Are C4 Pathway Plants Threatened by Global Climatic Change?
535
carboxylase) under a variety of short -term environmental perturbations of
CO 2 partial pressure, irradiance, and leaf temperature. The near constancy
of 21 % leakiness over the wide variety of conditions imposed is greater than
that estimated previously by Hatch and Osmond (1976). Evidently many
factors contribute to the regulation that occurs between the mesophyll
and the bundle-sheath cells. Table 25.1 also shows that leakiness did not
vary when measured during short-term variation in CO2 concentration with
Sorghum bicolor (L.) Moench and Amaranthus edulis. However, there was
a small increase in leakiness at irradiances of less than 250 J..lmol quanta
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Leaf Nitrogen Content
(mmol m- 2 )
Fig. 25.3a-d. The effect of leaf nitrogen content on a Rate of CO2 assimilation in
Sorghum bicolor (L.) Moench (cv. Tx61OSR). b The ratio of intercellular to ambient
partial pressure of CO2, P/Pa' c Carbon isotope discrimination measured during CO 2
exchange, d. d Bundle-sheath leakiness, <1>. Leakiness was calculated from the measurements of d in c, and measurements of P/Pa in b using the equation d = 4.4 + (28.2<1> -
9.6) P/Pa' The lines are drawn by eye. Measurements were made at an irradiance of
1600 ~mol quanta m -2S- I, an ambient CO2 partial pressure of 330 ~bar and a leaf-to-air
vapor pressure difference of 20 mbar and each symbols is the average of three to five
measurements per leaf. Leaf nitrogen was determined on an auto-analyzer after a
Kjehldahl digestion