PBIO*3110 – Crop Physiology Lecture #3 Fall Semester 2008 Lecture Notes for Thursday 11 September What is development? Introduction: Phenology I ­ The life cycle Learning Objectives 1. Learn to differentiate between growth and development. 2. Appreciate why phenology is important in crop production. 3. Be able to identify important stages of development.
Document1/8/13/2008 1 MORPHOGENESIS Morphogenesis (or the origin of form) may be thought of as consisting of two primary functions: growth and differentiation (or development) (See Fig. 1). Plant scientists use the term growth rather loosely: it may mean an increase in cell number, plant size, plant weight, or all of the above. In the context of this discussion, however, it is important to distinguish growth from differentiation (or development). Differentiation may be thought of as an increase in complexit y. Internal and external factors influencing cellular differentiation cause cell groups to become distinct tissue types and organs through expression of specific genes. The genes prescribe, according to the function, location, and phenological stage of development, the manufacturing of specific enzymes. Growth and differentiation are frequently associated, but this is not necessarily always the case. For instance, increase of dry matter can occur without any further differentiation (e.g., deposition of storage material in grain, stem or root) and differentiation can occur without a concurrent increase in weight (e.g., germination and "growth" of seedlings in the dark). In crop physiology, the term development is usually employed as the sequence of periodical phenomena of plants (or phenological development).
Document1/8/13/2008 2 PHENOLOGY Phenology is the qualitative and quantitative description of a plant's life cycle from seed to seed. Qualitative aspects of phenology include morphological development and the partitioning of the life cycle into distinct stages of development, such as seedling emergence, flowering, and physiological maturity. Quantitative aspects of development include rate of development and the duration of the life cycle. Phenology differs among plant species and varies among cultivars within a species. Environmental effects such as temperature and photoperiod influence phenology, and genotype x environment interactions for phenological development are an important factor in the selection of properly adapted crop cultivars (i.e., crop cultivars differ in their response of phenology to environmental factors, such as photoperiod­sensitive vs. photoperiod­insensitive genotypes). A good understanding of the phenology of a crop is essential in physiological and agronomic studies of the crop. Three reasons why phenology is important are: 1. Seasonal dry matter accumulation is a function of the duration of the life cycle of annual crops (see Equation1, Lecture # 1). In particular, the duration of the period of maximum light interception by the canopy is usually the most important determinant of seasonal dry matter accumulation (see Fig. 2, Lecture #1). 2. Rates of physiological processes can vary substantially among phases of the life cycle. For instance, (i) dry matter partitioning to the seeds or fruits occurs during the final phase or phases of the life cycle of annual crops. (ii) Potential leaf photosynthesis tends to peak when leaves have just fully expanded, declines gradually after full expansion, and declines rapidly before or after physiological maturity has been attained (e.g., Fig. 2). (iii) Leaf photosynthesis may be modulated by the demand of assimilates by the "sinks" (i.e., rate of partitioning can affect leaf photosynthesis). 3. Most crops are more susceptible to adverse environmental conditions during one or more phases or stages of phenological development. For instance, (i) the impact of adverse conditions on crop yield is particularly large during periods when florets are initiated or when seed number is established. (ii) Effects of growth regulators (or herbicides) on crop development and yield are highly depended on the stage of development at which the growth regulator (or herbicide) has been applied.
Document1/8/13/2008 3 PHASES OF DEVELOPMENT There are many ways to divide and subdivide the life cycles of crop species. First, division of the life cycle into the period before flowering and the period after flowering is relevant for most crops (e.g., Fig. 3). Partitioning of dry matter changes substantially after flowering (i.e., flowers, fruits and/or seeds become major "sinks" of photoassimilates), which can have an effect on canopy photosynthesis and respiration, leaf senescence and, in general, on a plant's response to stress. [Many authors call the period from planting to flowering the "vegetative period of development" and the period from flowering to seed maturity the "reproductive period of development." In grasses, however, this notation is not correct since reproductive development is initiated long before flowering (e.g., double­ridge formation in wheat and tassel initiation in maize)]. Second, the pattern of rate of dry matter accumulation is used to characterize the life cycle. Three more or less distinct phases of rate of dry matter accumulation can be distinguished in the sigmoidal pattern of dry matter accumulation which is common to most plants (Lecture 2, Fig. 1; Fig. 4): (i) a period of exponential dry matter accumulation during early phases of development, (ii) a period of more or less linear dry matter accumulation, and (iii) a period of declining rates of dry matter accumulation (and sometimes even a negative rate of dry matter accumulation) during the final phase of the life cycle when green leaf area declines due to leaf senescence. [Note, rate of dry matter accumulation at a particular time during the life cycle of the crop is the slope of the growth curve at that time]. A third approach to phenology is what I call the functional approach, in
Document1/8/13/2008 4 which phenology is treated in terms of the impact of each period on the economic product. In the functional approach to phenology, a crop's life cycle can be divided into three phases: (i) the period of the formation of the factory for the production of raw material (i.e., the factory is the leaf canopy that intercepts incident solar radiation and the raw material is reduced CO2), (ii) the period of the formation of the manufacturing factory (i.e., the establishment of seed/grain sink size), and (iii) the period during which the economic product is manufactured (i.e., filling of the seed/grain). The crop canopy is established during the first period and a reduction in rate of dry matter accumulation during this period will affect maximum LAI, which may or may not influence absorption of incident solar radiation during the period of complete leaf­area expansion. The number of seeds/grains per unit area and the potential size of each seed/grain determine yield potential. The period during which economic yield potential is established is called the critical period for yield establishment. Duration of this period varies among crops. For instance, duration of this period is approximately 3 weeks for maize and approximately 6 weeks for soybean. If potential yield of a crop is low due to a low rate of dry matter accumulation during the period of "formation of the manufacturing product", high rates of dry matter accumulation during other periods are irrelevant to final yield. The formation of the economic product occurs during the final period of the life cycle of the crop. This period is affected potentially
Document1/8/13/2008 5 most by the relative maturity of the crop. Because this period is relatively short (approximately 40 days for a maize crop in Ontario), any environmental factor that shortens this period (e.g., a killing frost) will affect yield relative to the reduction of this period and not relative to the duration of the total life cycle. This is the reason for the importance of the selection of the appropriate "relative maturity" of the hybrid or cultivar in the production of field crops. For horticultural crops similar constraints to production apply because of the economic necessity to bring the product to market during a certain period of the year. The duration of either the period of the formation the factory for the production of raw material, the period of the formation of the manufacturing factory, or the period of the formation of the economic product cannot be changed without affecting either yield or the time at which that yield is realized. An example of a model of the morphology and phenology of maize is presented in Table 1 (Tollenaar, 1990). Note that the framework of this model is functional (i.e., leaf growth phase = formation of factory for the production of raw material; flowering period = formation of the manufacturing factory; grain­filling period = formation of economic product), but that the sub­phases are morphological in nature. The last period in Table 1 is not usually included in phenological models because physiological maturity signals the end of the life cycle, but this period is of importance to agronomic production (i.e., harvestability and grain­drying cost). Various images of the plant or part of the plant at various stages of development are depicted in the extension updates "corn development" and "soybean development" for maize and soybean phenology, respectively. Phenological models of other crop and horticultural species may be similar or may be quite different from the one presented for maize in Table 1: the model will vary depending on (i) whether the species is determinant or indeterminant, (ii) what organ is of economic interest (seed, flower, roots, phytomass) and (iii) how and when environmental influences such as photoperiod and temperature affect phenology.
Document1/8/13/2008 6 SUMMARY In order to assess the impact of any physiological, morphological and/or biochemical change on crop productivity, the change has to be evaluated in terms of its effect on dry matter accumulations during the whole life cycle. For instance, increasing the number of ears in maize will have no impact on final yield if the rate of dry matter accumulation during the period of formation of the economic product has not been increased simultaneously. Also, increasing leaf photosynthesis or increasing LAI, during the period of the formation of the factory for raw material, will not increase final yield if this does not result in a higher absorptance of solar radiation by the canopy during the two subsequent phases (for instance, because the canopy already absorbed all incident solar radiation). Whereas the evaluation of plant responses to genetic, chemical, and/or environmental alterations is done most conveniently in petri dishes and at the seedling stage, any meaningfull assessment of a treatment effect requires an appreciation of the impact of phenology on crop yield. REFERENCES Tollenaar, M. 1990. The influence of developmental patterns on grain yield of maize. Pages 181­193 in S.K. Sinha, P.V. Sane, S.C. Bhargava and P.K. Agrawal, eds. Proc. Int. Congress Plant Physiol., 15­20 Feb. 1988, New Delhi, India.
Document1/8/13/2008 7 Table 1: Growth Staging of maize 1. The leaf growth phase 1.1 Imbibition of the seed 1.2 Plant emergence 1.3 Transition from predominantly heterotrophic to predominantly autotrophic growth 1.4 End of Juvenile phase 1.5 Tassel initiation 1.6 Initiation of topmost ear 1.7 Emergence of topmost leaf 2. The flowering period 2.1 Tassel emergence 2.2 Anthesis 2.3 Silking 2.4 Fertilization of the florets 3. The grain filling period 3.1 Onset of lag phase of grain d.m. accumulation 3.2 Onset of rapid grain d.m. accumulation 3.3 End of rapid grain d.m. accumulation 3.4 Half milk line 4. The period of physiological maturity 4.1 Black layer formation 4.2 25% grain moisture
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