28 results in Reaching for the Sun
Part II - Plant nutrition
- John King, University of Saskatchewan, Canada
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Summary
Plants must gain from the environment the raw materials they need to sustain their structure, growth, and development. In Part I, we learned how they use light energy in photosynthesis and chemical energy generated through respiration to manufacture a wide array of organic chemical compounds. The focus in Part I was very much on the role of carbon from carbon dioxide in this endeavor. Part II is devoted to the question of what else plants need to sustain their growth and development.
In contrast to animals, the nutritional requirements of plants are simple. In addition to carbon dioxide, they need only water and certain chemical elements, all of which they most often take up from the soil. In the first two chapters in Part II, we examine how plants take in and distribute the water and mineral elements they require. The third chapter deals separately with the special case of how plants acquire and use nitrogen. A final chapter describes how plants transport substances of all kinds within and between their various organs.
These physiological processes are often grouped under the collective heading of plant nutrition.
Part IV - Stress, defense, and decline
- John King, University of Saskatchewan, Canada
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Plants are exposed to unusual environmental conditions, daily and seasonally. Away from the equator, perennials such as trees and shrubs can be subjected to extreme cold in winter; plants growing at high altitude may experience, in addition to cold all year round (at least at night), drying winds and high levels of harmful ultraviolet radiation; desert plants must often suffer through long difficult periods of extreme high or low temperatures; in many locations, extended periods of drought or flooding may have to be endured; tolerance to increasingly saline soils may become necessary as we continue to abuse our arable lands; and soil, water, and air pollutants as a result of human activity may be encountered. Stress and how plants cope with it is the subject of the first chapter in Part IV.
Plants, both wild and cultivated, are surrounded by bacteria, fungi, nematodes, mites, insects, mammals, and other living hazards to their wellbeing, all hungry, many potentially harmful. Plants cannot easily avoid these enemies by moving away or hiding. The first part of the second chapter in Part IV focuses on the strategies plants use to combat the enemies in their environment, first and foremost, the ever-evolving chemical warfare that they wage against constantly adapting foes.
However, plants live in communities, as do other organisms, within which they compete with one another for moisture, light, and soil nutrients. Plants have evolved a variety of ways to create Lebensraum in their generally overcrowded world.
8 - The time of their lives
- John King, University of Saskatchewan, Canada
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INTRODUCTION
Plants growing in those areas of the world where there are definite seasons synchronize their activities to suit regular variations in the weather. It makes little sense for a seed in temperate regions to begin germinating as soon as it is produced if it is late summer. Onset of winter would likely freeze and kill the fragile seedling before it establishes itself. A tree or shrub faced with an approaching cold season begins, in advance, to make provision to protect tender growing points and leaves within buds (see Chapter 9).
In another example, annual herbs develop vegetatively before diverting their energies to forming flowers, fruits, and seeds. Leaves are most often produced first to supply enough food reserves through photosynthesis before energy-consuming tasks like reproduction begin. Such sequences of events are timed to occur in synchrony with the seasons.
For these, and many other reasons, plants have evolved ways to sense the passage of time; to measure the lengths of days and the onset of seasons. Rhythms in plant life are linked to such cycles in their environment through a number of remarkably exact time-measuring mechanisms.
DAILY AND SEASONAL RHYTHMS IN PLANT LIFE
Sleep movements
It has been recognized for a long time that rhythms in plants exist. Androsthenes, historian to Alexander the Great, was one of the first to observe, over 2300 years ago, that the leaves of some plants adopt different positions during the day than at night.
15 - Elemental cycles
- John King, University of Saskatchewan, Canada
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INTRODUCTION
For nearly all of its four and a half billion years of existence, the Earth's environment has followed a path and pace of change affected much less by human activities than by other, much larger, natural forces. Humans began having an impact a few thousand years ago through such activities as the slash and burn gardens in the tropics and the burning of grasslands by hunter/gatherer societies prior to forest clearing for agriculture in temperate regions. In the past few hundred years, however, burgeoning human populations and their activities in all parts of the globe have greatly increased the scale of impact.
The goal of this chapter is to examine the period prior to the advent of widespread human effects on the Earth's environment, especially factors important to plants and the roles they play in the biosphere.
For the first half billion years or so after it began forming, the Earth was shaped and reshaped by physical and chemical forces alone. Around 3.8 billion years before the present (BP), the outer crust, which formed as the Earth cooled sufficiently to form a solid, unbroken surface layer, cracked into a number of massive plates floating on a deeper mantle of molten rock. Ever since, these tectonic plates on which the continents are carried have continued to move from place to place at the Earth's surface and, at their edges, to sink beneath one another into the hot mantle below, a process called subduction.
Part III - Growth and development
- John King, University of Saskatchewan, Canada
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A plant goes through a highly organized series of stages as it progresses through its life cycle. A fertilized egg, the zygote, divides repeatedly, differentiating into a wide array of tissues and organs, giving rise to a mature plant.
In Part III, chapters on how plants respond to cues from their environment (changes in light, temperature, and gravity) as they grow and develop are set beside discussions of how they sense day and night, respond to the changing seasons, attract pollinators with color, fragrances, and flavors, and use dormancy as a survival strategy.
Preface to the Second Edition
- John King, University of Saskatchewan, Canada
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The rationale for the 1997 edition was that, the more people that know about the lifestyle of plants, the more likely it is that they will appreciate what has to be done to preserve this component of the biosphere upon which our survival depends. A secondary aim was the hope that people would discover that knowing more about how plants work is not just useful but also fun.
This second edition is addressed primarily to an audience of students of plant sciences but also to keen gardeners, naturalists, or anyone with questions about why the Earth is green, how plants defend themselves against diseases and predators, how they combat the stresses of constant exposure to the environment, and how climate change is affecting plants.
The general alarm voiced in the 1997 edition about the deleterious effects humans are having on our environment through global warming, deforestation, overgrazing by domestic animals, overcropping of arable lands, and pollution of land, water, and the atmosphere, is now, more than 10 years on, more clearly focused. Thus, in addition to making the book's original contents consistent with the most recent knowledge, material has been added to educate students of all ages about how human activities are impacting the lives of plants – how plants are affected at the physiological level by changes to our environment, such as through increasing concentrations of greenhouse gases, higher temperatures, longer growing seasons, increasing ocean acidity, changing water economy of the Earth, and pollutants such as ozone.
13 - Chemical warfare
- John King, University of Saskatchewan, Canada
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INTRODUCTION
Plants can be devasted by attacks from adversaries. Recall the near total damage to vegetation caused by plagues of locusts down the centuries or the wiping out of entire crops by disease as in the infamous example of the potato blight in Ireland in the mid-nineteenth century.
Yet, green plants still dominate the landscape in spite of their countless enemies; plants make up a major proportion of the world's biomass, the total weight of all living things. They have evolved an impressive array of strategies, physical and chemical, to defend themselves. Some plants even seem to use their weaponry to ward off competition from their own kind.
PLANT DEFENSES AGAINST PREDATORS
Indeterminate growth
Plants have an amazing ability to renew themselves even as they are being attacked. Grazing animals may spend major amounts of time cropping their preferred food sources, yet these same plants usually maintain healthy and vigorous growth as long as environmental conditions continue to be favorable. Disease may devastate a plant in the wild but rarely is the attack so complete as to wipe out an entire species. Renewal almost inevitably occurs, given enough time, because plants have an indeterminate growth style.
Physical defenses
Another partial answer to the question of why plants dominate the world is that many of them have developed effective physical defenses.
Part V - Plants and the environment
- John King, University of Saskatchewan, Canada
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There is increasing alarm around the world about how human activities are forcing the pace of change in the environment. A major concern is the effect we are having on plants.
Plants are major regulators of the environment through their ability to capture energy from the Sun and convert it to a form available to other organisms; through their influence on shaping the composition of our atmosphere by removal of carbon dioxide and addition of oxygen during photosynthesis; by their retention, circulation, and evaporation of water below, at, and above the Earth's surface; and in their ability to mine and redistribute mineral elements in soil via their roots, thus contributing to the weathering and breakdown of the surface layers of the planet. All of these critical functions of plants are under threat because of stresses imposed on them through global warming, deforestation, afforestation, agriculture, industrial and domestic pollution, irrigation, drainage, and flooding.
Before the Industrial Revolution began a few hundred years ago, the pace of changes to the Earth was affected by human activities in only limited, localized ways. Chapter 15 is devoted to an examination of this phase of development of our planet, particularly the cycling and recycling of some key chemical elements on which living organisms depend.
As industrialization took hold and grew, the pace of alterations to the Earth's environment began accelerating to what has now become an alarming rate, leading to changes to our land, water, and atmosphere resources, causing an increasingly wide array of stresses on plants.
6 - Transport of delights
- John King, University of Saskatchewan, Canada
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DEMANDS ON A PLANT TRANSPORT SYSTEM
Delivery to all tissues …
Growth can take place in any area of a plant throughout its lifetime (see Part III). However, only green parts of the plant can photosynthesize yet non-green parts also require food to provide energy and other needs. For example, buds at the tips of branches contain many young leaves which are too immature to carry out photosynthesis yet are growing rapidly. The trunks and stems of trees and many shrubs are not green yet contain substantial amounts of living tissues which must be provided with nourishment. Energy is needed to form flowers, fruits, and seeds, none of which may be green, at least when mature. Some plants form food storage organs like bulbs, corms, and tubers, to which, as they swell, food must be delivered.
… At different times
These various demands for food arise not only at widely separated locations in a plant but also at different times during a growing season. In temperate climates early in spring the main need may be to move food from mature leaves capable of photosynthesis or from storage organs to young, developing leaves at stem tips, and to rapidly growing roots. Later, as the plant begins reproductive growth, flowers must be nurtured, followed by fruits and seeds. Later still, if overwintering storage organs are formed they, too, must be supplied with significant amounts of food. All of these organs can be at different locations on the plant body.
16 - The human touch
- John King, University of Saskatchewan, Canada
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INTRODUCTION
The focus of the previous chapter was on factors that shaped and directed changes to the Earth's environment over the four and a half billion years before the start of the Industrial Revolution in the eighteenth century. But what effects have humans had since global industrialization began? How are the changes to the global environment caused by humans since then influencing plants, in particular, today?
Answers to these and other key questions are by no means complete. Even where answers have been provided, the data on which they are based and their interpretation are often contradictory or contentious. This is not surprising but is confusing even for those who have the broadest and deepest knowledge. Not only is human influence growing and changing constantly but the environment itself is composed of a complex array of components which interact with one another often in ways about which there is inadequate knowledge or none at all.
THE IMPACT OF HUMAN ACTIVITIES ON THE ATMOSPHERE
Central to any understanding of human influence on the global environment is our effect on the atmosphere. The composition of the Earth's atmosphere determines inward transmission of the Sun's energy, its distribution across the globe, and its radiation back into space. The difference between the incoming and outgoing energy from the Sun determines the surface temperature of the Earth, important to organisms because all have a temperature range to which they are adapted.
11 - Color, fragrance, and flavor
- John King, University of Saskatchewan, Canada
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INTRODUCTION
Plants produce a bewildering array of exotic chemicals: the pigments which give color to leaves, petals, fruits, and seeds; the substances that create aromas and tastes; those formed to help defend against attack by diseases, predators, and competitors; and others with no known function (Figure 11).
We take advantage of the munificence of plants as chemical factories in many different ways, most crucially in medicine. Worldwide, at least a quarter of medicines come directly from plants; in some places, much more.
SIGNALING
Plants and animals put a great deal of time and energy into attracting attention to themselves. By way of colors, scents, sounds, vibrations, and elaborate movements, such as those in mating displays, living things send signals to one another. The distances over which these signals are sent may be short (a few centimeters) or long (kilometers in the case of some chemical sex attractants; the male gypsy moth can detect the scent put out by a female over 3 km away!).
Signaling is one of the essentials of life. No plant or animal could afford the cost of giving off “pointless” signals; the energy cost is too great. In The Origin of Species, Charles Darwin put it as well as anyone could in the case of flower color:
Flowers rank amongst the most beautiful productions of nature; but they have been rendered conspicuous in contrast with the green leaves, and in consequence at the same time beautiful, so that they may be easily observed by insects. […]
12 - Stressful tranquility
- John King, University of Saskatchewan, Canada
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INTRODUCTION
Plants are exposed to unusual, even extreme, environmental conditions, daily, seasonally, or from time to time depending on where they live. Beneath the benign face of the natural green world, plants are waging battles constantly against difficulties posed by their environments. Because these stresses often lead to reduced health in plants, just as they do in animals, they are also of considerable interest to agricultural scientists. Stressed crops usually produce lower yields. Understanding how plants cope with and respond to environmental stresses (often called abiotic stresses to distinguish them from those caused by diseases and predators, which are biotic stresses) is, therefore, important to breeders whose job it is to develop crop varieties with resistance to stresses while maintaining high yields.
WHAT IS STRESS?
The word stress was used first by engineers to explain what happens when a force is applied to an object; strain is the change in the object caused by the stress. For example, an elastic band can be stressed by forcing it to expand; strain is how much the band is stretched by the force applied. Stresses and strains in the physical world can often be precisely applied and measured.
DEFINING “BIOLOGICAL” STRESS AND STRAIN
In a cultivated context
Anything that does not allow a plant to reach its full potential is a stress which will have a consequent strain, such as lower growth or seed production.
7 - Growth: the long and the short of it
- John King, University of Saskatchewan, Canada
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INTRODUCTION
The fastest growing trees are the eucalypts, one type of which, found in New Guinea, has been known to add nearly 8 m to its height in 1 year. Even these “sprinters” are eclipsed by giant bamboo, which can grow over 1 metre a day, 30 m in under 3 months.
At the other extreme, a Sitka spruce found at the tree limit in the Arctic had one of the slowest growth rates on record. From measurements of the annual growth rings in the trunk it was estimated to be about 100 years old yet was only 28 cm tall.
The total growth of which some plants are capable in a lifetime is startling. One of the largest giant redwood trees found had a wood volume of more than 1500 m3 and weighed over 1000 tonnes. Since the seed of the giant redwood weighs less than 0.005 g, the weight increase over the lifetime of this specimen was more than 250 billion times. Large trees like these can live for more than 4000 years, illustrating that plants often combine in their bodies tissues of great antiquity with others that are still youthful, producing new leaves, shoots, roots, fruits, and seeds.
CONTROL OF DEVELOPMENT AND GROWTH FORM
In animals, organs develop very early in life and become an integral part of the whole organism without which it cannot function.
Preface to the First Edition
- John King, University of Saskatchewan, Canada
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The idea for this book arose from a conversation I had one day with two of my neighbors. Neither is a plant specialist but both are keen gardeners. One has in his garden a number of large trees of which not everyone is an unqualified admirer, including the other neighbor. For one thing, the trees shade adjacent gardens from direct sunlight for much of the day, including that of the second neighbor, a somewhat sensitive matter at this northerly latitude where there is a relatively short growing season. In the autumn, immense numbers of leaves find their way into the general neighborhood, often late in the season since some of these particular trees continue shedding leaves even after the first snow. The task of cleaning up frozen, congealed, decaying leaves is not universally appreciated.
Not for the first time, then, the owner of one of the shaded gardens was trying to persuade the tree-loving neighbor to remove his trees which, to the former, were obstacles to productive gardening. As the conversation developed, it became obvious that the aggrieved party thought the main bulk of a tree came from the soil since he made repeated reference to the fact that the offending trees were taking in significant quantities of nutrients through their roots. Of course, plants do absorb many essential minerals and water from soil but we have known for a long time, for more than 200 years in fact, that air, not soil, is the source of the main building block (carbon) from which the bulk of green plants is manufactured.
Index
- John King, University of Saskatchewan, Canada
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10 - Dormancy: a matter of survival
- John King, University of Saskatchewan, Canada
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INTRODUCTION
In preparation for the most unfavorable weather, a plant may need special protection against the climate. These periods of recurring poor growing conditions must be anticipated well in advance. It would be no use for the plant to begin preparing for winter, for example, the morning of the first frost or for a long, dry, hot season in a desert when water was no longer available.
What a plant does in preparation for long periods of poor weather is often quite elaborate, requiring a long period of good weather after the signal is received that an unfavorable season is approaching. The signal received by the plant cannot be linked directly to future poor conditions. For example, it is not low temperature which triggers the processes inside a plant leading to preparations for winter. Preparations might begin in mid to late summer when the temperature is still high. Shortening day length is a more reliable signal than temperature for plants to use to anticipate winter.
SURVIVAL STRATEGIES
Winter buds
When forming winter buds a plant stops producing new leaves. Instead, small, tough scales are formed which tightly enclose the soft, delicate growing points in terminal buds on branches. They can withstand freezing and thawing many times over without disintegrating and they repel water while keeping the tender tissues inside moist and alive. Only in the spring, when their task is complete, are they shed as the growing points begin once more to grow.
2 - Plant respiration: breathing without lungs
- John King, University of Saskatchewan, Canada
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INTRODUCTION
An impressive achievement in biology in the twentieth century was gaining a comprehensive understanding of respiration. All living things respire. Still, the knowledge that most people have of what is involved often begins and ends with: “we inhale air rich in oxygen and exhale it enriched with carbon dioxide.” But there is much more to it than that.
The foods we eat are slowly burned or “combusted,” as Lavoisier described it more than 250 years ago (see Chapter 1). Using the O2 from the air, we slowly convert carbohydrates, fats, proteins, and other substances, finally, to CO2 and water. This releases the energy contained in foods, some of it in the form of chemical energy, which is useful to us; the rest is given off to our surroundings as heat. We put the useful energy to work to sustain our life support systems – to drive our muscles and other organs, keep us warm, feed our brains, and build our complex molecules (Figure 2).
PLANTS NEED RESPIRATION AS WELL AS PHOTOSYNTHESIS
We might suppose that because plants have access to an endless supply of energy from the sun they do not need any other source – not so. Not all parts of a plant photosynthesize, only those that are green; however, non-green parts also need energy. In addition, photosynthesis occurs only during the day but plants grow round the clock.
Appendix
- John King, University of Saskatchewan, Canada
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Attempts in the past to create genetically engineered (GE) crops have led to a great deal of negative publicity about and considerable public antipathy towards the technology; this is unfortunate. Improvements to the technology have moved well beyond the first generation of GE crops, those with resistance to herbicides. More recent advances in this field have considerable relevance to several topics covered here in Parts IV and V; further advances now in the early stages of exploration and exploitation promise to transform the world as we know it.
One major driver in the future of GE agriculture will be climate change. The number one tool that all living organisms use to respond to alterations to their environment which occur naturally over time is genetic change, which is adaptive. But the kind of genetic change dependent on simple mutations of existing endogenous genes will not bring about adaptation fast enough to meet the challenges posed by rapid climate change in the case of crop species. GE is the only technology that is both sufficiently rapid and targeted to bring about adaptation to and mitigation of the human forcing of changes to global and regional environments in crops.
For example, nitrous oxide, a greenhouse gas some 300 times more harmful than CO2, is percolating into the atmosphere from soil, to which we are adding increasing amounts of nitrogen fertilizer, worldwide.
4 - Nutrition for the healthy lifestyle
- John King, University of Saskatchewan, Canada
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INTRODUCTION
Increasingly, people are thinking about and acting on what they include in their diets. There is a constant bombardment from health specialists, through the media and the internet, exhorting us to maintain a balance in what we eat, an important component of a well regulated lifestyle. Increasingly, we are learning that to eat immoderately and injudiciously is harmful.
We are also generally aware that plants become sick, just as animals do, when not supplied with the nutrients needed for good health. For animals, these requirements are elaborate and include the balanced provision of complex molecules in their diets, such as carbohydrates, fats, and proteins, as well as vitamins and certain minerals. Plants are different in being able to produce their own organic molecules from simpler, inorganic ones.
But, in common with animals, plants need certain minerals for healthy growth. Some of these are essential to both plants and animals in greater or lesser amounts; others are essential either to animals or plants but not both; and some are of probable, but at present uncertain, value to either.
ESSENTIAL MINERAL ELEMENTS
We eat some plant materials for their mineral content, like bananas for their potassium and spinach for its iron, nutrients which are essential both to animals and plants. Sodium is essential to animals but is required for only C4 plants. Molybdenum is essential to plants but is toxic to animals when more than a trace is present in food.
Contents
- John King, University of Saskatchewan, Canada
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