6.3.3  Reproductive development

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Figure 6.22 A notional distribution of biomass during the vegetative growth and reproductive development in an idealised annual plant such as a cereal or grain legume over c. 125 d. Whole-plant biomass follows a sigmoidal pattern with a near-exponential increase during vegetative growth and an asymptotic increase during subsequent maturation. Reproductive structures have by then become dominants sinks for photoassimilate, drawing 90-95% of their carbon from current photosynthesis but also mobilising stored assimilate from leaves, stems and roots, which lose biomass during that process (Original drawing P.E. Kriedemann; based on various sources)


Figure 6.23 An unirrigated crop of lupin (Lupinus angustifolius cv. Unicrop) at Perth shows major redistribution of plant carbon from vegetative to reproductive structures during grain filling. This cultivar is indeterminate with successive cycles of reproductive development. FP, FS and FT indicate commencement of flowering on primary, secondary and tertiary shoots respectively. Seed carbon increased exponentially over the period 8-12 weeks after anthesis coinciding with leaf loss and some reduction in stem carbon. Nearby irrigated lupins retained leaves much longer (Based on Pate et al. 1980)


Table 6.10


Figure 6.24 A century of breeding and selection has produced some solid gains in harvest index (HI) (ratio of grain to whole-plant bioinass) for crop species including barley (dashed line), wheat (solid line) and rice (dotted line) as shown here. Introduction of dwarfing genes to reduce lodging under high-nutrient cultivation was a major factor in this achievement. Cereal architecture necessitates some trade off between stout stems to support heavy ears and a retention of leaf area to generate photoassimilate. HI will eventually reach a ceiling set by those constraints (Based on Evans 1993)


Table 6.11


Figure 6.25 Early growth of reproductive tissues relative to stem mass in dwarf genotypes foreshadows faster ear development and higher HI. The tall and productive Mexican spring wheat (Yaqui 50, designated rht) eventually produces heavier ears, but returns a lower HI at maturity. Introduction of two major dwarfing genes (Rht 1 + Rht 2, hence Rht 1 + 2 indicated here) resulted in shorter stems. Consequently developing ears were subject to less competition for photoassimilate during early differentiation and for grain filling subsequent to anthesis. Successive (coincident) harvests for these two lines are connected by broken lines. Bars represent standard errors (Based on Bush and Evans 1988)

Annual plants show a sigmoidal increase in total biomass during each life cycle (Figure 6.22) where a near-exponential vegetative phase (Phase 1) gives way to a reproductive phase (Phase 2) starting with flower initiation. In effect, Phase 1 sets a potential for reproductive yield whereas events during Phase 2 determine realisation of that potential because nearly all of the photoassimilate stored in reproductive structures (90–95% in cereal grains, for example) comes from carbon fixed subsequent to initiation. Reproductive organs then become dominant sinks for current photoassimilate as well as carbon-based resources previously stored in leaves and stems.

The carbon content of shoot components changes dramatically following onset of reproductive development (e.g. lupin in Figure 6.23) and the dynamic balance between leaves and stem that had been previously maintained during vegetative growth is now replaced by an accelerated senescence of leaves and loss of non-structural carbohydrates from leaves plus stems with a resultant loss in biomass (see also Rawson and Evans 1971). At full maturity (24–25 weeks after sowing in Figure 6.23) reproductive structures account for about 50% of above-ground biomass (represented in that case as plant carbon) with seeds accounting for about two-thirds of that investment). A ratio of harvested biomass to total shoot mass or shoot harvest index (shoot HI, sensu Donald 1962) for these lupin plants was thus about 0.33. Harvest index can apply equally well to the ratio of harvested biomass to total plant biomass (shoots plus roots) but shoot HI is more common in agronomy because root dry mass is so difficult to measure.

In nature, a combination of ecological factors and life cycle options has led to wide variation in reproductive effort by vascular plants so that dry matter invested in reproductive structures relative to vegetative biomass will vary accordingly. For example, late successional rainforest species which combine shade adaptation with longevity are characterised by large propagules where massive seed reserves buffer young seedlings against shortfalls in carbon supply due to deep shade or dry spells. By contrast, early successional (pioneer) species on disturbed sites benefit by producing a large number of widely disseminated seeds. Their reproductive effort is best invested in number rather than size, and carries an added advantage that at least some viable seed will be produced even under stressful conditions. Weedy barleygrass is a case in point where Chapin et al. (1989) report that these species produce 4.5-fold more grains, but they are only one-sixth the size of cultivated barley. Ripening patterns also differed where grains matured synchronously in cultivated barley, but matured and dehisced progressively from tip to base in ears of barleygrass.

Domesticated plants have been subjected to sustained selection pressures on reproductive development by humans (Table 6.10) and now reflect wide variation from tuber-forming species such as potato, where over 80% of plant biomass is harvested as storage organs, to high-value flower crops such as tulip where blooms might represent only 20% of the final biomass of whole plants. Mid-range are legumes, cereals and other grain crops where human selection for yield has led to a notable increase in HI. Wheat, for example (Figure 6.24), increased from between 0.30 and 0.35 to almost 0.55 over a century, while barley and rice have shown similar trends. Gifford (1986) documents yield improvement in cereals, cotton, peanuts and soybean which is similarly due to substantial increase in HI, emphasising (Gifford et al. 1984) that partitioning of photoassimilate rather than generation of whole-plant biomass was responsible for such yield improvement.

Carbon partitioning during reproductive development thus responds to sink strength which then impinges on final yield. Other important sources of variation in yield can be identified via a simple yield component model. Taking cereals as an example, final yield ((g grain) m–2) will be a product of ears per square metre (ears m–2), grains per ear and mass per grain. Ears m–2 is in turn an outcome of planting density (plants m–2), tillers per plant and ears per tiller.

Some yield components such as mass per grain are especially stable, others such as ears m–2 and grains per ear vary widely with seasonal conditions or according to original planting density (Table 6.11). In that case (Insignia wheat at Glen Osmond, South Australia), mass per grain was highly conserved (33–35 mg) whereas tillers per plant varied from 41 at lowest planting density to only three at highest density. Significantly, yield variation was buffered by compensatory responses in yield components. For example, effects of low planting density were offset by production of more tillers per plant and more ears per tiller. Grains per ear then determine potential yield so that growing conditions would have become crucial for realising such potential via grain retention and filling.

Genotype × environment interactions lead to huge variation in cereal grain yield and have been exploited for yield improvement. Universally, high grain number per square metre is a prerequisite for high yield and can be achieved via more ears per square metre and/or more grains per ear. In wheat and barley, grain number per ear has been primarily responsible for gains in yield; ears m–2 and mass per grain have not shown consistent increase (see Evans 1993 and literature cited).

Returning to collective outcomes represented by HI, one major impetus to improved shoot HI in cereals came from the introduction of dwarfing genes. In primitive wheats, and tall plants generally, reproductive structures have to compete with rapidly extending stems for photoassimilate, but dwarf cultivars alleviate such competition and enable a shift in carbon partitioning to ears. Early growth of ears and stems in two lines of a Mexican spring wheat (Figure 6.25) illustrate this principle. A steeper slope in the dwarf line (designated Rht 1+2) compared with the tall line (rht) implies greater allocation of photoassimilate to ear growth relative to stem growth. Expression of two dominant dwarfing genes in Rht 1+2 (i.e. Rht 1 plus Rht 2) resulted in stem shortening and was accompanied by an altered physiology where leaf and stem tissue proved insensitive to gibberellic acid. Such genotypes are reminiscent of dwarf wheats bred in Japan during the nineteenth century and used there for intensive cultivation (see Evans 1977).

Tall wheat commonly lodges in nitrogen-rich conditions, and dwarf wheats were originally developed to overcome this problem. Agronomists and crop physiologists subsequently recognised the yield advantage from improved partitioning of photoassimilate. Continuing selection for shoot HI in short bread wheats of northwest Mexico (Sayre et al. 1997) has resulted in grain yield increase from around 600 to almost 800 g m–2 between 1960 and 1990 (kernel number per square metre of land was also increased), while Watanabe et al. (1994) have documented comparative performance of Australian wheat cultivars developed between the 1850s and 1990s with similar conclusions. New cultivars outyielded old cultivars due to greater shoot HI rather than total biomass, while Austin et al. (1980) document genetic improvement of winter wheat in Britain over the preceding 80 years with a similar conclusion.

Modern hexaploid wheats are widely recognised as outyielding their diploid relatives due to higher HI and extended leaf area duration, but reduced photosynthetic capacity (area basis) has also been reported. Given such correlation, some researchers imagined there might even be a trade off between HI and leaf assimilation but lacked definitive information, and especially data on nitrogen-use efficiency. Leaf nitrogen is a key driving variable for photosynthetic activity and comparisons between genotypes or contrasts between successive developmental stages on a given plant need to accommodate variation in leaf nitrogen.

Evans (1985) clarified this issue by growing modern hexaploid wheat (Triticum aestivum) and less-developed diploid relatives (including T. monococcum) on high, medium and low nitrogen supply and then comparing light-saturated rates of photosynthesis as a function of leaf nitrogen on an area basis (Evans 1985). Genetic differences were apparent, but in the critical comparison between T. aestivum and T. monococcum, gas exchange data overlapped almost completely, indicating no intrinsic difference in photosynthetic properties between these two species.

As expected in Evans’s (1985) experiments, extensive tillering on high nitrogen in T. monococcum (52 heads per plant) resulted in higher grain yield (27.0 g per plant) compared with T. aestivum (12.3 g per plant). Nevertheless field trials showing greater HI in T. aestivum were confirmed by these pot experiments where T. aestivum returned 0.50 cf. 0.31 in T. monococcum. Superior field yield in hexaploid wheats can thus be attributed to a greater shoot HI and leaf area duration with no trade off in photosynthetic capacity.

Shoot HI has become an important selection criterion for plant breeders and focuses our attention on where shoot HI will eventually plateau. Enlarged ears or panicles call for robust stems, while generating photoassimilate necessitates a canopy, so that investment in vegetative organs will remain substantial and will impose a ceiling on shoot HI which is estimated at about 0.62 for wheat (Austin et al. 1980).

Clearly some room still exists for further improvement in shoot HI compared with 1980s values (Figure 6.24) but there is a corollary. If shoot biomass continues to remain unchanged, further improvement in HI implies some reduction in leaf and stem mass. Considering leaves, SLA will have a finite limit for structural reasons so that the area of CO2-assimilating tissue servicing those enlarged sinks must also reduce as mass is reduced. Net assimilation per unit area (NAR) will therefore need to increase even further if potentially higher yields are to be realised.

To this end, NAR can be regarded as a product of inherent capacity for net photosynthesis which is expressed to a greater or lesser extent according to canopy light climate. Significantly, net photosynthesis embodies respiratory losses where both gain and loss of photoassimilate are a further expression of genotype × environment interactions and are subject to human selection pressures. Variation in community NAR, and thus prospects for further improvement in net carbon assimilation, can come from either photosynthetic or respiratory sources. Crop growth analysis (Section 6.4) deals with canopy architecture and light climate as factors in carbon gain, while growth efficiency and respiration (Section 6.5) covers carbon losses.