Department of Plant Sciences, University of W. Australia, Nedlands, W. A., 6907.

Introduction

In a typical spring barley the accumulation of dry mass (growth) commences slowly with the emergence of the coleptile and increases exponentially until physiological maturity (Fig. 1). During that interval crop growth is characterized by an orderly sequence of ontogenic events (development), such as coleoptile emergence, floral initiation and anthesis (Fig. 2). These events precipitate the coordinated initiation, appearance and growth of tissue structures (such as leaves, stems, spikes and grain) characteristic of the mature plant. The timing of developmental events, together with rates of tissue structure initiation and appearance, determines the manner in which accumulating dry mass is partitioned into those structures and, to the proportion stored as grain (Figs. 1 & 2). Cultivar and growing season conditions impact on developmental progress, on growth and the manner of its partitioning. From an analysis of the latter the determination of yield involves three component steps. These are:

• The development a vegetative foundation for future yield determination over the vegetative phase.
• The translation of that vegetative foundation, over the pre-anthesis stage of the reproductive phase, to actual yield potential at the time of anthesis.
• The realization of yield potential over the post-anthesis stage of the reproductive phase.

This paper quantifies the numerous factors contributing to the accumulation and partitioning of dry matter among commercially available barley cultivars differing substantially in their phenology and in their agro-ecological regions of adaptation in Western Australia.

Materials and Methods

Plant growth and developmental progress were examined by frequent sampling from five replicated field trials over four growing seasons. All trials were sown in the interval of late May/early June under conditions of high soil fertility and at a density of 100 plants m^-2. At a wheatbelt location in 1994 and 1995 heavy opening rainfall ensured a fully charged soil profile at the time of sowing. With limited winter rainfall in 1994 soil moisture deficiency intensified from the time cv. Stirling was approaching anthesis. The 1995 season was very different. Persistent winter rainfall resulted in saturated soils until shortly before anthesis in cv. Stirling. With ample soil moisture reserves, and below average temperatures thereafter, physiological maturity occurred a month later. To overcome the confounding of soil moisture related problems, the 1996 and 1997 trials were located on the UWA Field Station in Perth (a costal location) where, on a free draining soil with the availability of irrigation and higher but less extreme temperatures, the inherent potential of cultivars was more fully expressed.

Results

a. Development of vegetative foundation for future yield

During the vegetative phase, from sowing (S) to coleoptile emergence (CE) and CE to floral initiation (FI), the barley plant initiates its total complement of leaves and primary tillers (Fig. 2). The mean duration of this phase varied with cultivar and to a greater extent with season (Table 1). Variation in temperature and radiation, together with cultivar differences in the timing of FI and leaf sizes, were considered the primary factors responsible for mean cultivar differences recorded for canopy development (LAI), shoot numbers and dry mass at FI.

b. Translation of vegetative foundation to actual yield potential at anthesis

The interval from FI to anthesis comprises two sub-stages. During the first (FI to MPN = maximum primordia number) spikes are formed and, during the second (MPN to Anthesis), those spikes grow in size concurrently with the elongation of the stem internodes subtending them (Fig. 2). Mean cultivar differences in the duration to FI (vegetative phase) expanded over the interval from FI – Anthesis and, unlike seasonal differences, were significantly correlated with one another (Table 2). Mean leaf area indices, shoot numbers and dry masses increased slowly during spike formation but exponentially thereafter (Fig. 1), with variation due to seasons greater than variation among cultivars. Similar comments apply to the rates of stem elongation and tiller appearance but not, spikelet primordia number. The imbalance between the exponential increases in LAI (= assimilate supply) and growth (= assimilate demand) generated competition between and within plants and tillers, restricting the number of fertile spikes and spikelets at anthesis to about 60% of the maximum formed; irrespective of dry mass (Fig. 1 and Table 2). Seasonal, but particularly, cultivar differences in dry mass at FI were reflected in dry mass at anthesis; indicating dominant influences of cultivar, temperature and radiation on dry matter accumulation over the pre-anthesis stage.

Table 1 : Season and cultivar means for measures of thermal durations (°Cd), leaves, shoot and dry mass over the vegetative phase (S – FI). It is over this interval that the vegetative foundation for future yield is established. Temp = mean for temperature, PTQ = photo-thermal quotient, S = sowing, CE = coleoptile emergence, FI = floral initiation, LN = leaf number on the main stem, LAI = leaf area index, DM = dry mass.

Table 2: Season and cultivar means for measures of thermal durations (°Cd), leaves, shoot and spikelet, and dry mass over the spike formation stage (FI - MPN) and spike/stem growth (MPN – Anth) sub-stages. It is over these intervals that the vegetative foundation at FI is translated into actual yield potential at anthesis. Temp = mean for temperature, PTQ = photo-thermal quotient, S = sowing, FI = floral initiation, MPN = maximum primordia number, Anth = anthesis, LN = leaf number on the main stem, LAI = leaf area index, RCE = rate of canopy expansion (LAI/°Cd), TI = tiller interval (°Cd/tiller), TS = tiller survival (%), DM = dry mass.

c. Realization of actual yield potential at the time of maturity

In the interval from anthesis to maturity grains form and grow in size (Figs. 1 and 2). The duration of this interval varied minimally with cultivar but greatly with season. Plant dry mass doubled, with contributions from current photosynthesis supplemented by the translocation of stored reserves as canopy senescence increased (Fig. 1 and Table 3). Seasonal and cultivar differences in dry mass at maturity was strongly correlated with dry masses at anthesis but not, with either grain yield or harvest index. This reflex the influence of temperature, evaporative demand and soil moisture availability on the realisation of potential yield.

Table 3: Season and cultivar means for measures of thermal durations (°Cd), leaves, shoot and spikelet, and dry mass over the post-anthesis innterval (Anth – PM). It is over this intervals that the potential yield at the time of anthesis is translated into realizeable yield at maturity. Temp = mean for temperature, PTQ = photo-thermal quotient, S = sowing, AA = awn appearance, Anth = anthesis, LN = leaf number on the main stem, MLAI = maximum leaf area index, RCS = rate of canopy senescence (LAI/°Cd), MShn = maximum shoot number, PT = primary tiller, FShn = fertile shoot number, DM = dry mass, GY = grain yield, HI = harvest index.

Conclusions

• Relative to growing season duration the timing of anthesis is a major factor determining the adaptation of cultivars and their classification into developmental groupings. Although correlated with the duration to FI, cultivar differences in the timing of anthesis were significantly greater due to subtle variation in the duration of sub-stages over the interval from FI to Anthesis.
• Developmental differences within cultivar groupings were inconsistently expressed due to interactions with transient and unpredictable variations in seasonal weather conditions. Such differences are of value in retrospectively explaining cultivar differences in performance but, of limited value as selection criteria or for predictive purposes.
• With results of any single trial reflecting specific adaptation to the seasonal weather conditions experienced, selection in early generations is therefore opportunistic, with recommendations unreliable unless replicated over a realistic number of seasons.

References

1. Boyd, W. J. R. (1996): In: Barley genetics VII. Proc. 7th Int. Barley Genet. Symp., Saskatoon, Canada. pp. 276-283

2. Jalal Kamali, M. R. (1999): Ph. D. Thesis (submitted), The University of Western Australia. pp. 380.