Comparative photosynthesis, growth and transpiration of two species of Atriplex

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Abstract
Throughout a period of 23 days, during which the photosynthesis, growth and transpiration of two species of Atriplex were compared, A. spongiosa, a “C4” species (first products of photosynthesis = 4-C dicarboxylic acids), maintained net rates of leaf photosynthesis as high as, or higher than, those of A. hastata, a “C3” species (photosynthesis exhibiting the Calvin-type characteristics). However, as the experiment progressed, the proportion of photosynthate which was used to produce new leaf material declined progressively in A. spongiosa, so that total plant growth rate, initially more than twice as high as in A. hastata, declined to only 0.8 of the A. hastata value. This result demonstrated clearly that more efficient photosynthesis is only one factor, and in this case a relatively minor factor, in total growth rate. Transpiration rates were consistently lower in A. spongiosa than in A. hastata and the ratio declined slightly during the experiment. In consequence, water-use efficiency, both on a single-leaf and whole-plant basis, was much greater in the C4 species. Levels of mesophyll resistance (r m ) were consistently lower in A. spongiosa and increased from about 0.4–0.6 to 1.2–1.5 s cm−1 during the experiment. In A. hastata there was more variability in r m levels but little overall trend towards a higher r m , initial and final values being of the order of 2.5–2.6 and 2.6–2.9 s cm−1, respectively. Levels of stomatal resistance (r l ) were higher in A. spongiosa (about 1.0–1.2 s cm−1) than in A. hastata (about 0.7–0.8 s cm−1) at the beginning of the experiment and increased to 2.0–2.6 s cm−1, whereas they remained relatively constant in A. hastata. The combination of relatively low r m levels and relatively high r l levels provide the explanation for the substantially greater water use efficiency in A. spongiosa. The progressive changes in these levels and in the pattern of leaf area development in A. spongiosa provide an elegant example of adaptation to arid conditions by this species.