum conditions of light, temperature and water supply because the plants need to go through a certain period of cold before they are prepared to emerge from dormancy and begin growing. Dormancy is ended when a sufficient amount of cold temperatures have broken down the growth inhibitor hormones.

All plants have hormones, which are internally secreted compounds carried to and which affect certain plant tissues. Plant hormones are produced in one place and active in another, are used in metabolism and coordinate internal processes in response to external events.

Plants also integrate information from light and temperature as to whether to flower on a daily basis. The closure of flower petals at night is a direct response to day and night. Cells expand in size on the petals’ upper surface to open the flower and on the lower surface to close. For flowering, there must be a conversion of the growth energy from the apical meristem (shoot tip) to the floral meristem (flower bud) and that depends on the plant’s maturity, temperature and day length. A plant must be in a minimum vegetative size to support the weight of flowers and it must have enough food in reserve to supply the demands of developing reproductive organs (the flower). Leaves produce florigen, which is theorized to be a combination of the three main growth hormones, in the upper most point of the stem and then the hormone moves through the vascular system to the apical meristem to start flowering.

Flowering and day length are directly connected. A plant’s light detector, the photochrome, induces the circadian rhythm which controls floral initiation. The proper exposure to light ensures environmentally timely flowering. Plants use the seasonal change in day length as one of the signals to flower. Some examples of this phenomenon include the Morning Glory (Ipomoea nil), whose buds unfurl as the day brightens with the rising sun. The California poppy (Eschscholzia californica) blooms from 1:00 p.m. until dusk, but only on sunny days.

Some plants bloom at a certain time of year or a specific time of the day because of the presence of a pollinator that needs optimum environmental conditions. The Four O’Clock (Miabilis jalap) plant opens at 4:00 on summer afternoons, after the heat of the day has passed and the moth which feasts on the blossoms leaves the area. The flower closes in the morning, when temperatures cool and the moth starts flying.

Understanding the science behind the effect of weather and day length on plants can help you predict the timing of your plants, thus allowing you to garden more confidently. If you would like more information about this or other garden topics, we’re here to help. UCCE/El Dorado Master Gardeners can be reached at by phone at 621-5512 or in person at 311 Fair Lane in Placerville from 9-12 every weekday.