MEASUREMENT AND MODELING OF PHOTOSYNTHETIC RESPONSE OF PEARL-MILLET TO SOIL-PHOSPHORUS ADDITION

There have been no studies of the effects of soil P deficiency on pear l miller (Pennisetum glaucum (L.) R. Br.) photosynthesis, despite the fact that P deficiency is the major constraint to pearl millet product ion in most regions of West Africa. Because current photosynthesis-bas ed crop simulation models do not explicitly take into account P defici ency effects on leaf photosynthesis, they cannot predict millet growth without extensive calibration. We studied the effects of soil additio n on leaf P content, photosynthetic rate (A), and whole-plant dry matt er production (DM) of non-water-stressed, 28 d pearl miller plants gro wn in pots containing 6.00 kg of a P-deficient soil. As soil P additio n increased from 0 to 155.2 mg P kg(-1) soil, leaf P content increased from 0.65 to 7.0 g kg(-1). Both A and DM had maximal values near 51.7 mg P kg(-1) soil, which corresponded to a leaf P content of 3.2 g kg( -1). Within this range of soil P addition, the slope of A plotted agai nst stomatal conductance (g,) tripled, and mean leaf internal CO2 conc entration ([CO2](i)) decreased from 260 to 92 mu L L(-1), thus indicat ing that P deficiency limited A through metabolic dysfunction rather t han stomatal regulation. Light response curves of A, which changed mar kedly with P leaf content, were modelled as a single substrate, Michae lis-Menten reaction, using quantum flux as the substrate for each leve l of soil P addition. An Eadie-Hofstee plot of light response data rev ealed that both K-M, which is mathematically equivalent to quantum eff iciency, and V-max, which is the light-saturated rate of photosynthesi s, increased sharply from leaf P contents of 0.6 to 3 g kg(-1), with p eak values between 4 and 5 g P kg(-1). Polynomial equations relating K -M and V-max, to leaf P content offered a simple and attractive way of modelling photosynthetic light response for plants of different P sta tus, but this approach is somewhat complicated by the decrease of leaf P content with ontogeny.