While the pineapple plant is a xerophyte and is capable of good crop production under relatively low water regimes, the plant responds well to as much as 5 cm (2 in.) of water per month from rain or irrigation (Fig. 6.16). Maintaining readily available soil moisture in the immediate root zone requires less water for pineapple than for other crops that have much higher transpiration rates.
The entire pineapple plant adapts to drought and thereby maintains a productive potential in dry soil. Pineapple roots survive drought by suberization, a survival mechanism that limits their capacity to explore the soil for nutrients and moisture. Drought also results in the closure of stomata during the entire day (see Malezieux et al., Chapter 5, this volume). However, plants that are grown under ample moisture in well-aerated soils have well-distributed roots with numerous white root tips. This enhances the consumptive use of water and carbon fixation, an adaptive feature that is partly responsible for the exceptional response that
pineapple shows to drip irrigation (Fig. 6.16A).
While much of the world's pineapple is grown without irrigation, long periods of inadequate rainfall seriously compromise both plant growth and fruit development. In the drier areas of Hawaii, irrigation was a necessity if predictable yields were to be obtained. Soil and leaf moisture readings have been used to develop guidelines for irrigation scheduling. Once the guidelines are understood, rainfall can be considered in the scheduling. Because all areas cannot be irrigated simultaneously on a large plantation, irrigation is usually scheduled to begin before it is actually needed in the most sensitive areas. These are usually young plant-crop fields. Irrigation is completed after the ideal time in the older ratoon fields, where the crop canopy minimizes soil drying and associated moisture loss.
The pineapple leaf is a major water-storage organ for the pineapple plant (see Coppens d'Eeckenbrugge and Leal, Chapter 2, and Malezieux et al., Chapter 5, this volume). The cells of the upper mesophyll layer can occupy almost 50% of the leaf volume when full of water. Under conditions of limited water-supply, the water in the mesophyll layer is withdrawn by the plant. The subsequent shrinkage of the leaf thickness can be used as an indicator of water deficiency. In practice, representative 'D'-leaves are pulled, nested together in a group of six to ten leaves and the total thickness is measured and compared with the thickness of fully turgid leaves.
A shortcoming of this method is that it does not take into account the viability of the root system and its ability to extract moisture from the soil. Roots infested with nematodes or symphylids, damaged by grubs or diseased with fungal pathogens may result in leaves showing moisture deficit, while soil moisture is sufficient for a healthy plant.
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