Gas Exchange

Crops with C3 and C4 type photosynthesis, which are the principal focus of this book and currently represent over 98% of cultivated crops in terms of land area utilized, inherently have a much lower water-use efficiency (WUE) than CAM plants. The WUE equals the net CO2 fixed by photosynthesis divided by the water lost via transpiration on an instantaneous, daily or seasonal basis. The water vapour content of saturated air, which is the case for nearly all the air spaces within the shoots of plants, increases essentially exponentially with temperature. For example, air saturated with water vapour contains 6.8 g of water vapour m-3 at 5°C, 17.3 g m-3 at 20°C, and 39.7 g m-3 at 35°C. The water vapour content of the air surrounding plants is generally far below the saturation value and does not change much during the course of a day unless a major change in weather occurs. For air containing 4.0 g of water vapour m-3 (relative humidity of 59% at 5°C, 23% at 20°C, and 10% at 35°C), the shoot-to-air difference in water vapour content is 2.8gm-3, 13.3 g m-3, and 35.7 g m-3 at the respective temperatures. The rate of water loss depends on this shoot-to-air difference in water vapour content and the amount of stomatal opening; for the same amount of stomatal opening, transpiration is thus 13.3/2.8 or 4.8-fold higher at 20°C than at 5°C and 35.7/13.3 or 2.7-fold higher at 35°C than at 20°C. If the shoot and air temperatures are 15°C cooler during the night than during the day (which is realistic where CAM plants are grown) transpiration would be three to five times lower during the night than during the day for the same amount of stomatal opening. This underscores the importance of nocturnal stomatal opening for water conservation by CAM plants. CAM crops also tend to have a lower maximal stomatal conductance than do C3 and C4 crops, which further reduces water loss (Nobel, 1988, 1994).

Plants using the C3 or the C4 photosynthetic pathway have a net CO2 uptake only during the day, whereas CAM plants take up CO2 predominantly during the night. However, they can also take up CO2 during the day, especially when soil water is not limiting (Fig. 14.1). Any evaluation of shoot gas exchange over 24 h periods, as is necessary and conventional for CAM plants (Fig. 14.1), automatically includes the contribution of respiration to net CO2 uptake. The maximal instantaneous rates of net CO2 uptake by Agave mapisaga and O. ficus-indica occur at night and can be greater than those

Fig. 14.1. Net CO2 uptake over 24 h periods for various cultivated CAM species under approximately optimal conditions. Data for leaves of Agave mapisaga are from Nobel et al. (1992); for leaves of Agave fourcroydes from Nobel (1985); for leaves of Ananas comosus at a suboptimal PPF averaging 360 mmol m-2 s-1 from Medina et al. (1991); for stems of Opuntia ficus-indica from Nobel (1988) and P.S. Nobel (unpublished observations); and for stems of Stenocereus queretaroensis from Nobel and Pimienta-Barrios (1995) and P.S. Nobel (unpublished observations).

Fig. 14.1. Net CO2 uptake over 24 h periods for various cultivated CAM species under approximately optimal conditions. Data for leaves of Agave mapisaga are from Nobel et al. (1992); for leaves of Agave fourcroydes from Nobel (1985); for leaves of Ananas comosus at a suboptimal PPF averaging 360 mmol m-2 s-1 from Medina et al. (1991); for stems of Opuntia ficus-indica from Nobel (1988) and P.S. Nobel (unpublished observations); and for stems of Stenocereus queretaroensis from Nobel and Pimienta-Barrios (1995) and P.S. Nobel (unpublished observations).

of most other perennials whose maximal rates occur during the day. The instantaneous net CO2 uptake rates of these highly productive CAM species can exceed 25 |mmolm-2s-1 (Fig. 14.1), whereas nearly all ferns, shrubs and trees, as well as many C3 and C4 crops, have lower maximal uptake rates (Nobel, 1991b). The maximal net CO2 uptake rates for A. fourcroydes and Stenocereus queretaroensis are about 10 |mmol m-2 s-1 and are similar to those of many other perennials. Maximum instantaneous values for net CO2 uptake by Ananas comosus examined over 24 h periods by various research groups are relatively low (Bartholomew and Malezieux, 1994); for example, 2.2 |mmol m-2 s-1 (Borland and Griffiths, 1989), 2.4 |mmol m-2 s-1 (Nose et al, 1986), 3.6 |mmol m-2 s-1 (Neales et al., 1980) and 4.9 |mmol m-2 s-1 with a suboptimal PPF (Fig. 14.1; Medina et al., 1991). Indeed, net CO2 uptake by A. comosus has apparently not been measured under optimal conditions. Yet because of the high WUE of all of these CAM species, they can have a relatively high productivity when cultivated under dryland (non-irrigated) conditions. In the future, the most important and relevant comparisons among crops differing in photosynthetic pathway may be based on WUE, because the available supply of groundwater is steadily decreasing, as is the supply of uncontaminated surface water available for irrigation at a reasonable cost.

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