The effect of drought on the photosynthetic functioning of two C3 plants, Phaseolus vulgaris and Elatostema repens, has been examined. Leaf net CO2 uptake measured in normal air was negligible at a leaf water deficit of about 30% while the calculated leaf intercellular CO2 concentration (Ci) was unchanged. However, both the maximal photosynthetic capacity (CO2-dependent O2 evolution) and apparent quantum yield, measured in the presence of saturating CO2 levels (5 to 14%), only started to decrease within the range of 25 to 30% leaf water deficit. This shows that the drought-induced inhibition seen in normal air is not caused by an inhibition of the photosynthetic mechanism, and that in this case Ci values can be misleading. Both 77 K and room-temperature fluorescence measurements indicate that the functioning of the photosystem-II reaction centre is hardly modified by water shortage. Furthermore, an analysis of photochemical chlorophyll fluorescence quenching shows, in the absence of CO2, that O2 can be an efficient acceptor of photosynthetic energy, even in severly dehydrated plants which do not show net CO2 uptake in normal air. In these plants, O2 is probably reduced mainly via Mehler-type reactions. High-light treatment given at low O2 increases photoinhibition as measured by the decrease of apparent quantum yield in dehydrated P. vulgaris, whereas, interestingly, 1% O2 protects dehydrated E. repens against high-light damage. The two plants could have different protective mechanisms depending upon the O2 level or different photoinhibitory sites or mechanisms.
The data presented here deal with the effects of high-light exposure on the 77 K fluorescence characteristics of Elatostema repens. It is shown that the decrease of the variable fluorescence during the treatment is biphasic. The reactions responsible for the first phase of fluorescence quenching are saturated under 700 μmol photon m(-2) s(-1) and insensitive to streptomycin, whereas those responsible for the second phase are not yet saturated under 700 μ mol photon m(-2) s(-1) and sensitive to streptomycin. It is concluded that only the second phase of fluorescence quenching is associated with photoinhibitory processes. Rate and amplitude of recovery from photoinhibition are maximum under very low light (3.5 μ mol photon m(-2) s(-1)), and very small at a moderate light (160 μ mol photon m(-2) s(-1)) which does not cause photoinhibition. It is concluded that recovery processes are inhibited during photoinhibition. It is suggested that they could be associated with damage occuring on the oxidizing side of PSII.
We report the photosynthetic characteristics of a C3 shade plant native to the tropical rain forest understory. It was shown that Elatostema repens Lour. (Hall) f. (Urticaceae) presents a large light adjustment capacity. The effects of several lightfleck sequences on photoinhibition of photosynthesis and carbon gain are analyzed. Photoinhibition is measured both as a decrease in leaf net CO2 uptake in limiting light (shown to be linearly correlated to quantum yield of O2 evolution measured at saturating CO2) and as a decrease of the ratio of variable fluorescence (Fv) to maximum fluorescence (Fmax) measured in liquid nitrogen. It is shown that lightflecks (from 10 to 30 min in duration) of 700 μmol m–2 s–1 (high light) induce photoinhibition, and that the effects of those successive high light periods are additive; there is apparently no recovery from photoinhibition during the low light periods (from 10 to 45 min in duration). In contrast, the Fv/Fmax ratio, though decreasing similarly to quantum yield of net CO2 uptake on leaves submitted to a continuous illumination of 700 μmol m–2 s–1, is only decreased a little on leaves submitted to lightfleck sequences of the same photon flux density. Lightflecks of 250 μmol m–2 s–1 are not photoinhibitory. Compared to the control maintained under light growth condition (40 μmol m–2 s–1) carbon gain is increased on leaves submitted to lightflecks; this gain remains high throughout the light cycles on leaves submitted to nonphotoinhibitory lightflecks and to the photoinhibitory lightflecks followed by the shortest low light period. In the other cases, carbon gain, higher than that of the control at the beginning of the treatments, decreases and becomes lower than the control carbon gain. Finally, the relevance of photoinhibition in the tropical rain forest understory environment is discussed.
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