Environmental factors greatly influence the structural and physiological adaptations of leaves in plants. Some of the factors like light, temperature and water greatly affect the structural and biochemical change in the leaf. Thus plants growing under forest canopy and deserts (Table 13-6) have different structural adaptabilities.
Xerophytic plants have adaptations which help the plant to minimize water loss and at the same time maintain sufficient net uptake of CO2 from the surroundings. In these plants, either the cuticle is extremely thick, abundant sclerenchymatous tissue is present and their leaves are small or well dissected or their leaves are swollen, and succulent.
In both the categories, the stomata are few, and sunken. Thus transpiration is reduced but the diffusion resistance for CO2 intake is increased. As a result xerophytes are less efficient photosynthetically as far as per unit area is concerned.
Most of the succulents have Crassulacean acid metabolism (CAM). Here stomata remain closed during the day and open at night when water loss is little due to prevailing low temperature. This CO2 is fixed via PEP—carboxylase initially into oxaloacetic acid and then to other 4-carbon acids e.g., malic acid, etc. (Fig. 13-38).
This is called dark carbon dioxide fixation and is most efficient at 10- 15°C. CAM plants have parenchyma cells which are large and vacuolated. These vacuoles are used for storing malic and other acids in large amounts.
RuBP + CO2 → 2 PGA → 2 PEP
2 PEP + CO2 → 2 oxaloacetic acid → malic acid
These organic acids temporarily keep CO2 in bound form. During the day when the stomata are closed, the CO2 is released from these organic acids and photosynthesized through the Calvin cycle. These acids are decarboxylated and the CO2 released is fixed to produce PGA.
CAM has several advantages e.g., it increases water-use efficiency of the plant and secondly through its enzyme PEP carboxylase, they are adapted to extreme hot climates. Further, CAM plants can also obtain a CO2 compensation point of zero at night and in this way accomplish a steeper gradient for CO2 uptake compared with C3 plants.
Recent studies have shown sufficient similarity between the CAM and C4 plants in regard to carbon fixation and photosynthesis. For instance in both category of plants, during photosynthesis CO2 is fixed into C4dicarboxylic acids. However, they differ in regard to the place and time of carboxylation and decarboxylation.
In CAM plants, carboxylation occurs during the night while decarboxylation takes place during the day time. In C4 plants on the contrary, carboxylation reaction occurs in mesophyll cells while the carbohydrate synthesis takes place in the bundle sheath cells.
Moreover, both these reactions occur during the day or night conditions. Further, organic acids formation in CAM plants is temporary during the night/dark. Figure 13-39 shows broad scheme of CO2 fixation in Sedum sp. CAM plant.
Plants growing in shade and sun are differently adapted structurally and physiologically to different levels of available light or irradiance. Plants growing in shade or below the forest canopy have leaves which are thin with large surface area. The chlorophyll is more and stomata are few per unit area. Since they possess higher leaf area ratio, they have the ability to use small amounts of light more efficiently.
They also have following additional attributes:
(i) Low light compensation point;
(ii) Greater amount of photosynthetic tissue;
(iii) Ability to maintain a net uptake of CO2;
(iv) More efficient to use incident light, and
(v) Lower level of ribulose 1, 5-bisphosphate carboxylase enzyme.
Plants growing in sunlight have attributes opposite to those mentioned in (i)-(v) as above.