Data are for fruits midway between floral bud appearance and fruit maturation and are from Nobel and De la Barrera (2000).
derlying cladodes, the sucrose, other sugars, and amino acids delivered by the phloem to the fruits (Wang and Nobel 1995) are probably polymerized into osmotically inactive proteins, mucilage, and starch (Nobel and De la Barrera 2000). This allows the fruits to continue receiving water and solutes from the phloem, thus increasing their sugar and starch content.
The curves developed for environmental responses of net CO2 uptake (Fig. 4.2) were obtained in the laboratory, where one factor at a time was varied under controlled conditions. However, variations in air temperature, drought duration, and total daily PPF occur simultaneously in the field. For instance, to examine the effect of elevation on net CO2 uptake and biomass productivity, the decreasing air temperature, generally increasing water availability, and variable changes in PPF with increasing elevation must all be considered.
One method of dealing with the simultaneous variation of environmental factors is to multiply the influence of the three main environmental factors (Fig. 4.2) on net CO2 uptake:
Eq. 4.1 Environmental Productivity Index (EPI) = Fraction of maximal total daily net
CO2 uptake = Temperature Index x Water Index x PPF Index where each component index ranges from zero (the point at which that factor completely inhibits total daily net CO2 uptake per unit area) to one (where that factor is op timal for net CO2 uptake; Nobel 1984, 1988, 1999). Thus, each component index is normalized to one under the optimal conditions for that environmental factor. EPI can be used to estimate the fraction of maximal total daily net CO2 uptake per unit area expected for certain prevailing conditions and should be calculated for various areas on the plant. This fraction is multiplied by the total daily net CO2 uptake per unit area under optimal conditions to predict the actual daily net CO2 uptake. The total daily net CO2 uptake per plant can be obtained by the actual uptake values per area times the respective areas into which the plant has been divided. The total daily net CO2 uptake per unit ground area can be obtained from the total net CO2 uptake per plant and the ground area per plant. Finally, CO2 uptake can be numerically converted to biomass, assuming that the CO2 is converted to carbohydrate (30 g mol-1) or some other biomass equivalent, leading to a biomass productivity in kg m-2 day-1. This daily biomass productivity is then summed up over a year to obtain the annual productivity, which is conventionally expressed in tons (1,000 kg) dry mass hectare-1 year-1 (Nobel 1991).
For Ferocactus acanthodes in the northwestern Sonoran Desert, the Temperature Index approaches 1.00 for the moderately cool parts of the year in late autumn and the spring, as the summer is too hot and the winter is too cold for maximal net CO2 uptake by this cactus (Fig. 4.4A). The Water Index approaches 1.00 for the rainy periods in late summer and the early winter (Fig. 4.4B). The seasonal variation in the PPF Index averaged over the plant surface (Fig. 4.4C) reflects the variation in the sun's trajectory with the time of year, being lowest in the winter and highest near the early summer. The product of these three indices, EPI,
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