Light response curve

To look closer to light response kinetics specific growth rates have been determined experimentally (Lehr et al, 2012). In Fig. 5 a typical growth vs light response curve is shown. 

The light response curve of the whole leaf displays the normal hyberbolic shape (Fig. 1). It represents the sum of the light response... 

The slope of the initial linear part of the light response curve (quantum yield region) is proportional to both absorptance of a leaf and the quantum yield. Absorptance of a leaf is affected by the pigment content and the structure of the leaf tissues. Normally, healthy green leaves absorb 75 to 95% of incident quanta of white light. 

The quantum yield is expressed as the number of moles of O2 evolved (or CO2 fixed) per mole absorbed quanta of photosynthetically active radiation (400-700 nm). Obviously the quantum yield is constrained by biophysical/biochemical processes. Healthy leaves of C3 plants show almost identical quantum yields for O2 evolution at a saturating partial pressure of C02f irrespective of taxa and habitats of the materials, which approach the theoretical maximum of around 0.11 mol O2 mol absorbed quanta (2,3). Compared with this, the quantum yield is considerably lower at the CO2 partial pressure of around 350 bar due to photorespiration. Nevertheless, variation in the quantum yield among various C3 plants remains small also at this CO2 partial pressure. Thus, initial slopes of the light response curves are not markedly different among "healthy C3 plants. 

Isobilateral dicot leaves, conifer needles, and many monocot leaves, receive light on both surfaces. The light response curves of photosynthesis of such leaves have sharper transition when the leaves are irradiated from both sides simultaneously than when irradiated unilaterally (18-20). This may also be ascribed to the differentiation of tissues in terms of optical properties and the gradient in properties of chloroplasts. Thus, it can be generalised that the leaf is constructed so as to maximise the sharpness of the transient region under natural light conditions. 

In the previous sections, we have seen that light response curves of "healthy leaves have similar initial slopes and similar sharpnesses, when the leaves are irradiated appropriately. This is quite reasonable because the higher the quantum yield and the sharper the trcuisition are, the more efficient the leaf photosynthesis is, provided that there are... 

Table 2. The apparent ("Qy") and absolute (Qy) quantum yields of CO2 fixation and O2 evolution. The apparent quantum yields were obtained from slopes of the linear part (50-150 /umolm s" PPFD) of light response curves then corrected to the absolute quantum yield using leaf absorptivity.

In fig. 1 representative light response curves are shown of 4 plants with different treatments. Electron transport rates were calculated from chlorophyll fluorescence measurements and are represented by the symbols (circles = FA,... 

Figure 1 Light response curves in 4 different plants. 0=FA, J =559, M=0.88,...

Figure 2 Maximum electron transport rates versus needle age from light response curve fits of 6 treatments and 10 plants per treatment. Open stars= NH3+NO2, filled stars= SO2+NO2, other symbols as in fig. 1...

Table 1 averaged values (n=10) of light response curves fitted to electron transport rates measured in needles of Douglas fir. Needles of different age were used for these measurements. The values given after is the standard error of the mean. , 0, , significantly different at P = 0.05, 0.01 and 0.001 respectively (one way analysis of variance)... 

FIGURE 5. Effect of illumination in air of control and PPT treated leaves on the subsequent photosynthesis oxygen evolution at high EC023. Light response curves. 

This speaks of the possibility that the midday depression in these cases is due to the damage of the photosynthetic apparatus by photoinhibition and not to the decrease of stomatal conductance. The diurnal change of the light response curve of bamboo photosynthesis has been measured(Fig.8)(9). From Fig.8, it may be seen that not... 

Fig.12 Light response curves of wheat leaf photosynthesis at different atmospheric pressure...

An alternative approach to the use of light response curves is to normalise this measure (1), and to define the light limitation (L,2) as ... 

Other problems with the sensitivity approach occur when, as is usual, the light response curve does not cross both axes at zero. Furthermore, a linear photosynthetic response to light does not necessarily imply that the availability of light is the sole limiting factor, as two photosynthetic response curves with different quantum yields may both have L, equal to one (Fig. 1). 

FIGURE 1. Light response curve for photosynthesis (solid), L,2 nd the corresponding changes in c, (dashed line). The response for a leaf with a very low quantum yield is shown dotted. 

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