Leaf Conductance in Relation to Rate of CO 2 Assimilation

Abstract

Plants of Zea mays were grown with different concentrations of nitrate (0.6, 4, 12, and 24 millimolar) and phosphate (0.04, 0.13, 0.53, and 1.33 millimolar) supplied to the roots, photon flux densities (0.12, 0.5, and 2 millimoles per square meter per second), and ambient partial pressures of CO2 (305 and 610 microbars). Differences in mineral nutrition and irradiance led to a large variation in rate of CO2 assimilation per unit leaf area (A, 11 to 58 micromoles per square meter per second) when measured under standard conditions. The variation was shown, with the plants that had received different amounts of nitrate, to be related to variations in the nitrogen and chlorophyll contents, and phosphoenolpyr-uvate and ribulose-1,5-bisphosphate carboxylase activities per unit leaf area. Irrespective of growth treatment, A and leaf conductance to CO2 transfer (g), measured under standard conditions were in almost constant proportion, implying that intercellular partial pressure of CO2 (pJ) was almost constant at 95 microbars. The same proportionality was maintained as A and g increased in an initially nitrogen-deficient plant that had been supplied with abundant nitrate. It was shown that Pi measured at a given ambient partial pressure was not affected by the ambient partial pressure at which the plants had been grown, although it was different when measured at different ambient partial pressures. This suggests that the close coupling between A and g in these experiments is not associated with sensitivity of stomata to change in pi. Similar, though less comprehensive, experiments were done with Gos-sypium hirsutum, and yielded similar conclusions, except that the pro-portionality between A and g at normal ambient partial pressure of CO2 implied Pi = 200 microbars. In a previous paper (8) we found that response of leaf con-ductance, g,' in leaves of Eucalyptus pauciflora, to change in photon flux density, I, was similar to response of rate of CO2 assimilation, A, to I. Thus, intercellular partial pressure of C02, pi, remained almost constant (257-243,ubar) when I changed 8-fold from 0.25 to 2 mmol m-2 s-'. We also showed that the sensitivity of stomata to change in partial pressure of CO2 was too small to maintain pi so nearly constant; the variation in aperture was almost entirely due to a direct response of the variation of I. Following that study, we designed several experiments to further explore the relationship between leaf conduct-'Abbreviations: g, conductance to CO2 transfer, mol m-2 s-'; A, rate of CO2 assimilation, umol m-2 s-'; I, photon flux density (400-700 nm), mmol m-2 s-'; Pa, ambient partial pressure of CO2, gbar; Pi, intercellular partial pressure of CO2Z gbar; PEP, phosphoenolpyruvate; RuP2, ribulose-1 ,5-bisphosphate. ance and rate of assimilation. A brief summary of the results has been published (9). In these experiments, to be described in detail in this and subsequent papers (10, 11), the intrinsic capacity of leaf mesophyll tissue for photosynthesis was varied in a number of ways, involving different time scales. Although the means by which changes in conductance were attuned to changes in mes-ophyll capacity may have been quite different over the different time scales, the effect was that A and g were uniquely related in a given species, in such a way that pi was almost constant. In this paper we describe measurements of A and g in plants in which different photosynthetic capacities had been brought about by different nitrogen and phosphorus nutrition treatments, different photon flux densities during growth, and differing partial pressures of CO2 during growth. MATERIALS AND METHODS Plant Material. Seeds ofZea mays L. and Gossypium hirsutum L. were sown in 5-L plastic pots containing sterilized garden soil. After emergence, seedlings were thinned from four to one per pot to obtain uniform plants. Plants were grown in a glasshouse under full sunlight, the photoperiod being 12 to 13 h, and the midday photon flux density (400-700 nm) being about 2 mmol m-2 s_'. Air temperature in the glasshouse was 32 ± 2°C during the day and 18 ± 2°C at night. RH varied between 50 and 70%. The soil in each pot was flushed daily with 1 L of nutrient solution in the late afternoon. During the daytime the plants were watered lightly every 3 h to compensate for loss by transpi-ration. Nutrient solutions were based on Hewitt's nitrate-type nutrient solution, consisting of 12 mM NO3-, 4 mm K+, 4 mM Ca2+, 1.5 mM Mg2+, 1.33 mM PO43-with balancing Na+, SO42-, Cl-and micronutrients. Nitrogen nutrition experiments comprised four groups of 24 Z. mays plants and 10 G. hirsutum plants treated with 24, 12, 4, and 0.6 mM NO3-in nutrient solutions. Phosphorus nutrition experiments comprised four groups of 6 Z. mays plants treated with 1.33, 0.53, 0.133, and 0.040 mM PO43-in nutrient solutions. In experiments on the effect of ambient partial pressure of CO2 during ontogeny, plants were grown in two glasshouses. One glasshouse was well ventilated, the partial pressure of CO2 being about 320 ± 20 Mbar (i.e. normal atmospheric partial pressure in Canberra). The partial pressure of CO2 in the other glasshouse was maintained at 640 ± 15 Mbar by injecting pure CO2. The partial pressure of CO2 was monitored and controlled with an URAS II (Hartman and Braun, Frankfurt, West Germany) IR gas analyzer. The plants in each glasshouse were divided into four groups, 8 Z. mays plants and 6 G. hirsutum plants in each group, which were given different nitrogen nutrient treatments as described previously. Gas Exchange Methods. Measurements were made on G. 821