Phytomers, Phyllochrons, Phenology and Temperate Cereal Development

Journal of Agricultural Science (2005), 143, 137–150. f 2005 Cambridge University Press 137 doi:10.1017/S0021859605005083 Printed in the United Kingdom CENTENARY REVIEW Phytomers, phyllochrons, phenology and temperate cereal development G. S. M C MASTER 2150 Centre Ave., Bldg D, Suite 200, Fort Collins, CO, 80526, USA (Revised MS received 16 September 2004) SUMMARY Extensive research has been conducted on temperate cereal development since the inception of the Journal of Agricultural Science, Cambridge in 1905. This review presents an overview of the orderly and predictable development of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.). It begins with the concept of building canopies by the formation, growth and senescence of phytomers (the unit comprised of the leaf, axillary bud, node and internode). Morphological naming schemes for uniquely identifying each plant part are then extended to uniquely name each phytomer unit. The role of the phyllochron (rate of leaf appearance) in synchronizing cereal development and phytomer formation is discussed, as is the use of phenology to predict the timing of the formation, growth and senescence of individual components. The complete developmental sequence of the winter wheat shoot apex correlated with growth stages is extended to spring barley. This overview discusses the abiotic factors controlling cereal development, with special attention given to key questions regarding the critical role of temperature. The review concludes with some cautious glances forward to the exciting possibilities for better understanding of mechanisms controlling the phyllochron and phenology being gained from advances in functional genomics and molecular biology. INTRODUCTION Although Goethe first coined the term ‘morphology’ in the late eighteenth century, much of our scientific The grasses are a vast group, and a corr- understanding of grasses has occurred since the later espondingly vast mass of information about nineteenth century. Dissemination of knowledge was them has been accumulated by botanists. If this greatly expanded by the creation of agricultural information were brought together and corr- journals; the Journal of Agricultural Science, elated, it would form a library rather than a Cambridge was one of the earliest when first pub- volume. lished in 1905. As Agnes Arbor pointed out, by 1934 a A. Arber (1934) vast accumulation of information about grasses had been accomplished. The state of knowledge in 1934 From earliest times, humans have gathered infor- has been dwarfed by the exponential explosion of mation on plants, albeit a somewhat informal form of research in the latter half of the twentieth century, documentation until the past few centuries. Interest and one can only imagine what Agnes Arber would has been great regarding the progression of plants write today about our accumulated information. A though their life cycle and ultimately providing seeds, significant focus of the work on grasses has been on fruits, leaves, stems and tubers. In addition, knowl- the agriculturally significant temperate small-grain edge of the stage at which plants should be harvested cereals, wheat (Triticum aestivum L.) and barley for medicinal value was of vital interest. These inter- (Hordeum vulgare L.). ests increased as groups moved from hunting and An extensive conceptual framework of cereal gathering societies to agrarian cultures. Grasses have development has been devised, resulting in many played an increasingly important role in agriculture, tools for predicting cereal development with remark- and today are the dominant agricultural crops. able accuracy. The foundation of this work is that cereals develop in an orderly and predictable manner, Email: [email protected] especially when environmental conditions are not 138 G. S. M C MASTER S15 MS S9 T1 MS S1 MS L6 L5 T2 S13 MS T11 L4 S1 T1 MS L4 T2 L3 L3 T11 L1 MS L2 MS L1 Fig. 1. Building plant canopies by the addition of phytomer units. Morphological naming schemes for vegetative tissues according to Klepper et al. (1983a) and reproductive tissue by Wilhelm & McMaster (1996). Adapted from McMaster (2002). limiting. The pattern of development remains the plastochron and phyllochron in coordinating the same for different genotypes with variation mainly phytomers is combined with the vast work on phe- occurring in the rate or duration of specific develop- nology to predict the orderly development of cereals. mental events, suggesting a strong genetic control. The review finishes with a wary glance towards the The pattern remains the same for varying environ- future and possible role of functional genomics in ments, again with variation primarily in the rate or better explaining the mechanisms controlling cereal duration of specific developmental events within a development. genotype. Some genotypes and developmental events vary more than others to specific environ- BUILDING CANOPIES mental stresses. Centennial journal issues provide opportunities Just as anatomy often views the cell as the funda- and incentives to consider the historical path leading mental building block of tissues, the phytomer can be to our current knowledge and where the path may considered the fundamental building block of plant lead in the future. Yet restraint is necessary, because canopies. Plants construct their canopies by the temperate cereal development is too vast a subject to repeated formation, expansion and senescence of a review in complete detail here, and readers are direc- basic unit, the phytomer. The phytomer concept dates ted to more detailed reviews that offer varying per- at least to Gray (1879), and the phytomer unit usually spectives (e.g. Engledow & Ramiah 1930; Evans & is defined as consisting of a leaf and the associated Grover 1940; Bonnett 1966; Milthorpe & Ivins 1966; axillary bud, node and internode (Evans & Grover Langer 1979; Kirby & Appleyard 1984; Hay & Kirby 1940). The phytomer unit originates at the shoot apex 1991; Rickman & Klepper 1995; McMaster 1997). A with the leaf initial. A shoot is constructed by the more modest goal is to present a general overview of sequential addition of phytomer units at the shoot selected key concepts of wheat and barley develop- apex. Each component of the phytomer unit can ment that emerge from these reviews. Most concepts continue to differentiate and grow, resulting in its apply to other grasses, and often all plants, and are visible appearance (Fig. 1). For instance, the leaf based on the premise that development is orderly and initial can continue to differentiate and grow, result- predictable. The fundamental purpose of this review ing in the appearance of the leaf blade from the is to describe how wheat and barley (and by inference, subtending whorl of leaves. If the axillary bud other grass) canopies are built by the formation, differentiates and grows, then a tiller may appear growth and senescence of phytomers. The role of the from the axil of the leaf in the form of a leaf blade. Cereal development and temperature 139 The associated internode may grow from cells derived the whorl of leaves and the blade length is half the from the intercalary meristem under certain condi- blade length of the penultimate leaf, the Haun stage tions leading to the separation of the nodes of each would be 2.5. phytomer on the shoot. At any stage (depending on A variety of shoot naming schemes for annual environment and genetics) of differentiation and grasses have been proposed (Katayama 1931; Jewiss growth, a phytomer component may then begin to 1972; Klepper et al. 1982, 1983a). In these naming senesce. Generally, the leaf is the primary phytomer schemes, the first culm to emerge from the seed is component that senesces, and towards the death of considered the main shoot (MS). Primary tillers are the shoot the internode tissue senesces. Each shoot is those derived from the axils of leaves on the main built in the same manner, and a field could be viewed shoot, with a designation of T#, where # refers to leaf as a population of phytomers forming, growing and number of the main shoot. For instance, T1 is the senescencing. primary tiller emerging from the axil of L1 on the MS One important consideration of this model is how (Fig. 1). Secondary tillers are those derived from pri- the axillary bud differentiates and grows. In the model mary tiller leaves and designated as T##, where the described in the previous paragraph for wheat and first # refers to the parent primary tiller number and barley, the axillary bud forms a new shoot. In some the second # is the leaf number of the parent primary annual grasses such as maize (Zea mays L.), the axil- tiller that subtends the secondary tiller. An example lary bud can either develop into another shoot or a would be the T11 tiller which emerges from the first female inflorescence (e.g. the ear). Another variation leaf of the primary tiller T1 (Fig. 1). Similar designa- of significance is whether the leaf initial continues to tions are given for tertiary (e.g. T111) and higher differentiate and grow. Once the shoot apex switches order tillers. An anomalous tiller, the coleoptile tiller, from vegetative to reproductive events at the time of is derived from the axil of the coleoptile leaf. This single double ridge, leaf initials stop further develop- primary tiller is designated as TC by some (Kirby ment. At double ridge, the ridge formed above the & Eisenberg 1966) and T0 by Klepper et al. (1982, leaf initial which develops into the spikelet could be 1983a). viewed as the ‘axillary bud’. One then could extend Once the leaf on each shoot can be uniquely named, the phytomer unit concept in the developing spike as this naming scheme is easily adapted to nodes and the leaf initial, the spikelet initial leading to the internodes, where node number (and the internode spikelet primordium, and the node and inter- above the node) is the same as the leaf number.

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