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factor of conductance to P; l, layer; LAI, leaf area index; Lflux, calculated equally weighted light dose; N, leaf nitrogen content; N b , residual leaf nitrogen content; P, net photosynthetic rate; P ml , maximum photosynthetic capacity under saturating light and CO 2 ; PPFD, photosynthetic photon flux density; PS II, photosystem II; Rubisco, ribulose-1,5-bisphosphate-carboxylase-oxygenase; c, ratio of P ml to N. INTRODUCTIONWithin the past two decades, considerable information has been collected on the occurrence of photoinhibition under natural conditions among species, environments and ecotypes. It is now well recognized that exposure of leaves to excessive light, particularly in combination with other environmental stresses, can result in enhanced photoinhibition (e.g. Powles & Critchley 1980; Long, Humphries & Falkowski 1994;Werner & Correia 1996). Furthermore, there is increasing evidence that natural light levels alone are sufficient to cause photoinhibition (Bolhár-Nordenkampf, Hofer & Lerchner 1991; Ögren & Evans 1992;Raven 1994). Despite the rather good understanding of the phenomenon of photoinhibition, little is known about its importance for photosynthetic carbon assimilation, whole-plant primary production and growth under natural field conditions (Ögren 1994). Few studies have yet attempted to quantitatively estimate the effect of photoinhibition on plant primary production (Ögren & Sjöström 1990; Long et al. 1994).Whereas in former studies photoinhibition was frequently considered as a damaging process (e.g. Osmond & Chow 1988), there is now increasing evidence for its importance as a protective process which mediates the controlled dissipation of excessive light energy (Krause 1988;Demmig-Adams 1990; Demmig-Adams & Adams 1992). This comprises a variety of processes at different sites in the chloroplast, as for example non-radiative dissipation of the excess excitation energy in the antenna or degradation of the D1-protein and D1-turnover (Bilger, Schreiber & Bock 1995). The development of new molecular techniques stimulated intense research on the physiological mechanisms of photoinhibition and elucidate our understanding of the related processes. However, many of these studies ABSTRACT A canopy photosynthesis model was modified to assess the effect of photoinhibition on whole-plant carbon gain. Photoinhibitory changes in maximum quantum yield of photosystem II (F v /F m ) could be explained solely from a parameter (Lflux) calculated from the light microenvironment of the leaves. This relationship between F v /F m and the intercepted cumulative light dose, integrated and equally weighted over several hours was incorporated into the model. The effect of photoinhibition on net photosynthesis was described through relationships between photoinhibition and the shaping parameters of the photosynthetic light-response curve (quantum use efficiency, convexity, and maximum capacity). This new aspect of the model was then validated by comparing measured field data (diurnal courses of F v /F m ) with si...

To analyse characteristic patterns of dynamic and chronic photoinhibition within a plant community, a new technique is proposed, which is based on the long- and short-term recovery time of maximum photochemical efficiency of PSII (F v/F m) after environmental stress. Chronic photoinhibition was determined as a sustainable decrease in predawn F v/F m, occurring during periods of prolonged stress, whereas dynamic photoinhibition was assessed from the fully reversible diurnal decline in F v/F m. Applied to a Mediterranean macchia ecosystem, this definition allowed the characterization of typical annual patterns of chronic and dynamic photoinhibition. Both types of photoinhibition were highest during summer drought. However, differences emerged among the ten dominant macchia species regarding their susceptibility to chronic photoinhibition during different seasons. Chronic and dynamic photoinhibition were dependent on leaf orientation. Semi-deciduous species avoided enhanced chronic photoinhibition through a reduction of excessive light interception by vertical foliage orientation during summer, whereas evergreen sclerophylls did not exhibit pronounced structural photoprotective mechanisms. Chronic and total photoinhibition were significantly correlated with predawn and midday water potentials, respectively, and a grouping of the macchia species into three functional groups is proposed according to this relationship.

The carbon-dioxide response of photosynthesis of leaves of Quercus suber, a sclerophyllous species of the European Mediterranean region, was studied as a function of time of day at the end of the summer dry season in the natural habitat. To examine the response experimentally, a "standard" time course for temperature and humidity, which resembled natural conditions, was imposed on the leaves, and the CO2 pressure external to the leaves on subsequent days was varied. The particular temperature and humidity conditions chosen were those which elicited a strong stomatal closure at midday and the simultaneous depression of net CO2 uptake. Midday depression of CO2 uptake is the result of i) a decrease in CO2-saturated photosynthetic capacity after light saturation is reached in the early morning, ii) a decrease in the initial slope of the CO2 response curve (carboxylation efficiency), and iii) a substantial increase in the CO2 compensation point caused by an increase in leaf temperature and a decrease in humidity. As a consequence of the changes in photosynthesis, the internal leaf CO2 pressure remained essentially constant despite stomatal closure. The effects on capacity, slope, and compensation point were reversed by lowering the temperature and increasing the humidity in the afternoon. Constant internal CO2 may aid in minimizing photoinhibition during stomatal closure at midday. The results are discussed in terms of possible temperature, humidity, and hormonal effects on photosynthesis.

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