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Preillumination o f intact pea leaves with a strong blue-green light o f 400 W m "2 markedly inhibited both photoacoustically monitored 0 2-evolution activity and PS II photochemistry as estimated from chlorophyll fluorescence measurements. The aim o f the present work was to examine, with the help o f the photoacoustic technique, whether this high-light treatment dete riorated the in vivo PS I function too. High-frequency photoacoustic measurements indicated that photochemical conversion o f far-red light energy in PS I was preserved (and even tran siently stimulated) whereas photochemical energy storage monitored in light exciting both PS I and PS II was markedly diminished. Low-frequency photoacoustic measurements of the Emerson enhancement showed a spectacular change in the PS II/PS I activity balance in favor o f PS I. It was also observed that the linear portion o f the saturation curve of the far-red light effect in the Emerson enhancement was not changed by the light treatment. Those results lead to the conclusion that, in contrast to PS II, the in vivo PS I photofunctioning was resistant to strong light stress, thus confirming previous suggestions derived from in vitro studies. Estima tion of the redox state of the PS I reaction center by leaf absorbance measurements at ca. 820 nm suggested that, under steady illum ination, a considerably larger fraction o f PS I cen ters were in the closed state in high-light pretreated leaves as compared to control leaves, pre sumably allowing passive adjustment o f the macroscopic quantum yield o f PS I photochemis try to the strongly reduced photochemical efficiency o f photoinhibited PS II. Abbreviations: B,, fraction o f closed PS I reaction cen ters; E, Emerson enhancement; P700, reaction center o f PS I; V, relative variable chlorophyll fluorescence; PS, photosystem; 4>p, quantum yield for photochemistry in PS II; PES, photochemical energy storage.

In the Sahelian zone, low soil N could be as limiting as drought in pearl millet production. Although growth and crop productivity depend on several biochemical reactions in which the nitrogen metabolism plays a great role, there is little information available on how N uptake and key enzymes, nitrate reductase and glutamine synthetase, are affected by nitrogen and water interaction in millet. For this purpose, the millet variety cv. Souna III was grown in the field during the dry season under three levels of nitrogen fertilization (N0 = 0.0, N1 = 17.13, and N2 = 68.50 kg N ha-1) and different water regimes (well-watered and water-stressed) in a split-plot experimental design. Irrigation was stopped for water-stressed plants during tillering, and the grain formation and filling phases, thereby giving rise to two water deficit cycles. A major quantity of mobilized N (79-100%) was taken up before flowering in all N treatments. Nitrogen uptake declined significantly only during the second water deficit cycle. During the first water deficit cycle, aboveground biomass was reduced and the maintenance of the N uptake resulted in increased N and nitrate concentrations. The water deficit reduced nitrate reductase activity in all treatments and the effect was greater under high N. The increase in nitrate concentration under water deficit conditions showed that the reduction in nitrate reductase activity was probably not due to limiting nitrates. Glutamine synthetase activity was higher under the low N treatments, N1 and N0, showing the absence of a stimulating effect of glutamine synthetase activity by nitrate or ammonium. These results are discussed on the basis of their effect on grain N and grain yield.

Water stress is known to induce active oxygen species in plants. The accumulation of these harmful species must be prevented by plants as rapidly as possible to maintain growth and productivity. The aim of this study was to determine the effect of water stress on superoxide dismutase isozymes (SOD, EC 1.15.1.1.) in two cowpea cultivars [Vigna unguiculata L. Walp., cv. Bambey 21 (B21) and cv. TN88-63]. Plants were submitted to water stress by withholding water supply and the expression of SOD was characterized during stress induction. In the same time, photosynthesis characteristics were determined through the measurement of the quantum yield of PS II photochemistry and the energy absorption rate per reaction centre. Results show how water stress regulates the synthesis and the activity of superoxide dismutase isoforms and how these enzymes contribute to protect photosynthesis against the damageable effects of superoxide radicals in cowpea. Increased MnSOD and FeSOD activity and concentration were shown to be induced by water stress and associated with protection of photosystem II photochemistry and whole plant growth against oxidative stress in these plants. On the contrary, plants unable to express high MnSOD and/or FeSOD isoforms showed more sensitivity to water stress.