Plant population density and plant size

Effect of plant population density on average fruit size and yield

The effects of planting density, and thus of competition for light, on average fruit weight and yield have been demonstrated many times (Bartholomew and Paull, 1986; Py et al., 1987) and have been reconfirmed by recent studies (E. Malezieux, 1992, unpublished results; Scott, 1992; Zhang, 1992; Christensen, 1994). Virtually all studies show that average fruit weight decreases approximately linearly with increasing planting density (Bartholomew and Paull, 1986; see Fig. 6.10), while yield increased linearly or in a curvilinear manner if densities were high enough (see Fig. 6.11). Fruit diameter (Treto et al., 1974; Zhang, 1992) and fruit length (Norman, 1978) decreased as the planting density increased. Although yield components were not reported in most studies, over a moderate range of densities the decrease in fruit weight seems to be due mainly to a decrease in average fruitlet weight rather than in fruitlet number (Sanford, 1962; Pinon, 1981; E. Malezieux, 1992, unpublished results). However, at very high densities, fruit are so small that it is likely that both fruitlet number and weight decrease. In recent studies (Scott, 1992; Zhang, 1992; Christensen, 1994), total yield increased with planting density to populations as high as 128,000 plants ha-1, but the numbers of smaller fruit, which have a lower commercial value, increased at the highest densities. All varieties of pineapple examined respond similarly to increasing plant population density, although the slopes of the lines fitted to the data were different for 'Queen', 'Spanish' and 'Smooth Cayenne' (see Fig. 6.10). The differences in slope are probably due in part to differences in the efficiency of plants in producing a fruit, but differences such as those for 'Smooth Cayenne' grown at different locations are probably due largely to differences in environment or cultural practices.

Effect of plant population density on crop duration and spread in maturity

The period from forcing to harvest is prolonged as planting density increases (Py et al, 1987; Scott, 1992; Zhang, 1992; D. Christensen, 1994, personal communication). In Hawaii (Zhang, 1992), when plants were forced in September, no delay in maturity was found at plant population densities below 75,000 plant ha-1. In southern Queensland, days from induction in January, 1990 to peak harvest increased from 253 at a density of 46,100 plants ha-1 to 265 at a density of 80,700 plants ha-1. A second study in 1994 had similar results, with days from induction to harvest increasing from 256 at 62,500 plants ha-1 to 272 at a density of 85,200 plants ha-1. Christensen (1995) found that there was about a 1-day delay in maturity for every 1000 plant ha-1 increase in density, though presumably there is a lower threshold where this effect is not observed.

Christensen (1995) reported that peak harvest was not determined at a density of 93,700 plant ha-1 because plants segregated into two populations, one of large, unshaded fruit that matured early and one of small, shaded fruit that matured much later. This segregation was probably due to the increased plant-to-plant variability that occurs at high plant population densities. Small variations in plant weight at planting become magnified as density increases because large plants overtop smaller ones. Development of small fruit buried in the canopy are delayed because such fruits have less sun exposure than do large fruit borne on large plants. Shading could retard development by lowering the average fruit temperature (Malezieux et al., 1994) or by reducing the supply of assimilates allocated to the developing fruit. It is clear that fruit development is delayed, particularly where higher plant populations densities are used.

Plant-weight-fruit-weight relationships

Plant or leaf weight at the time of forcing and fruit weight at harvest are generally highly correlated for a given variety of pineapple within a given environment (Bartholomew and Paull, 1986; Py et al., 1987; Malézieux, 1988, 1993; Zhang and Bartholomew, 1997). However, the relationship between plant weight at induction and fruit weight at harvest is complex and not always predictable. The strength of the relationship between plant or 'D'-leaf weight and fruit weight depends on the growing conditions, including climatic conditions, prevailing during a specific crop and hence it is not extrapolatable. Linford (1933) reported that the number of floret buds on plants from two fields was well correlated with stem and peduncle diameter, but less well correlated with stem weight and not significantly correlated with stem length. Further, the number of florets was greater for each stem-diameter class for plants from one field than from the other. Stem weight per floret was 1.65 g for plants from one field and 10.45 g from the second field, causing Linford (1933) to suggest that the factors that determine floret numbers may not be proportional to the plant's ability to carry its fruitlets to maturity.

In regions near the equator, where the environment is relatively uniform throughout the year, the correlation between plant or 'D'-leaf weight at forcing and fruit weight at harvest might be expected to be high during most months of the year. However, in Côte d'Ivoire, within a particular field variability in fruit weight might or might not be well correlated with plant weight at forcing (Malézieux, 1988). In these equatorial regions, it seems likely that the primary effect of plant weight is to determine fruitlet number rather than fruitlet weight at harvest (Malézieux, 1988). In an experiment where plants of different ages, and consequently plant weights, were forced at the same date (Malézieux and Sébillotte, 1990a), the increase in plant weight at forcing was associated with an increase in fruitlets per fruit (Fig. 8.7). The leaf-area index (LAI) in this experiment ranged from 2.0 to 10. The number of fruitlets in this experiment was linearly correlated with plant growth during the month following forcing (Fig. 8.8).

A positive and significant relationship between plant weight and fruitlet number was also shown in an experiment in Côte d'Ivoire, where plots planted monthly where forced systematically at 8 months (Malézieux and Sebillotte, 1990b). Part of the residual variation might be related to climatic conditions in the month following forcing, because drought and low radiation reduced the expected number of fruitlets (Fig. 8.9). The fact that there was no direct relationship between plant weight at forcing and fruitlet weight at harvest in these experiments might be related to the fact that fruitlet filling is the result of the balance between the source (whole-plant capacity to provide assimilates for the fruit) and the sink (number for fruitlets to be filled).

Fig. 8.7. Effect of plant weight at forcing, dry-mass basis, on fruitlets per 'Smooth Cayenne' fruit grown in Côte d'Ivoire (from Malézieux and Sébillotte, 1990a).

Fig. 8.8. Relationship between plant growth in the month following forcing and fruitlets initiated per fruit in Côte d'Ivoire (from Malézieux and Sébillotte, 1990a).

Fig. 8.7. Effect of plant weight at forcing, dry-mass basis, on fruitlets per 'Smooth Cayenne' fruit grown in Côte d'Ivoire (from Malézieux and Sébillotte, 1990a).

Fig. 8.8. Relationship between plant growth in the month following forcing and fruitlets initiated per fruit in Côte d'Ivoire (from Malézieux and Sébillotte, 1990a).

Statistical relationships between plant weight at forcing and fruit weight at harvest may also be established. In Côte d'Ivoire, a linear relationship was found for the data obtained from the monthly-planting trial previously referred to (Malézieux, 1993; Fig. 8.10). Part of the residual variation may be explained by climatic conditions after forcing, which influence fruitlet number and fruitlet filling. Data points that fall below the regression line are due to inadequate fruit filling for plantings made in May, June and July, because of drought and low irradiance during fruit development, while data points located above the line are due to good growing conditions during fruit development for plantings made in January and February. This relationship is not universal, but depends on a variety of factors including the climatic conditions prevailing after forcing,

Plant fresh weight (g)

Fig. 8.9. Relationship between plant fresh weight (PW) and fruitlets per long spiral (NF) for 'Smooth Cayenne' pineapple grown in Côte d'Ivoire (from Malézieux and Sébillotte,1990b).

Plant fresh weight (g)

Plant weight at forcing (g)

Fig. 8.10. Relationship between plant fresh weight at forcing (PW) and fruit weight at harvest (FW) for 'Smooth Cayenne' pineapple in Côte d'Ivoire (from Malézieux, 1 993).

Plant weight at forcing (g)

Fig. 8.9. Relationship between plant fresh weight (PW) and fruitlets per long spiral (NF) for 'Smooth Cayenne' pineapple grown in Côte d'Ivoire (from Malézieux and Sébillotte,1990b).

Fig. 8.10. Relationship between plant fresh weight at forcing (PW) and fruit weight at harvest (FW) for 'Smooth Cayenne' pineapple in Côte d'Ivoire (from Malézieux, 1 993).

the mineral status of the plant and the quality of pest and disease control.

In a time-of-planting trial conducted in Queensland, Australia (Sinclair, 1992b), which included multiple dates of forcing, the relationship between plant weight at forcing and fruit weight at harvest was not significant if no account was taken of season of induction. Seasonal influence was a major determinant of fruit weight in this experiment. Fruit weight was mainly determined by climatic factors - primarily temperature -that occurred during flower induction and fruit development rather than plant weight at forcing (Sinclair, 1992b). In this trial, plants weighing 3.0 to almost 4.5 kg that were forced in autumn had low (1.0 kg) fruit weight at harvest because fruit development occurred during the winter. Plants weighing only about 2.5 kg that were forced in spring produced fruit that had a fresh weight of about 2.0 kg at harvest.

Was this article helpful?

0 0
Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Post a comment