Quiz 6: Water Relations

Geology/Geography/Oceanography/Atmospheric Sciences

Ultraviolet light has a shorter wave length than visible range. The range is 10nm-400nm. The shorter wave length also consists of a higher amount of energy than that of the visible range. This much energy can produce the following effects on photosynthetic plants: • It leads to a breakdown in the organic molecules that are involved in photosynthesis. This also includes the final product. • The high energy of UV light destroys the biochemical machinery required for photosynthesis. • UV radiation reduces the photosynthetic ability in plants by reducing carbon dioxide uptake from the atmosphere. This reduces the plant's growth. Therefore, UV light cannot be used for photosynthesis. It would not be possible to evolve a photosynthetic system which uses ultra violet light because of the following reasons: • Chlorophyll is the most important pigment in plants. It has a potent role in the absorption of light for photosynthesis. There are two types of subtypes of chlorophyll: chlorophyll a and chlorophyll b. Chlorophyll c is seen in algae. • Chlorophyll a absorbs light in the range of 450-600 nm and chlorophyll b absorbs light best in the range of 500-600 nm. • Since this range is above the range of UV light, it is impossible to think of machinery without the chlorophyll pigments which can use UV light. UV light is invisible to humans and is not tolerable by plants for best survival, but there are few insects that can see this UV range. This light draws their attention towards pollen and nectar. The 365 nm range is best absorbed by insects such as flies and wasps. However, this does not imply any change in the above facts about the effects of UV on plants and photosynthesis. Infrared light has a longer wavelength than that of visible range, but it has a less energy when compared to the visible range. The visible range carries energy as photons, which are the essential minimum requirement for photosynthesis. This much energy is not present in infrared rays. Hence, they cannot be used for photosynthesis by plants.

Range is the most appropriate measure of dispersion. It shows the difference between the smallest and the largest value of the data set. Hence, is calculated as img . Standard deviation is the calculation of the value by which each sample differs from the mean of the total samples present in the data set. It is denoted by sigma (s), and it is calculated as img Variance is the measure of the dispersion of set of samples around the mean of the samples of the data set. It is the square of the differences between the sample and the mean of the samples of the data set. Standard deviation and variance represent the variation in a sample better than range. This is because of the following reasons: • The range gives a total difference between the highest and the lowest value, but the samples inside the data set can have a smaller range than this. The range cannot give the exact variation. Also, there may be sample sets that have same range but differ in variation amongst the respective samples in it. • The standard deviation or the variation calculates the difference on the basis of the median. Thus, the samples that vary greatly and that do not vary much can both be represented by accurate variation, not approximation as in a range calculation. • The range cannot help in the statistical comparison of two samples, whereas variation and standard deviation can produce a statistical comparison chart, as it has individual data of each sample's variation. On basis of the above points, we can conclude that standard deviation and variance are a better representation of variation than range.

C 3 plants are found predominantly in moist climates because carbon dioxide enters leaves through the stomata. This process is accompanied with the exit of water. The water movement from plants to the atmosphere is more rapid than the movement of carbon dioxide from the atmosphere to plants. This is further increased if the climate is dry. Hence, if more water flows out, the leaves will be devoid of water very soon. This leads to a shutdown of photosynthesis. For this, the stomata are closed. This in turn stops the carbon dioxide intake, and thus the C 3 pathway is inhibited. Such an adverse effect can be avoided if the climate is moist. A moist climate has an ample amount of water in the atmosphere. Thus, this reduces the gradient and slows down water flow from leaves to the atmosphere. C 4 plants are found in dry and hot climates because C 4 plants lose less water as they open a smaller amount of stomata at a time for this purpose. CO 2 uptake would not stop in C 4 plants even if the temperature increases. CAM plants are found in arid and semiarid environments and along epiphytes growing in the canopies of forests. The plants can survive in very dry environments, as the carbon dioxide fixation takes place at night when the rate of water loss is highly reduced. Thus, CAM plants loose only 50 g of water with per gram of new tissue being produced. The cooccurrence of C 3 , C 4, and CAM plants is possible in areas with microclimates. A microclimate is a local atmosphere zone where the climate differs from the surrounding areas. This can be a few square feet or many miles in coverage. Such microclimates where the C 3 , C 4 , and CAM plants can co-occur can be a desert, which can have winter and summer annuals. In this, C 3 can occur in the winter climate and C 4 and CAM plants would be seen in semi-arid or arid hot months. Apart from microclimates, seasonal variation due to the drought cycles would favor different C 3 , C 4 , and CAM plants in a single area at different times.