7.2.1 Timing of reproduction

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Timely reproduction is the essence of success for plants as individuals, populations or species. Plant life cycles are attuned to cyclic seasonal environments and in the next chapter we examine the physical factors responsible. Having considered the basic blueprint of axial growth from apical meristems, we now turn to the reproductive options available, with their consequences for survival, multiplication and genetic adapt-ability. The Australasian continent provides some of the most diverse climates on earth, from cool moist temperate of New Zealand’s South Island and Tasmania to harsh hot deserts of central Australia and wet tropics of North Queensland and the Northern Territory. Climatic extremes pose enormous hazards for many stages of plant life cycles, so appropriate reproductive strategies and adaptations are vital.

(a)  Short or long life cycles: annuals, biennials and perennials

Annual plants complete their life cycle within one year, some-times much less. Many weeds multiply rapidly, including the model plant Arabidopsis, which can go from seed to seed within six weeks. Some desert annuals achieve similarly impressive speeds, but for different reasons — making opportunistic use of infrequent water supplies before drought returns. In contrast, perennials take a number of years to progress from seed germination through to plant maturity, flowering, seed and fruit formation, and finally senescence and death. Biennials have an intermediate life cycle with vegetative growth in the first year, and reproduction followed by death in the second. In temperate climates, annual cycles are attuned to seasonal changes, with a necessary period of rest or dormancy (see Section 8.1) during the cold winter. In many tropical climates where there is less yearly temperature or daylength change, a dormant period may relate to other climatic factors, especially rainfall.

     Typical annual plants survive through winter as dormant seeds and then germinate when temperatures increase in spring. Flowering and seed formation are achieved within the favourable growing periods of spring, summer and autumn, with seed dispersal in autumn and plant death during the cold winter. In contrast, perennial plants need to be adapted to exist through adverse seasonal climes, such as temperate winter and tropical dry season. Instead of the whole plant dying in autumn, metabolism slows down, as in frost-resistant leaves of evergreen trees, or leaves are shed, as in deciduous trees, or above-ground plant parts die leaving underground storage organs to resume growth in spring, as in herbaceous perennials. As with annual species, seed frequently germinates in spring, but flowering may not commence for a number of years. This phenomenon is called juvenility. Once the juvenile period is over, flowering and seed production are generally synchronised with the amenable seasons, as with annuals. In the tropical dry season, many perennial plants also undergo a period of environmentally induced dormancy until rains commence and growth can continue. Subtropical climates are favourable enough for many tropical species, but often have more pronounced seasons of temperature, rainfall and daylength than the true tropics. The limiting factor is often winter cold, which although usually non-freezing can still be fatal to unadapted tropical plants.

    The timing of onset of flowering is crucial to plant survival. For example, premature break of dormancy and early flower opening may risk exposed new soft tissues to frost damage, or essential pollinating insects may be absent or inactive. In many species, the environmental factors that influence timing of dormancy and flowering are similar, and resumption of growth in spring frequently coincides with flowering.

(b)  Floral initiation

figure

Figure 7.14 Scanning electron micrographs show changes in shoot apex geometry on transition from vegetative to floral state. (a)-(c) Sunflower (Helianthus annuus) initially increases in diameter, followed by appearance of bract (B) then floret (F) primordia; L = leaf. (d)-(f) Maize (Zea mays) shows increasing apex (A) height, then appearance of floral branches (lateral exes, LA) and spikelets (S).

(Based on Moncur 1981)

An increase in dimensions, either height or diameter or both, of the shoot apical meristem usually marks the transition to the floral state (Figure 7.14). Subsequent development of the four whorls of floral organs occurs in the order sepals, petals, stamens and pistil. Most temperate perennials initiate floral buds in summer or autumn, often overlapping with the previous phase of fruit development. Floral buds then lie dormant over winter, and dormancy is broken by the extended cold period allowing rapid resumption of growth and flowering when permissive temperatures commence in spring, often nine months after initiation, and up to 12 months in the case of male pecan flowers. However, a long floral bud dormancy is not universal: female kiwifruit and pecan flowers initiate after the dormant winter period only two months prior to anthesis.

(c)  Dormancy and chilling

In most temperate fruits, floral initiation and early flower development in late summer and autumn are followed by a period of winter dormancy. The term dormancy embraces a wide range of mechanisms that all relate to cessation of growth. Three classes of dormancy have been identified: endodormancy, paradormancy and ecodormancy (Lang et al. 1987). These are discussed in detail in Section 8.1. Normally, floral and vegetative buds of deciduous woody perennials will not burst until they have experienced a period of low temperature.

A major achievement of horticultural research is the mani-pulation of chilling requirement allowing for yield improve-ment across an extended climatic range. Low-chill peach, nectarine and apple cultivars can produce two crops per year under tropical conditions, for example in Indonesia, provided trees are defoliated after harvest. This procedure modifies bud endodormancy, resulting in budburst a few weeks later. Low-chill peaches can also be managed for out of season greenhouse production in temperate climates. Trees are trained to a trellis, pruned and treated with paclobutrazol (an inhibitor of gibberellin biosynthesis) to reduce vegetative vigour. Hand defoliation induces early flowering and fruiting, but the timing of treatment is important. Premature defoliation results in an unacceptably high incidence of abnormal and sterile flowers.

Budburst is a complex set of physiological processes which follows the fulfilment of the chilling requirement. The first phase is typically a lack of growth (ecodormancy) imposed simply by the low temperature of early spring, as most deciduous species require a certain amount of warmth, measured as a ‘heat sum’, before budburst can proceed. Much effort has been put into models to enable prediction of budburst in spring, and these are based on accumulated ‘chill’ and/or ‘heat’ units (Section 8.1).

(d)  Freezing survival in winter and cold damage in spring

Survival of freezing temperatures by overwintering flower buds can be a problem in areas that experience extremely cold winters. The ability to supercool is an adaptation for freezing avoidance, and flower buds of deciduous fruit trees achieve this by avoiding ice nucleation in the sensitive primordia (Andrews and Proebsting 1986). Tissue dehydration occurs via migration of water from primordia and vascular traces into the flower bud scales. The high sucrose content of the primordia reduces the ice nucleation risk even further.

In areas which experience low temperatures during spring flowering, damage to delicate floral organs can result in reduced fertility. Dormant flower buds are generally more resistant to low-temperature damage, but flowers gradually lose this tolerance with increasing differentiation. In most plants, open flowers are the most sensitive stage, and cold can cause partial infertility or complete abortion of the generative tissue. Supercooling in open flowers operates in a similar manner to that described above for dormant winter buds. Subtropical and tropical species, although rarely exposed to frosts, may also suffer floral defects due to cold but non-freezing temperatures at these same stages. Rare frosts on Florida citrus or Brazilian coffee dramatically affect the world market for these commodities.

(e)  Irregular bearing

Irregular bearing is an unpredictable feature of many tree crops which causes an overall reduction in yield. Typically, a heavy crop one year is followed by a low yield the next. For this reason it is often known as alternate or biennial bearing. A heavy apple crop can reduce subsequent flower numbers, but also decreases cell numbers in the cortical tissue of these developing flowers. The consequence may be low numbers of smaller fruit in the following year’s crop, and hence a very poor yield, referred to as an ‘off’ year.

Two theories on control of irregular bearing relate to endogenous plant hormone status and carbohydrate status. Floral initiation may be inhibited by the presence of seeded fruits on the tree, but not by seedless fruits. The inhibitory influence of fruit is possibly linked to gibberellins, a class of plant hormone (Section 9.1), produced by seeds. Gibberellins are floral inhibitors in many woody perennials including apple, stone fruit and mango if applied to shoots before floral initiation. Alternatively, continued presence of fruit will deplete carbo-hydrate reserves, perhaps below a threshold required for normal floral initiation. In some mandarin cultivars, massive crop loads can be fatal to the tree, presumably due to resource exhaustion, and in coffee can lead to branch dieback. Irregular bearing is a severe cultural problem that, once entrained as an on–off rhythm, is often difficult to overcome, although early harvest is an intervention which can restore a normal bearing pattern.

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