The intention of this review is to discuss floral initiation of horticultural trees. Floral initiation is best understood for herbaceous species, especially at the molecular level, so a brief overview of the control of floral initiation of Arabidopsis (Arabidopsis thaliana (L.) Heynh.) precedes the discussion of trees. Four major pathways to flowering have been characterized in Arabidopsis, including environmental induction through photoperiod and temperature, autonomous floral initiation, and regulation by gibberellins. Tropical trees are generally induced to flower through environmental cues, whereas floral initiation of temperate deciduous trees is often autonomous. In the tropical evergreen tree mango, Mangifera indica L., cool temperature is the only factor known to induce flowering, but does not ensure floral initiation will occur because there are important interactions with vegetative growth. The temperate deciduous tree apple, Malus domestica Borkh., flowers autonomously, with floral initiation dependent on aspects of vegetative development in the growing season before anthesis, although with respect to the floral initiation of trees in general: the effect of the environment, interactions with vegetative growth, the roles of plant growth regulators and carbohydrates, and recent advances in molecular biology, are discussed.

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Regulation of floral initiation in horticultural trees

Litchi and Longan: Botany, Production and Uses

The management of mature macadamia orchards has evolved largely through the need to control inter-row crowding to maintain machinery access. The current study was undertaken to identify physiological constraints to production and priorities for future research and development. The two components of the study were a preliminary field study to determine the impact of current pruning and hedging management strategies on canopy photosynthetic performance and a literature review to identify physiological issues affecting orchard productivity. The field photosynthesis study demonstrated that emerging flush leaves have a negative light saturation net assimilation rate (Amax) that increases to that of mature leaves (Amax 8–10 µmol CO2/m2.s) over 28 days. Leaf age has no effect on Amax of light-adapted leaves. Shade-adapted macadamia leaves cannot attain the photosynthetic capacity of light-adapted leaves. This means that a late hedging strategy to remove around 1 m of canopy from the side of trees to improve orchard access reduces the photosynthetic capacity of the orchard until sufficient flushing occurs to restore the canopy. The literature review focussed on light interception and distribution, photosynthesis, carbohydrate and nitrogen cycling, environmental response, flowering and fruit set, nut abscission, canopy management, and nutrition and disease control. Light interception modelling work, which has been widely adopted for deciduous temperate fruit crops, has immediate application to macadamia production and can explain many of the yield responses recorded by macadamias to canopy management. Macadamia yields appear to increase up to about 96% light interception. Variation in light distribution within canopies that affects the yield and quality of temperate crops is also a present problem with macadamias. It leads to uneven distribution of leaf and fruit throughout the canopy and to a heavily shaded void developing in the middle of trees. The relationship between irradiance, leaf photosynthetic capacity and longevity, flushing, flowering, and fruit set is poorly understood. No information is available on the training of young trees to improve light distribution and canopy photosynthetic efficiency. The current practice of light annual hedging of mature macadamias appears to restrict the production of fruiting wood. Earlier nut abscission can be achieved by the use of ethephon, which offers greater flexibility in the timing of hedging. Research work is required on the effects of timing, frequency and severity of hedging on fruiting wood production, flowering, and fruit set of macadamias. Information on the need to supplement mechanical hedging with manual pruning to improve light distribution throughout canopies is also required. The cyclical and highly variable nature of macadamia yields needs to be further analysed to determine whether a consistent pattern exists and whether environmental factors are an influence. A simple measure of the storage levels of the major assimilates, carbohydrates and nitrogen compounds, is unlikely to predict cyclical yield patterns because both are continually cycled within the tree and it appears that, from comprehensive studies on another evergreen fruit crop, avocado, the major supply of carbohydrate during the fruit-filling stage is from current photosynthesis. Several studies have failed to demonstrate irrigation responses by field-grown macadamias. The cyclical and variable yield of macadamias demonstrates that yield responses from field experiments cannot be expected in less than 5 years, even assuming adequate plot size, buffering, and replication. On the drier sites with light-textured soils in south-eastern Queensland, yield responses could be expected. Following the success of temperate fruit tree crop breeding programs and trends with other evergreen tree crops, macadamia breeding needs to focus on dwarfing clonal rootstocks that provide uniformity, and vigour control to improve cropping efficiency, to reduce canopy management costs and to minimise the reduction in yield following tree topping.

Macadamia physiology review: a canopy light response study and literature review

Post-pruning shoot growth increases fruit abscission and reduces stem carbohydrates and yield in macadamia

Orchard trees of A4 macadamia (Macadamia integrifolia × tetraphylla) and Hass avocado (Persea americana) were thinned and tip-pruned at different times to generate a range of trees with different stages of leafy flush development. The stagger in the start of the first flush following pruning was repeated in the start of subsequent flushes, so pruning was an effective means of changing the phase of the cycle of flush development. The difference in phase affected flowering. In macadamia, over 80% of the variance in flowering was explained by regression against pruning time. Flowering ranged from 0 to 43% of tip-pruned branches, with the most profuse flowering on flushes appearing in July. In avocado, over 45% of the variance in flowering of the first flush following pruning, and over 35% of the second flush, was explained by regression against pruning time, with more flowering on those flushes starting nearer to the winter solstice. The control trees flowered better than the pruned trees but there was too little information to explain the difference. The macadamia control trees had less synchronised flushing than the pruned trees, and were not subject to internal thinning. The avocado control trees appeared to have a phase of flush development different from the pruned trees, with a summer flush commencing before the first pruning date and maturing before the maturation of the first post-pruning flushes. More research is needed for related canopy management guidelines.

The timing of flush development affects the flowering of avocado (Persea americana) and macadamia (Macadamia integrifolia × tetraphylla)

We investigated the effects of the timing of shoot elongation on the flowering of lychee (Litchi chinensis Sonn.) in eastern Australia. Trees of cv. Kwai May Pink growing in Alstonville (lat. 28.9° S) were pruned during spring and summer, and subsequent shoot elongation was measured until the following spring. New shoots grew by discrete flushes, with the trees initiating 3, 2, or 1 vegetative shoots prior to winter, according to the pruning sequence. Shoots were vegetative when the mean temperature during early flush development was above 17-19°C, and floral at lower temperatures. Trees with successive flushes commencing in February (late summer) and June (early winter) were more likely to flower than trees with flushes commencing in April and August, because the weather conditions in June were cooler than those in August and more likely to favour induction. The importance of cool weather conditions during early flush development for floral determination was not significantly affected by the number of vegetative flushes to develop between pruning and winter. Having shown that the phase of recurrent flushing affects flowering, we sought to model the process in order to recognise reproductive and non-reproductive cycles along Australia's north-eastern seaboard, and to develop a management strategy for the promotion of flowering. From the results of the Alstonville pruning trial, the interval between successive flushes was regressed against the mean product of daily irradiation and mean daily temperature (°C.MJ/[m2.day]) during the interval. The regression was used in conjunction with long-term weather records to estimate the flush commencement dates required for the completion of 1 or 2 vegetative flushes by the winter solstice at different latitudes. The earliest date for the completion of 1 flush ranged from 16 February in northern New South Wales (lat. 30° S) to 13 March in northern Queensland (lat. 17° S). To test the model, a pruning trial was conducted near Mareeba (lat. 17° S). Trees pruned on 10 February, estimated to produce ≈ 1.5 flushes prior to winter (i.e. flushes in late autumn and early spring, but not in winter), flowered poorly and had low yields. In contrast, trees pruned on 11 March, estimated to produce 1 vegetative flush by winter, had good flowering and yields. Thus, strategic pruning after harvest can be used to manipulate flushing times, so that new, potentially flowering shoots emerge in winter. Cool temperatures are still required for successful flowering, and we provide estimates of the likelihood of such weather in the major growing areas by calculating the annual number of days with a mean temperature <20°C. For Cairns (lat. 16.9° S) the number of such days varied from 0 to 39 from 1888 to 1993, which is consistent with the irregular flowering of lychee in coastal northern Queensland. Our work is the first demonstration for any species that the phase of recurrent flushing affects flowering, and emphasises the interplay between a plant's endogenous developmental cycle and seasonal variations in weather.

C. M. Menzel

Papers

Flowering and shoot elongation of lychee in eastern Australia