Phenology

A seedling becomes independent of its maternal parent when it has used up all the endosperm nutrient resources (when cotyledons drop off). Following this a plant will grow, producing stems, leaves and roots. Eventually, a plant will flower and produce

© 2003 CAB International. Weed Ecology in Natural and Agricultural Systems (B.D. Booth, S.D. Murphy and C.J. Swanton)

seeds, and then either die (monocarpic) or have repeated periods of flowering (poly-carpic) throughout its life. The study of these regular life cycle events is called phenology. To a certain extent, a plant's phenology is determined by its genetics (a maple tree will never be an annual); however, plants also respond to their environment and therefore the environment influences phenology.

Effect of abiotic factors on phenology

The abiotic environment will have direct and indirect effects on phenology. A pheno-logical event may be directly triggered by a specific environmental cue; for example, a seed may not germinate until a specific temperature is reached. A plant's direct responses may be immediate or delayed (inductive effect). In addition, environmental cues may be qualitative (absolute), where they are necessary for the effect to occur at all, or they may be quantitative (facultative), where they increase the rate of occurrence but are not necessary for it to occur. In some cases, a series of environmental cues may be required before a phenological event is triggered.

Indirectly, the environment can influence phenology because it can influence the rate of important physiological processes (e.g. photosynthesis, cell division). Generally, a plant's response increases as the level of factor increases until an optimal level is reached and then begins to decrease (Fig. 7.1). Beyond an upper limit, elevating the level of the factor will inhibit growth and yield. For example, most plants will not survive in either very cold or very hot temperatures and will have optimum growth at intermediate temperatures. Every species (population or individual) will have a different set of minimum, maximum and optimum responses to a factor. Because plants respond to their environment, the rate at which they pass through their life cycle can vary. Thus, a plant growing in optimal conditions will grow faster than one under stressful conditions, and then be larger when it flowers.

Factors that influence phenology include light, temperature, water, nutrients, gases (oxygen and carbon dioxide), and soil characteristics (pH and texture). In the next sections we look at how temperature and

Zone of tolerance optimum range <->

upper limit

Zone of tolerance optimum range <->

upper limit species absent

Fig. 7.1. Effect of environment factors (e.g. temperature and light) on the rate of important physiological processes (e.g. photosynthesis, cell division). A plant's response increases as the level of factor increases until an optimal level is reached and then begins to decrease. Beyond an upper and lower limit of the factor, the plant will not survive.

species absent high

Environmental gradient

Fig. 7.1. Effect of environment factors (e.g. temperature and light) on the rate of important physiological processes (e.g. photosynthesis, cell division). A plant's response increases as the level of factor increases until an optimal level is reached and then begins to decrease. Beyond an upper and lower limit of the factor, the plant will not survive.

light influence phenology, and then go on to discuss how abiotic factors interact. Note that plants are influenced by other factors, but the effects of these are beyond the scope of this book.

Temperature

Temperature has a strong influence on plant growth rate. We see this when garden plants appear to stop growing on cold spring days, and then 'grow before our eyes' on the next warm day. Below the threshold level, plants will die from freezing damage (or chilling in warmer climates). As temperature increases the metabolic rate increases and growth increases; however, a few degrees beyond the optimum, proteins begin to be denatured (destroyed) and the plant dies. The critical thresholds will change depending on the plant's life stage and environment. For example, a seed will have different limits from those of a seedling or mature plant. Furthermore, individuals growing in colder climates may be adapted or acclimated to lower temperatures than those in warmer climates.

Temperature influences other processes such as dormancy, germination, and bud and flower initiation. Canada thistle, for example, produces new shoots from overwintering roots only when temperatures reach 5°C (Sheley and Petroff, 1999). Some species require a cold period to promote flowering; if kept in warm temperatures, they will continue to grow vegetatively but will not flower (Salisbury and Ross, 1985). Dalmatian toadflax (Linaria dalmatica), for example, requires a winter dormancy period and then exposure to temperatures between 10 and 20°C to produce floral stems (Sheley and Petroff, 1999). Temperature is often an important factor controlling the overall distribution of a species, because it influences many important processes.

The critical temperatures will differ among various processes in the plant. For example, seedling emergence of wild mustard (Sinapis arvensis) is fastest under temperatures of 29/19°C and 35/25°C, while plants grow taller at 23/13°C (Huang et al., 2001) (Fig. 7.2). For shoot elongation the optimum temperature (and high and low

Temperature regime

— 12/2 --■□-■17/7 —c^ 23/13 —a—29/19 -A- 35/25

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