About Leaves!

Jolene Adams

Master Rosarian, NCNH

ARS Past President

This article is a 2009 Award of Merit winner.

Roses & You, June 2020

Your roses are actively growing now and pushing out leaves – burgundy, dark green, light green, shiny – clean new leaves. The leaves are more than just lovely foliage that covers the plant and keeps the sun from burning the tender bark of the stems.

Leaves provide the surface area needed for the rose to collect sunlight and conduct photosynthesis, which produces food for the plant. Rose leaves are described as “pinnate” – that means there is a central rib and then leaflets off to each side, with one terminal leaflet. Rose leaves can have 2, 3, 5, 7, 9, 11, 13 leaflets. Most modern roses have a 5-leaflet leave with perhaps a few 3-leaflet leaves close to the bloom.

The leaves grow on alternate sides of the stem. There is a short, broad blade called the stipule at the base of the long “rib” which is the petiole. The stipule often has two long projections called auricles. Some auricles are curly, some are straight. The leaflets themselves are attached to the petiole (the “rib”) by short petiolels.

Have you noticed that roses have different shaped leaflets? Some are very oval, others are long and slender, and you can find leaflets on some roses that are just about round. The shape of the leaflets are a good identifier for some roses.

Then there are the edges of the leaflets. The edge is called the leaf margin. The margin can be smooth, slightly saw-toothed, or even very deeply saw-toothed. This is another identifier.

Leaflets have a thin, protective layer of cells on the upper and lower surface, called the epidermis. Some leaflets also have a glossy coating on top called a cuticle. This helps prevent fungus disease organisms from puncturing the skin of the leaf and getting inside. Just beneath the epidermis are thickly packed palisade cells which contain chlorophyll. On the bottom of the leaflet, the palisade cells have big gaps between them so water vapor, carbon dioxide and oxygen can move around. There are guard cells on the epidermis that can close the opening (the stomate) to these open area so nothing gets in or out.

The inside of the leaflet is a spongy mass of cells used for storage and transporting food. Running through the middle of this area are the “veins” which are vascular bundles - the same xylem and phloem that we find in the stems. These transport water and food.

Leaves are a ‘service organization’ for photosynthesis, respiration and transpiration – a veritable HMO for your rose.

PHOTOSYNTHESIS

Photosynthesis happens when the green leaves produce nutrients (carbohydrates). When there is sunlight and chlorophyll present, the leaves convert carbon dioxide (CO2) and water (H2O) into carbohydrates (C6H12O6). No Atkins diet for these guys!! These carbohydrates are simple sugars – that’s why sap tastes slightly sweet! Oxygen (O2) is left over and released into the atmosphere. Without green plants getting rid of excess oxygen – we wouldn’t have breathable air on this planet.

RESPIRATION

Once the carbohydrates are created in the leaves, they are moved around in the plant and broken up into other compounds by enzymes within the cells. This releases chemical energy which is used in many other cell processes. Water and carbon dioxide are also released. The rate of respiration depends on temperature and the availability of oxygen and the simple carbohydrates. Respiration speeds up when it is warm and slows way down when it gets cold. But - - it never stops! The plant is continually feeding off of the chemical energy created during respiration. Parts of the plant that are cut off or harvested – like a long-stemmed rose – still respire!

So – photosynthesis makes the food (the simple carbohydrates), and respiration consumes the food. This goes on in a cycle. The photosynthesis part happens in the day when there is light and it peaks during midday and ceases at night. Meanwhile, respiration occurs 24/7, at variable rates depending on temperature. In order for the bush to grow and develop normally, photosynthesis must occur at a rate that greatly exceeds the rate of respiration.

TRANSPIRATION

Water moves through the plant, going from an area of high water concentration (like the roots) to areas of low water concentration (like the leaves and stem tips). Water evaporates from the leaves through the stomata on the underside of the leaves. A continuous flowing column of water is maintained in the xylem from root to stomata, as long as there is water in the soil and the stomata are open. Transpiration helps to cool the rose bush on hot days, and transports minerals and organic compound from the roots to the leaves. It stops at night when the stomata close up.

Your rose is a living, breathing (in the sense that oxygen and carbon dioxide are exchanged during the life processes) food producing machine that relies on YOU to supply water and adequate minerals and organic compounds to the roots so its leaves can do their job of producing energy for the life processes of the plant. You are responsible – after all, your rose can’t borrow the keys and drive to the nursery to shop for nutrients!