Irradiance slope and aspect

Though few controlled studies were found, the available data indicate that fruit weight is positively correlated with increasing solar irradiance (Bartholomew and Malézieux, 1994). Artificial shade delayed fruit development in Hawaii (Sideris et al., 1936), but it seems likely that shading reduced fruit temperature, thus slowing the rate of development. Covering both the fruit and crown reduced starch accumulation in the stem of the crown relative to stems of uncovered crowns (Sideris, 1933). The data allow the conclusion that crowns become autotrophic and sunlight exposure is required for starch accumulation in the crown stem. Starch reserves in planting material are considered to be an important factor in establishment (see Hepton, Chapter 6, this volume). Shading due to interplant competition reduces the weight of fruit from the centre row of three-row plantings below that of adjacent rows, and removal of plants at the time of forcing, when vegetative weight is fixed, increases the fruit weight (Bartholomew and Malézieux, 1994). In a plant-removal experiment in Côte d'Ivoire, planting densities of 50,000 and 100,000 plants ha-1 were established (Malézieux and Sébillotte, 1990c). At the time of forcing, every other plant was removed in half of the plots. Plant removal significantly increased fruit weight by 50 and 14% in the high- and low-density treatments, respectively. Similar results were obtained in Hawaii (W.G. Sanford, 1966, personal communication). Since plant weight in all plots was equal at the time of forcing, the improved exposure of plants in plots where plants were removed at the time of induction resulted in increased fruit weight. Increased fruit weight resulted from an increase in fruitlet weight in Côte d'Ivoire, while in the Hawaii study, increased fruit weight resulted from a significant increase in fruitlet number (Sanford, 1962). The differences between the two studies may be explained by the cooler temperatures prevailing in Hawaii, which would prolong the period of fruitlet initiation and thus allow enhanced rates of photosynthesis to be reflected in the number of fruitlets initiated.

In Côte d'Ivoire, where temperature is relatively stable and irradiance fluctuates significantly with seasonal changes in cloud cover and rainfall, potential plant weight at forcing and fruit weight at harvest were correlated with irradiance intercepted during the vegetative growth period and for the entire cycle (Fig. 8.6; Malezieux, 1988). In these unirrigated conditions, yield was depressed when drought was prolonged. Seasonal changes in irradiance increase with distance north and south of the equator and the effect of such changes on pineapple growth and yield are probably confounded with simultaneous changes in temperature. Southern Queensland, Australia and South Africa are further from the equator than most pineapple-growing regions and large seasonal effects on crop and fruit characteristics are observed in both regions. In southern Queensland (D. Christensen, 1999, personal communication), there is an approximately 2-month difference in time from flowering to fruit maturity between fruit induced in April and maturing over winter (9 months) and fruit induced in October and maturing over summer (7 months). In winter when the sun angle and total irradiance are low, plant layout in the field can affect fruit maturity. D. Christensen (1999, personal communication) observed that, where two-row beds were planted in an east-west orientation, plants in the north-facing row made greater growth than shaded plants in the south-facing row of two-row beds. Differences in the rate of fruit development in fields forced on the same day that have both north- and south-facing slopes help growers to manage peak harvests because areas with north-facing slopes mature about 2 weeks earlier than those with south-facing slopes (D. Christensen, 1999, personal communication). While there is only limited direct evidence of an effect of irradiance on yield, a wide range of experience indicates that growth, yield and fruit quality are all poorer when irradi-ance is limiting.

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