In the previous chapter we considered temperature as one important abiotic factor affecting plant performance; in this chapter we discuss some other important environmental stresses, though concentrating on water deficits. Over large areas of the Earth's surface, lack of water is the major factor limiting plant productivity. The average primary productivity of deserts is less than 0.1 tonne ha−1 yr−1; this is at least two orders of magnitude less than the productivity achieved when water is non-limiting. Even in relatively moist climates, such as that of southern England, drought lowers the yields of crops such as barley by an average of 10 to 15% each year, while the yields of more sensitive crops such as salads and potatoes may be reduced even further if unirrigated.

There are many possible definitions of drought and aridity that range from meteorological droughts defined in terms of the length of the rainless period, through definitions that allow for the water storage capacity of the soil and the evaporative demand of the atmosphere, to those that include some aspect of plant performance. In the following, drought is used to refer to any combination of restricted water supply (e.g. as a result of low rainfall or poor soil water storage) and/or enhanced rate of water loss (resulting from high evaporative demand) that tends to reduce plant productivity.

Chapter 2 considered radiative energy exchange between plants and their environment. Other ways in which plants interact with their aerial environment include the transfer of matter, heat and momentum. The mechanisms involved in mass transfer processes, such as the exchanges of CO2 and water vapour between plant leaves and the atmosphere, and in heat transfer, are very closely related so will be treated together. These can be broadly divided into those operating at a molecular level that do not involve mass movement of the medium (i.e. diffusion of matter and conduction of heat) and those processes, generally termed convection, where the entity is transported by mass movement of the fluid. The forces exerted on plants by the wind are a manifestation of momentum transfer.

Clear discussion of heat and mass transfer processes may be found in Campbell (1998) and in Monteith and Unsworth (2008) and a number of more advanced treatments (Cussler, 2007; Garratt, 1992; Kaimal & Finnigan, 1994; Monteith, 1975, 1976). The physical principles underlying these transfer processes and the analogies between them are outlined in this chapter, and this information is used to analyse transfer between the atmosphere and both single leaves and whole canopies. Although the principles described are applicable to transfer in any fluid, the examples in this chapter will be confined to transfer in air.

Measures of concentration

Before going into details of the different mechanisms of heat and mass transfer it is necessary to define what is meant by concentration.

Introduction

The ability of plants to modify their patterns of development appropriately in response to changes in the aerial environment is a major factor in their adaptation to specific habitats. These morphogenetic responses are usually taken to include quantitative changes in growth (both cell division and cell expansion), as well as differentiation of cells and organs and even changes in metabolic pathways. Important examples include: the tendency for stem elongation to be greater in certain classes of shade plants, thus enabling them to outgrow competitors; the development of characteristic ‘sun’ and ‘shade’ leaves with appropriate biochemical and physiological characteristics (see Chapter 7); the induction of flowering or other reproductive growth at an appropriate season and the induction of dormancy. For many of these developmental responses some feature of the light environment provides the main external signal, though other important signals can include temperature (see Chapter 9) and water availability.

In addition to affecting development through effects on photosynthesis (for example, rapid growth depends in part on high rates of photosynthesis) and on cellular damage (e.g. DNA damage caused by high irradiance UV-B), light can influence growth and development in a number of ways. These photomorphogenic responses, which are summarised in Table 8.1 include:

1. Phototropism – those directional alterations in growth that occur in response to directional light stimuli, as shown by the growth of shoots towards the light.

2. Photonasty – reversible light movements and related phenomena that occur in response to directional and non-directional light stimuli.

3. […]

This chapter introduces some of the ways in which information of the type discussed in earlier chapters can be applied to the improvement of crop yields. Farm yields have been improving over hundreds of years, though the rate of increase has been particularly rapid in the last 70 years or so (Figure 12.1(a)). These yield increases have resulted both from the introduction of new varieties and from advances in crop management (agronomy), including both the widespread use of fertilisers, herbicides, pesticides and fungicides, and improvements in machinery and irrigation. In addition to their increased yield potential, the new varieties that have been developed by plant breeders often incorporate improved pest or disease resistance and the ability to benefit from increased levels of fertiliser application; in cereals this is partly because newer dwarf genotypes are resistant to lodging. The ‘green revolution’ in the 1960s and 1970s was based on both the incorporation of dwarfing genes that conferred resistance to lodging and improved the harvest index (HI; the proportion of dry matter that is in harvestable yield) and on the introduction of photoperiod insensitivity genes that allowed the improved crops to be grown over a wide range of environments.

It is useful when discussing yield increases to distinguish between potential yields achieved with optimal agronomy under experiment station conditions, and farm yields as obtained by typical farmers and reported in national yield statistics (Figure 12.1(b)).

Introduction

There are four main ways in which radiation is important for plant life:

1. Thermal effects. Radiation is the major mode of energy exchange between plants and the aerial environment: solar radiation provides the main energy input to plants, with much of this energy being converted to heat and driving other radiation exchanges and processes such as transpiration, as well as being involved in determining tissue temperatures with consequences for rates of metabolic processes and the balance between them (see particularly Chapters 5 and 9).

2. Photosynthesis. Some of the solar radiation absorbed by plants is used to generate ‘energy-rich’ compounds that can drive energy-requiring (endergonic) biochemical reactions. These energy-rich compounds include those derived by dehydration (e.g. in the reaction of inorganic phosphate and ADP to form ATP) or reduction (e.g. of NADP+ to NADPH). This harnessing of the energy in solar radiation in photosynthesis is characteristic of plants and provides the main input of free energy into the biosphere (see Chapter 7).

3. Photomorphogenesis. The amount, direction, timing and spectral distribution of shortwave radiation also plays an important role in the regulation of growth and development (see Chapter 8).

4. Mutagenesis. Very shortwave, highly energetic radiation, including the ultraviolet, as well as X- and γ-radiation, can have damaging effects on living cells, particularly affecting the structure of the genetic material and causing mutations.

Progress in environmental plant physiology, as in other scientific disciplines, involves repeated cycles of observation or experimentation followed by data analysis and the construction and refining of hypotheses concerning the behaviour of the plant-environment system. This process is illustrated in very simplified form in Figure 1.1. At any stage the information and hypotheses may be qualitative or quantitative, and there may be more or less emphasis on the use of controlled experiments for providing the necessary data.

The initial stages of an investigation tend to provide a more qualitative description of system behaviour: much early ecological research, for example, was concerned with the description and classification of vegetation types, with a relatively small proportion of effort being devoted to understanding the underlying processes determining plant distribution. Further improvements in the understanding of any system, however, require a more quantitative approach based on a knowledge of the underlying mechanisms.

It is at this second level that this book is aimed: I have attempted to provide an introduction to environmental biophysics and to the physiology of plant responses that can be used to provide a quantitative basis for the study of ecological and agricultural problems. Further information on specific topics may be found in specialised texts referred to throughout the book.