This study addresses the role of the circadian clock in the seasonal growth cycle of trees: growth cessation, bud set, freezing tolerance, and bud burst. Populus tremula 3 Populus tremuloides (Ptt) LATE ELONGATED HYPOCOTYL1 (PttLHY1), PttLHY2, and TIMING OF CAB EXPRESSION1 constitute regulatory clock components because down-regulation by RNA interference of these genes leads to altered phase and period of clock-controlled gene expression as compared to the wild type. Also, both RNA interference lines show about 1-h-shorter critical daylength for growth cessation as compared to the wild type, extending their period of growth. During winter dormancy, when the diurnal variation in clock gene expression stops altogether, downregulation of PttLHY1 and PttLHY2 expression compromises freezing tolerance and the expression of C-REPEAT BINDING FACTOR1, suggesting a role of these genes in cold hardiness. Moreover, down-regulation of PttLHY1 and PttLHY2 causes a delay in bud burst. This evidence shows that in addition to a role in daylength-controlled processes, PttLHY plays a role in the temperature-dependent processes of dormancy in Populus such as cold hardiness and bud burst.

The occurrence of photoinhibition of photosynthesis in leaves of a willow canopy was examined by measuring the chlorophyll-a fluorescence ratio of F V/F M (FM is the maximum fluorescence level of the induction curve, and FV is the variable fluorescence, F V=F M-F 0, where F0 is the minimal fluorescence). The majority of the leaves situated on the upper parts of peripheral shoots showed an afternoon inhibition of this ratio on clear days. This was the consequence of both a decrease in F M and a rise in F O. In the same leaves the diurnal variation in intercepted photosynthetic photon flux density (PPFD) was monitored using leaf-mounted sensors. Using the multivariate method, partial least squares in latent variables, it is shown that the dose of PPFD, integrated and linearly weighted over the last 6-h period, best predicts photoinhibition. Photoinhibition occurred even among leaves that did not intercept PPFDs above 1000 μmol·m(-2)·s(-1). Exposure of leaves to a standard photoinhibitory treatment demonstrated that the depression in the F V/F M ratio was paralleled by an equal depression in the maximal quantum yield of CO2 uptake and a nearly equal depression in the rate of bending (convexity) of the light-response curve of CO2 uptake. As a result, the rate of net photosynthesis is depressed over the whole natural range of PPFD. By simulating the daily course in the rate of net photosynthesis, it is estimated that in the order of one-tenth of the potential carbon gain of peripheral willow shoots is lost on clear days as a result of photoinhibition. This applies to conditions of optimal temperatures. Photoinhibition is even more pronounced at air temperatures below 23° C, as judged from measurements of the FV/FM ratio on clear days: the afternoon inhibition of this ratio increased in a curvilinear manner from 15% to 25% with a temperature decrease from 23° to 14° C.

Photosynthesis in the intermediate light range is most efficientwhen the convexity of the photosynthetic light-response curve is high. Factors determining the convexity were examined for intact leaves using Salix sp. and for a plant cell culture using the green microalga Coccomyra sp. I t was found that the leaf had lower convexity than diluted plant cells because the light gradient through the leaf was not fully matched by a corresponding gradient in photosynthetic capacity. l h e degree to which the leaf gradients were matched was quantified by measuring photosynthesis at both leaf surfaces using modulated fluorescence. Two principal growth conditions were identified as those causing mismatch of leaf gradients and lowering of the convexity relative to cells. l h e first was growth under low light, where leaves did not develop any noteworthy gradient in photosynthetic capacity. This led to decreased convexity, particularly in old leaves with high chlorophyll content and, hence, steep light gradients. Second and less conspicuous was growth under high light conditions when light was given bilaterally rather than unilaterally, which yielded leaves of high photosynthetic capacity at both surfaces. Two situations were also identified that caused the convexity to decrease at the chloroplast levd: (a) increased light during growth, for both leaves and cells, and (b) increased CO, concentration during measurement of high-lightgrown leaves. These changes of the intrinsic convexity were interpreted to indicate that the convexity declines with increased capacity of ribulose-1,5-bisphosphate carboxylase/oxygenase relative to the capacity of electron transport.The light-response curve of photosynthesis is accurately described by a nonrectangular hyperbola (Prioul and Chartier, 1977; Leverenz and Jarvis, 1979; Marshall and Biscoe, 1980) that has the form e p 2 -(91 + P,,,) P + +IP,,, = O (1) where P is the rate of photosynthesis ( y variable), I is the irradiance ( x variable), CP is the maximum quantum yield, 8 is the convexity, and P,,, is the light-saturated rate of photosynthesis. @ and 8 are parameters of particular importance for photosynthetic productivity under natural conditions, because it is believed that the rate of photosynthesis is limited by light most of the time (Long, 1985; Ort and Baker, 1988). As can be seen in Figure 1 from the initial line set by @ to the plateau set by P,,, (the so-called Blackman curve). This is never realized for cells and leaves that typically show 8 values within the range of 0.7 to 0.99. It should be noted that 8 does not follow a linear scale. Therefore, a curve of 8 = 0.90 is more different from a curve of 8 = 0.99 than it is from a curve of 8 = 0.75