Surviving the sun
A smouldering summer day has just simmered down to a pleasant evening. Pranav and Aunt Piya are strolling in the park, taking a much-needed breather.
Pranav: Look at us. Parched and irritable, though we barely stepped out the whole day. And look at those stunning orange blossoms. Flame of the forest, isn’t it? And that lush green neem! How do these trees brave the blast of the sun and still look so amazingly fresh?
Aunty Piya: Well, these plants are luckier. They have their own defence mechanisms that kick in now to control and restrict the amount of water lost from their bodies. Not only in our city, you can spot a huge variety of these summer survivors in all parts of India, adapting to different soils, temperatures and rainfall. They not only survive but flourish during these punishingly hot months.
Pranav: But how? Read on Pranav and you will know.
Armour for the hot summer
Trees are wise. As long as they have sufficient water in the soil, they ward off heat easily. But when the temperature rises too high and water is scarce, trees restrict the amount of water they lose from their bodies. The commonest is by closing their stomata. So that no water is lost from them. Another way is through transpiration – plant sweating. Most effective method is, by shedding their leaves during the dry, hot season so that transpiration does not take place. No leaves, means less transpiration.
Swadeshi or not?
A large percentage of the trees we see today, especially in cities are exotic species, are not native to India. For eg, the gulmohar belongs to Madagascar and was brought to India by the British as other ‘good looking’ trees were brought in to act ‘avenue trees’. Other exotic species found in India include jacaranda, acacia, rain tree, baobab, tamarind, casuarina, cannonball tree, and many more.
The flowering season coincides with water loss in the plant because flowers draw a lot of nutrients (for color and nectar). In order to do this, in the case of severe water shortage, sometimes trees shed leaves so that nutrients are directed to flowers. However, flowering in spring and summer has strong reproductive and consequently, selective advantage. This is also one of the reasons why trees don’t flower until they become big enough such that their roots can access deep ground water sources.
A necessary evil
A plant with large leaves in the bright sunlight, still air and dry soil is faced with a critical dilemma. Opening of stomata to allow cooling through latent heat loss will result in loss of large volumes of water, while if it keeps its stomata closed to restrict water eventually faces starvation as it can no longer take in CO2. This is why transpiration has been termed as a “necessary evil.”
What if it gets hotter? (Global warming)
If it gets hotter, the change in temperature will be too rapid for plants to adapt to. Even a 2°C temperature increase in the next 50-200 years will be too much for them. They won’t be able to adapt hat fast. In nature, a plant species take about 30-40 million years to adapt to a 1°C rise, so how can they possibly adjust to this?