Alternative Pathways

Many plant species that evolved in hot, dry climates fix carbon through alternative pathways. Under hot and dry conditions, plants can rapidly lose water to the air through small pores called stomata (STOH-muh-tuh). Stomata (singular, stoma), shown in Figure 6-10, are usually located on the undersurface of the leaves. Plants can reduce water loss by partially closing their stomata when the air is hot and dry.

Stomata are the major passageways through which CO2 enters and O2 leaves a plant. When a plant's stomata are partly closed, the level of CO2 in the plant falls as CO2 is consumed in the Calvin cycle. At the same time, the level of O2 in the plant rises as the light reactions generate O2. Both a low CO2 level and a high O2 level inhibit carbon fixation by the Calvin cycle. Alternative pathways for carbon fixation help plants deal with this problem.

figure 6-10

(a) OPEN STOMA

(b) CLOSED STOMA

figure 6-10

(a) OPEN STOMA

(b) CLOSED STOMA

These photos show stomata in the leaf of a tobacco plant, Nicotiana tabacum. (a) When a stoma is open, water, carbon dioxide, and other gases can pass through it to enter or leave a plant (814X). (b) When a stoma is closed, passage through it is greatly restricted (878X).

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