Park S Nobel and Edward G Bobich

Introduction Early Research Net CO2 Uptake Water-Use Efficiency Net CO2 Uptake: Stems Temperature Water PPF

Nutrients and Salinity Atmospheric CO2 Net CO2 Uptake: Fruits

Productivity Responses to Environmental Factors Environmental Productivity Index Application of EPI to a Barrel Cactus Application of EPI to a CultivatedPlatyopuntia Survival

Temperature

Water

Salinity

Gravity and Wind Conclusions and Future Prospects Literature Cited

Introduction

Early Research

The Desert Botanical Laboratory of the Carnegie Institution of Washington located just outside Tucson, Arizona, was a major location for early environmental research on cacti, with approximately 100 publications on environmental responses of cacti from its founding in 1903 to its closure in 1940 (McGinnies 1981). The topics studied in cluded root growth, drought tolerance, temperature tolerance, and photosynthesis. Early experiments showed cacti to be limited in growth by low but not by high temperatures, to conserve water, to require much light, and to have special metabolic activity at night.

For instance, root growth of Opuntia versicolor was shown to be maximal at 33°C, decreasing 50% at i9°C (Cannon 1925). Ferocactus wislizenii could sustain 18 months of drought, during which it lost 70% of its initial water content, and could survive indoors for 6 years without water (MacDougal et al. 1915). Its osmotic pressure was about 0.4 MPa under wet conditions, increasing to 1.0 MPa after 6 months of drought (MacDougal and Cannon 1910). Maximum tissue acidity was shown to occur in Carnegiea gigantea and Ferocactus acanthodes in the morning (Long 1915). Indeed, tissue acidity steadily increases about threefold during the night and reversibly decreases during the daytime for Mammillaria grahamii and O. versicolor (Richards 1915). When the stomates in stems of C. gigantea open at night, the stems contract (MacDougal 1924; MacDougal and Working 1933). Stems of Opuntia phaea-cantha var. discata tolerate 1 hour at 62°C (MacDougal and Working 1922). With respect to low temperature, C. gigantea tolerates -8°C but not -io°C (Shreve 1931). Cacti also "require all the light that it is possible to give them, the more direct sunshine the better" (Shreve 1931).

In addition to the insights gained from research at the Desert Botanical Garden, other early studies had relevance to the environmental responses of cacti. For instance, cladodes of Opuntia leucotricha tend to orient perpendicularly to the direction of a light beam (Goebel 1895). Opuntia ficus-indica is injured at -5°C by ice crystals that are initiated extracellularly (Uphof 1916). Moreover, O. ficus-indica has a productivity of up to 20 tons dry mass hectare-1 year-1 (Griffiths 1915). However, the main research on the gas exchange and other environmental responses of cacti did not occur until after World War II, when instruments were developed to measure net CO2 uptake, transpiration, radiation, temperature, and water potential, using newly invented sensors and electronic circuitry.

Net CO2 Uptake

Although O. ficus-indica has received the most research attention, net CO2 uptake has been measured for species in all three traditional subfamilies of the Cactaceae (Table 4.1) and for all four subfamilies if Maihuenioideae is separated out of the Pereskioideae. For the Pereskioideae broadly defined, the leaves are the dominant photosynthetic organs, and net CO2 uptake occurs almost entirely during the daytime using the C3 photosynthetic pathway. Maximal rates of net CO2 uptake of 4 to 6 pmol m-2 s-1 for Maihuenia poeppigii, Pereskia aculeata, and P grandifolia occur in the early morning, whereas the stems always have a net CO2 loss (Nobel and Hartsock 1987). Similar to the Pereskioideae, many of the more primitive species of Opuntioideae have prominent leaves. Maximum net CO2 uptake rates for leaves of Austrocylindropuntia subulata, Pereskiopsis porteri, and Quiabentia chacoensis are about 3 pmol m-2 s-1 during the daytime (Table 4.1). However, in

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