Dr. Lakshmi Sridharan
This article is a 2009 Award of Merit winner
Roses& You, June 2020
Just as a human face reflects the emotions, feelings, and state of health of an individual, the rose foliage reflects the emotions and state of health of the rose plant. Lovely, green, spotless, and blemish-less foliage means the rose plant is perfectly happy and healthy as a well-fed baby is. Malnutrition, microbial infections, and pest problems- all express themselves on the leaves. It is, therefore, absolutely essential that a rosarian has a thorough understanding of rose foliage. Leaf acts as a stage on which both villains and heroes have well-defined roles to play. It is the center of all kinds of activities, the good, the bad, and the ugly.
Leaf is a masterpiece of Nature’s creation. No architect can come anywhere closer to Nature in designing something as perfect as leaf. There is a perfect correlation between structure and function. Each and every cell and its components reflect well thought-out, well-planned, and well-executed work of the Master architect-Mother Nature. The arrangement of leaves on the stem (phyllotaxy) reflects the architectural excellence of Nature. Phyllotaxy in rose is alternate and spiral. At every node (the place of attachment of leaf to the stem) there is just one leaf. As a result of this carefully planned strategy, each and every leaf exposes itself completely to sun. Complete exposure of leaves to sunlight increases their photosynthetic efficiency. The flat lamina or leaf blade increases the receptive surface for absorption of solar energy, the energy that drives physiological activities, such as photosynthesis, and floral development.
How does a leaf absorb solar energy? Within the leaf, there are photoreceptors (pigments that receive light) and photosensors (photodetectors- pigments that detect light) in highly specialized cells. The photoreceptors absorb light at different wavelengths. In plants, chlorophyll absorbs light in the violet, blue, and red wavelengths. Chlorophyll reflects and transmits green light and therefore, appears to be green. Chlorophyll is present in highly specialized organelle, the chloroplasts that are present in green leaves and stems of the plants. Chlorophyll a is the pigment directly involved in transformation of light energy into chemical energy. In addition to chlorophyll a, plants possess differently colored accessory pigments (chlorophyll b and xanthophylls) that absorb light in different wave lengths from that of chlorophyll a and pass the absorbed energy to chlorophyll a. This allows maximum absorption of solar energy for synthesis of carbohydrates during photosynthesis. The raw Materials for synthesis of carbohydrates are water, and carbon dioxide. Carbon dioxide enters the leaf through stomata, the pores on the leaf surface. On either side of the stoma is a specialized cell called guard cell that regulates the opening and closure of the stoma. The photodetectors, special pigments known as phytochromes that sense the light, now take charge of the chemical process. Light activated phytochromes direct the guard cells to open the stoma and induct carbon di oxide and chloroplasts to synthesize carbohydrates.
Photosynthesis involves a number of stepwise reactions that take place in two stages. Chloroplasts contain a host of enzymes, which carry out the biochemical reactions involved in photosynthesis. Reactions in the first stage are light dependent. Chlorophyll acts like an antenna, receiving the light waves. The absorbed light energy boosts up the electrons in the chlorophyll to a higher energy level. This process generates energy rich organic molecules. These energy-rich molecules are absolutely essential for synthesis of carbohydrate molecules that takes place in the second stage of photosynthesis. The light dependent reaction, thus, converts light energy into chemical energy. The second stage of photosynthesis is light independent. The chemical reactions during this stage convert Carbon dioxide into carbohydrate. Plants use these six carbon sugars to make starch, cellulose, sucrose and other carbon containing compounds that are absolutely essential for supporting plant life. Cellulose is one of the important constituents of plant cell wall. As sucrose, sugar moves from photosynthetic centers to other parts of the plant body.
The stomata on the leaves are the respiratory organs of the plant. During respiration, oxygen enters through the stomata and carbon dioxide moves out of the stomata, exactly opposite of photosynthesis. Respiration releases the energy locked up in the carbohydrates for the plant to carry on other activities. Stomata are the air conditioners as well as water pumps of the plants. Evaporation of water through the stomata is transpiration. Loss of water into the atmosphere cools the immediate vicinity of the leaf. If it were not for transpiration, plant will die of hyperthermia (overheating). Transpiration creates "transpiration pull", that helps in ascent of water from the root system to the shoot system. Rosarians, make a note of this---use of anti transpirants has adverse effects on physiological activities of the plant. By blocking the opening of stomata, antitranspirants decrease photosynthetic efficiency, respiration, and transpiration. It is the price you pay for keeping the microbes off the leaves.
Healthy leaves guarantee lovely blooms, as flowering is induced by the phytochromes that are present within the leaves. Phytochromes are photosensitive signal molecules, which turn on and off flower production. Flowering results from the interaction of two different types of phytochromes, which absorb light in two different regions, one in the red region of the spectrum (P 660) and the other in the far red region of the spectrum (P 735). These two different types of phytochromes are inter-convertible. Light starts the chemical reaction that gets completed in the dark. Sun light converts P 660 to P 735. When darkness sets in, the process reverses. At the end of dark period, all the phytochromes are P 660. Conversion of P 735 to P660 triggers the hormone system that induces flowering. The exact mechanism of triggering of the hormone is not known. Flower production is the result of interaction between these endogenous (internal) signal molecules and exogenous (external) light. Defoliation during microbial infection undoubtedly decreases photosynthetic efficiency, flower initiation, and development.
Leaf is also an excellent absorptive organ. Foliar application of liquid fertilizers is an excellent supplement to root feeding and at times, even an alternative to root feeding. Minerals in the soil may not be available for root absorption because of unfavorable soil pH, low temperature, drought or water logging. Mineral deficiency may also result from insufficient supply of minerals in the soil or from leaching of minerals from leaves during prolonged heavy rainfall. There is a great demand of mineral nutrients when roses put forth-new shoots and produce flowers. Leaves absorb minerals quickly and therefore one can easily correct any deficiency by foliar application.
Close look at the leaves will reveal whether the plant suffers from mineral deficiency, water stress or microbial infection. Yellowing of leaves or chlorosis results from deficiency of nitrogen, iron (veinal chlorosis), sulfur, magnesium (marginal chlorosis), calcium (marginal chlorosis), manganese (interveinal chlorosis) and molybdenum (interveinal chlorosis) or microbial infection (black spot, rust) or inadequate exposure to light. Potassium deficiency causes necrosis (death of cells) along leaf margins. Manganese deficiency results in necrotic spots on leaves. Zinc deficiency leads to leaves forming a rosette, twisting of leaves, and necrosis. Leaf curling, and mottling are due to Molybdenum deficiency. Foliar spraying with the deficient mineral or well-balanced liquid fertilizer will immediately take care of mineral deficiencies.
Adequate supply of water keeps the cells turgid and the lamina flat. When transpiration exceeds absorption, leaves wilt (droop). Prolonged water deficit may kill the plant. Water stress lowers plant defense against pathogens.
Fungal Pathogens express themselves as spots of various colors (Black Spot, orange cushions of rust, white powdery masses of mildew) after successful establishment within the host by reproducing spores. One never knows when a pathogen will land on the rose leaf. Washing the leaves for prevention of pathogen penetration is waste of water and time. Leaves absorb systemic fungicides just as it absorbs liquid fertilizers. Absorbed fungicides are, then, transported to other parts of the plants unlike surfactants like antitranspirants. Application of systemic fungicides is, therefore, better than use of surfactants. Diseased leaves are often chlorotic (yellowing) as in the case of black spot, or viral infection. Yellowing and defoliation reduce photosynthetic efficiency of plant. It is imperative that the rose gardener takes notice of any kind of discoloration or deformation of leaves for appropriate preventive as well as control measures.
Cutin and lignin present on the leaf surface offer protection against pathogen penetration. Does washing the leaves regularly prevent microbial infection? Definitely not. Leaf surface has billions and billions of microorganisms. (If you take a little piece of leaf and place it on nutrient agar, you will see a variety of microorganisms growing.) Most of these microorganisms are real warriors that will compete with an invader and protect the plant.
Mutilation, holes, stippling, presence of slimy materials, or discoloration result from pest attack. The pest may or may not be visible but their attack will be visible on the leaves for taking necessary counter attacks.
To get the bloom to the trophy table, it is absolutely essential to have healthy foliage. The leaf tells you what the plant needs. Listen to your leaves. Give the plant plenty of water, nutrients, air circulation, and adequate exposure to light for development of healthy foliage and better blooms.