Values for Component Indices of Environmental Productivity Index

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To determine the component indices of EPI (equation 14.1), plants are generally placed in environmental chambers, and one environmental factor is varied while the others are maintained at optimal values. Net CO2 uptake is then measured over 24 h periods. The only commercial CAM species for which the environmental indices relating to soil water status, temperature and PPF (equation 14.1) have been fully determined are A. fourcroydes, O. ficus-indica and S. queretaroensis; partial results are available for Ananas comosus (Neales et al., 1980; Sale and Neales, 1980; Nose et al., 1986) and Agave salmiana (Nobel et al., 1996). Thus, these are the species whose net CO2 uptake over 24 h periods can most readily be predicted in response to the environmental changes accompanying global climatic change. Extrapolations to other CAM species can be made if differences among the examined species can be rationalized.

At about 9 days of drought for A. fourcroydes and 13 days for A. salmiana the water index decreases 50% from its value of 1 under wet conditions (Fig. 14.2). The water index decreases more slowly for the stems of the two cacti used as crops because they have a much greater volume of tissue water storage per unit area across which water can be transpired. Drought of 23 days for O. ficus-indica and 36 days for S. queretaroensis reduces the water index by 50% (Fig. 14.2). The volume available for water storage per unit surface area (which indicates the average tissue depth supplying water for transpiration) is about 4 mm for leaves of A. fourcroydes (Nobel, 1985), 8 mm for leaves of A. salmiana (Nobel et al., 1996), 22 mm for the stems of O. ficus-indica (Nobel, 1988) and 78 mm for stems of 5. queretaroensis (Nobel and Pimienta-Barrios, 1995). A water index based on nocturnal acid accumulation (an indirect indication of total daily net CO2 uptake) decreases 50% in 12 days for A. salmiana, which has an average water storage depth of 7 mm in its leaves (Nobel and Meyer, 1985), and in 7 days for A. tequilana, which has an average water storage depth of 3 mm in its leaves (Nobel and Valenzuela, 1987). For these five species, an empirical relation of 4.8 times the square root of the tissue depth for water storage (in millimetres) accurately predicts the drought duration in days that will cause the daily net CO2 uptake to be halved. Unfortunately, the response of the water index to drought has not been studied for Ananas comosus (Bartholomew and Malézieux, 1994).

The optimal temperatures for net CO2 uptake by the five cultivated CAM species presented in Fig. 14.3 fall within the range found for other CAM species (Nobel, 1988), but the temperatures are lower than for nearly all other cultivated crops. In particular, most net CO2 uptake for commercial CAM species occurs at night; consequently, nocturnal temperatures are more important than are diurnal ones with respect to total daily net CO2 uptake.

Drought duration (days)

Fig. 14.2. Responses of the water index (see equation 14.1) to drought duration for Agave fourcroydes, A. salmiana, O. ficus-indica, and S. queretaroensis. Drought refers to the period when the shoot has a lower water potential than the soil just outside the roots in the centre of the root zone. Data for A. fourcroydes are from Nobel (1985); for A. salmiana from Nobel et al. (1996); for O. ficus-indica from Nobel and Hartsock (1983, 1984); and for S. queretaroensis from Nobel and Pimienta-Barrios (1995) and P.S. Nobel (unpublished observations).

Drought duration (days)

Fig. 14.2. Responses of the water index (see equation 14.1) to drought duration for Agave fourcroydes, A. salmiana, O. ficus-indica, and S. queretaroensis. Drought refers to the period when the shoot has a lower water potential than the soil just outside the roots in the centre of the root zone. Data for A. fourcroydes are from Nobel (1985); for A. salmiana from Nobel et al. (1996); for O. ficus-indica from Nobel and Hartsock (1983, 1984); and for S. queretaroensis from Nobel and Pimienta-Barrios (1995) and P.S. Nobel (unpublished observations).

Maximal daily net CO2 uptake occurs at night-time temperatures of about 18°C for A. fourcroydes, 13°C for A. salmiana, 15°C for Ananas comosus, 15°C for O. ficus-indica, and 16°C for 5. queretaroensis (Fig. 14.3). The optimal temperature for nocturnal net CO2 uptake is about 15°C for Agave americana (Neales, 1973), and the optimal temperatures for nocturnal acid accumulation are about 12°C for A. salmiana (Nobel and Meyer, 1985; Nobel et al., 1996) and 15°C for A. tequilana (Nobel and Valenzuela, 1987). Thus the optimal night temperature for daily net CO2 uptake is about 15°C for all of these cultivated CAM species. This is a major consideration when determining where CAM plants will be cultivated. Temperature cannot be easily manipulated in the field, other than by location of the cultivated plots, whereas the water status can be controlled by irrigation and by light interception through spacing of plants, which affects interplant shading.

The responses of daily net CO2 uptake to PPF are remarkably similar for the commercial CAM species examined, with 50% of maximal uptake occurring at a total daily PPF of 10 mol m-2 day-1 for A. fourcroydes, Ananas comosus and 5. queretaroensis and at 11 mol m-2 day-1 for O. ficus-indica (Fig. 14.4). Moreover, the total daily PPF at which total daily net CO2 uptake was zero was about 2 mol m-2 day-1 for all four species, with 95% of the

Day/night air temperature (°C)

Fig. 14.3. Responses of the temperature index (see equation 14.1) to day/night air temperatures for Agave fourcroydes, A. salmiana, Ananas comosus, O. ficus-indica and S. queretaroensis. The plants were routinely kept at a particular day/night temperature regime for 10 days to allow for acclimation (Nobel, 1988). Data are for mean night temperatures for S. queretaroensis and for constant night temperatures for the other species. They are from the references cited in Fig. 14.2, plus Connelly (1972), Neales et al. (1980) and Bartholomew and Malezieux (1994) for Ananas comosus.

Day/night air temperature (°C)

Fig. 14.3. Responses of the temperature index (see equation 14.1) to day/night air temperatures for Agave fourcroydes, A. salmiana, Ananas comosus, O. ficus-indica and S. queretaroensis. The plants were routinely kept at a particular day/night temperature regime for 10 days to allow for acclimation (Nobel, 1988). Data are for mean night temperatures for S. queretaroensis and for constant night temperatures for the other species. They are from the references cited in Fig. 14.2, plus Connelly (1972), Neales et al. (1980) and Bartholomew and Malezieux (1994) for Ananas comosus.

maximal net CO2 uptake being achieved at a total daily PPF of about 25 mol m-2 day-1 (Fig. 14.4). Half of maximal nocturnal acid accumulation occurred at a total daily PPF of 9 mol m-2 day-1 for A. salmiana (Nobel and Meyer, 1985) and at 11 mol m-2 day-1 for A. tequilana (Nobel and Valenzuela, 1987); therefore the PPF responses of such cultivated CAM species are nearly identical. In this regard, the chlorenchyma for all of the agave and cactus species is a relatively thick 3-5 mm, and the amount of chlorophyll per unit surface area is a relatively high 0.7-0.9 g m-2 (Nobel, 1988). Thus, the light-absorbing properties of the photosynthetic organs are similar, leading to their similar responses of net CO2 uptake to incident PPF as quantified by the PPF index (Fig. 14.4).

As mentioned above, a nutrient index can also be incorporated as a multiplicative factor in equation 14.1, although relatively little information on nutrient responses of CAM crops is available. Based on studies with four agave species and eleven cactus species, most of which are not crops, the five soil elements having the greatest effect on net CO2 uptake, growth and biomass productivity are N, P, K, B and Na (Nobel, 1989); other macronutrients and micronutrients will likely have important effects as well and may be

Ppf Equation

Fig. 14.4. Responses of the PPF index (see equation 14.1) to the total daily PPF for Agave fourcroydes, Ananas comosus, O. ficus-indica, and S. queretaroensis. Data for the agave and the cacti are for the PPF in the planes of the photosynthetic surfaces and are from the references cited in Fig. 14.2. For pineapple, data are for the PPF in a horizontal plane incident on the canopy and are from Sale and Neales (1980) and Nose et al. (1986).

Fig. 14.4. Responses of the PPF index (see equation 14.1) to the total daily PPF for Agave fourcroydes, Ananas comosus, O. ficus-indica, and S. queretaroensis. Data for the agave and the cacti are for the PPF in the planes of the photosynthetic surfaces and are from the references cited in Fig. 14.2. For pineapple, data are for the PPF in a horizontal plane incident on the canopy and are from Sale and Neales (1980) and Nose et al. (1986).

particularly limiting in certain soils. Growth and biomass productivity are optimal at about 3 mg N g-1 (0.3% by soil mass), 60 ||g P g-1, 250 ||g K g-1 and 1.0 |mg Bg-1 (Nobel, 1989). Suboptimal levels leading to half the maximal values are about 0.7 mg N g-1, 5 |g P g-1, 3 |g K g-1 and 0.04 |g B g-1.

Soil Na inhibits biomass productivity for the agaves and cacti tested with 20% inhibition occurring at about 60 |lg Na g-1 and 50% inhibition occurring at about 150 |mg Nag-1 (Nobel, 1989). The decrease in growth resulting from increasing soil salinity is apparently less for Ananas comosus than for agaves and cacti, but little quantitative data are available (Bartholomew and Malézieux, 1994). In this regard, salinity can increase in arid and semi-arid regions as a consequence of poor land management practices and even global climatic change (Schlesinger et al., 1990; Vitousek, 1994).

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