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Fig. 8.15A,B Effects of NaCl solutions supplied to the roots of small seedlings of Cereus validus (see Fig. 6.14D-F) in pot culture with sand on Na+ and Cl- levels in the roots, the water storage stem parenchyma and the peripheral green chlorenchyma. A NaCl concentrations in the watering solution were increased by daily increments of 50 mM up to 600 mM (upper abscissa), and the plants were analyzed as soon as the respective concentrations were reached at the times indicated (lower abscissa). B NaCl concentrations in the watering solution were increased by daily increments of 50 mM. At any given concentration indicated on the abscissa, plants were kept for 14 days after this concentration was reached and then analyzed. (Nobel et al. 1984)

Fig. 8.16A,B Root system of Subpilosocereus ottonis as shown on a seedling (A) and a tall fallen cactus (B) with extended horizontal root system and vertical tap roots

to overcome the dry season when salinity-stress is present, particularly if they are able to form functional roots again in the wet season. Indeed, cacti are known to be capable of rapid adventitious root regeneration (Fig. 8.14C). S. ottonis also develops a large horizontal root system from which strong vertical tap roots protrude into the soil, and from which fine absorptive roots have a seasonal turnover related to substratum salinity (Fig. 8.16).

To overcome an extended dry season, the insulated stems of the cacti use the possibility of nocturnal recycling of respiratory CO2 provided by the CAM-mechanism (see Sect. 5.2.2.2 and Box 5.1). The experimental seedlings of C. validus under salt stress reduced photosynthetic gas exchange (Fig. 8.17), and the contribution of nocturnal CO2-recycling to total night-time malate accumulation increased from 20% in the controls to 50% in the NaCl-treated plants. In the extreme case of total insulation, stomata may close permanently during both day and night, reducing gas exchange to an absolute minimum. In this way the plants do not gain carbon, but they minimize loss. Carbon from respiratory CO2 is recycled into malate during the night and, after decarboxylation of malate, in photosynthetic CO2-fixation and carbohydrate synthesis through the Calvin cycle during the day. Thus, metabolism

Fig. 8.17 Effect of NaCl on photosynthetic gas exchange of small seedlings of Cereus validus. The control plants were irrigated with water; the NaCl-treated plants received NaCl solutions of daily increments of 50 mM until 400 mM NaCl was reached and were then kept for 16 days at this NaCl level. The horizontal black bar indicates the dark period. (Nobel et al. 1984)

Fig. 8.17 Effect of NaCl on photosynthetic gas exchange of small seedlings of Cereus validus. The control plants were irrigated with water; the NaCl-treated plants received NaCl solutions of daily increments of 50 mM until 400 mM NaCl was reached and were then kept for 16 days at this NaCl level. The horizontal black bar indicates the dark period. (Nobel et al. 1984)

Fig. 8.18 Loss of fresh weight, i.e. loss of water, by small plants of Subpi-losocereus ottonis (about 0.3 m tall) derooted and placed in full sun exposure in the dry season starting on day 0. Errors are SD, n was 6 to 18. (Luttge et al. 1989a)

is maintained by respiratory and photosynthetic energy turnover, and the only input under these conditions is light energy, which keeps them alive.

Naturally, some loss of water vapour occurs via cuticular transpiration and leads to a gradual reduction of the water reserves in the water-storage parenchyma of the cactus stems (Fig. 8.18) and a decline of vitality. Its has been shown that cacti survive when up to 54% of tissue water content is lost, although any subsequent loss is lethal (Holthe and Szarek 1985). Thus, the chance of a small cactus to survive the salinity stress of the dry season is much smaller than that of a large cactus. A certain minimal biomass, with a sufficiently large water reserve in the water-storage parenchyma, is required. Of course, survival also depends on the length of the dry and wet season, respectively. If the wet season is longer and the dry season relatively short, newly established seedlings have a better chance of survival. Thus, as shown for cacti and agaves in the deserts of North America (Jordan and Nobel 1979, 1982) seedlings do not survive every year, and one observes age classes of larger plants, which indicates the wetter periods when seedlings were able to become established.

For the larger cacti growing on the vegetation islands, salinity stress in the top soil is reduced even in the dry season. The major tap roots of 6 - 7 m tall plants of S.

Fig. 8.19 Profile of Cl- levels in the soil of a large plant of Subpilosocereus ottonis on a vegetation island in the sand plain of Chichiriviche, Venezuela, during the dry season. (Medina et al. 1989)

ottonis are no longer than 50 cm, and even during the dry season they do not extend below the point where salinity becomes 150 meq Cl- kg-1 air-dried soil (Fig. 8.19). In conclusion, the major problem for the cacti in the salinas really is to survive the vulnerable seedling stage.

8.2.3.2.2 Tank-Forming Bromeliads

Some tank-forming terrestrial bromeliads occur on the salinas. At the northern coast of Venezuela by far the most frequent is Bromelia humilis, with the type II or tank-root life form (see Sect. 6.4 and Fig. 6.15B). Since it does not necessarily need to form soil roots, the leaf rosettes may simply lie on the ground, and thus, B. humilis is effectively also a salt excluder and stress avoider.

Within tanks and through tank roots B. humilis can collect and utilize water. Therefore, in contrast to the columnar cacti (Sect. 8.2.3.2.1), B. humilis can replenish water reserves even from very small spells of rain during the dry season. There is a marked peripheral water-storage parenchyma of thin-walled, non-green and highly vacuolated cells, with little cytoplasm at the adaxial surface of the leaves (see Fig. 6.22C). The leaf tissue loses water during the dry season, and the leaves become less succulent and have increased dry weight: fresh weight ratios (Fig. 8.20). Overall, the CAM plant B. humilis demonstrates water storage at three different time scales:

• short term storage based on the osmotic effects of nocturnal malate accumulation in the leaf cells (see Sect. 6.6.2.2),

• medium term storage in the tanks,

• long term storage in the water parenchyma.

Fig. 8.20 The degree of succulence (fresh weight: area) and dry weight: fresh weight (DW/FW) ratios in leaves of shaded and exposed plants of Bromelia humilis in the wet season (W) and in the dry season (D) in the sand plain of Chichiriviche, Venezuela. (Lee et al. 1989)

B. humilis occurs in different vegetation units on the sand plain and expresses three different phenotypes of growth form and pigmentation (see Sect. 4.1.2), namely the dark green phenotype shaded under shrubs and trees of the deciduous forest, the yellow phenotype exposed on bare soil islands or in the grassland of the sand plain, and a light-green intermediate phenotype, which also grows in relatively exposed conditions (Fig. 4.3). The differential characteristics of shade and sun plants (Sect. 4.1.2) are fully expressed in these phenotypes. In the dry season net CO2 exchange was reduced with 7.2mmolm-2 day-1 in the yellow and 22.2mmolm-2 day-1 in the green and shaded plants (Fig. 8.21), while an average rate obtained for all phenotypes in the rainy season was 33.0mmolm-2day-1.

In the dry season the plants operated with increasing internal CO2-recycling which corresponded to 56% in the green and 87% in the yellow phenotype (Fig.

Fig. 8.21 Net CO2 exchange ( Jco2) during the night and in the early morning of a green shaded (•) and a yellow exposed plant (o) of Bromelia humilis in the sand plain of Chichiriviche, Venezuela. The horizontal black bar indicates the night-period in the dry season. (Lee et al. 1989)

Table 8.2 Data on productivity of exposed and shaded plants of Bromelia humilis in the alluvial plain of Chichiriviche, Venezuela. (Lee et al. 1989)

Exposed Shaded

Table 8.2 Data on productivity of exposed and shaded plants of Bromelia humilis in the alluvial plain of Chichiriviche, Venezuela. (Lee et al. 1989)

Exposed Shaded

Potential max. productivity (t DW ha 1 year 1)

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