By Russell T. Nagata
University of Hawaii at Manoa College of Tropical Agriculture and Human Resources Komohana Research and Extension Center, Hilo
Color in the garden is something that many of us living in Hawaii take for granted, since we tend to have spectacular colors year round in every shade of the rainbow, including all shades of white.
And for those of you who don’t believe this, live in an area with seasons and you will know the meaning and feeling of spring fever. For those fortunate to have experienced the splendors of fall colors firsthand, you know that it is a sight to behold.
For many of us, fall colors are just something on the mainland and for nice glossy magazine pictures. However, the basis of color is rooted in the same pigments, whether in our tropical garden, spring flowers, or fall colors.
Speaking about fall colors: About the only common tree in which leaves become showy red prior to them dropping in mass, is the tropical almond or false kamani, Terminalia catapappa.
All the colors we see in plants are due to pigments within the plant tissues. Chlorophyll contributes to the green, xanthophylls produce the yellow colors, carotene (think carrots) is responsible for the orange color, and the anthocyanin produces the red and purple colors.
The colors that these pigments produce are affected by the location of the pigment, concentration of each pigment, the cellular environment where it resides, and external environmental factors, especially light intensity and temperature.
Deeply embedded pigments have a muted color, more pigment results in deeper color. The cellular pH influences the folding and bending of the pigment molecule which ultimately influence color reflectance.
We all know that leaves are green due to chlorophyll within the leaf. In general the more chlorophyll you have within the leaf cell, the darker green the leaf will be. Chlorophyll is responsible for converting sunlight, water, carbon dioxide and mineral nutrients into sugars/carbohydrates which the plant uses for growth, and the primary reason we derive calories from the plants we eat.
Chlorophyll is continually being replaced during the life of the leaf. Ironic as it may seem, light is the primary cause in the demise of chlorophyll within the leaf.
An interesting side note is that the chlorophyll molecule is very similar to the hemoglobin molecule which gives blood its red color. At the center of the chlorophyll molecule is a magnesium atom, and for hemoglobin we find an iron atom. (If I were a science fiction writer I would give this as the reason for green Martians.)
Xanthophylls and carotenes are also present within the leaves of plants, but are normally masked by the green color of chlorophyll. In some cases they are in tissue layers found above the chlorophyll layer within the leaf and give rise to yellow to orange leaves.
Both pigments are also more stable than chlorophyll and tend to last longer within the leaf. It is only when chlorophyll concentrations decrease or disappear that we see the yellow or orange colors in fall leaves. For fall colors, the shortening days and cooler temperatures signal the plant to prepare for the coming winter. Chlorophyll production slows and the concentration declines, whereupon the yellow and orange pigments are allowed to be seen.
Another situation where chlorophyll concentration declines and yellow or orange pigments may be seen are in leaves that are injured mechanically, infected by disease, infested by insects, or suffer from abiotic stresses, including nutrient deficiencies. These injuries result in chlorophyll concentration declining locally around the damaged site and are symptomatic of the cause.
The red and purple color in flowers and leaves are due to the presence of anthocyanin pigments. The brightness or vibrancies of these colors are influenced by many factors including temperature, light intensity, mineral nutrient status, and general plant health.
Some plants have genetically red or purple leaves from the start and are cultivated for that reason. Examples of these plants are red lettuce, red cabbage, red banana, tropical hibiscus, and many other ornamental plants.
Unlike xanthophyll and carotene, the anthocyanin is not usually pre-formed in the leaves of plants that display fall colors. The red color due to anthocyanin in fall leaf color are formed in response to the shortening day light and the physiological changes occurring within the plant to get it ready for winter. Trapped sugars within the leaf are converted into anthocyanin and are favored by sunny days, cool night and adequate moisture.
Enjoy your garden and don’t take its beauty for granted.
For more information on this and other gardening topics, please visit the CTAHR electronic publication website at http://www.ctahr.hawaii.edu/Site/Info.aspx or visit any of the local Cooperative Extension Service offices around the Island. I can be reached at firstname.lastname@example.org.