New Delhi
The capital's toxic air could be detrimental for plant life, reducing chlorophyll levels that are "directly proportional" to the amount of oxygen produced by a tree, a Delhi University (DU) study has found.
Researchers at DU's Acharya Narendra Dev College examined ten species of trees found widely in the city. These include 'Bael' (Aegle marmelos), 'Saptparni' (Alstonia scholaris), 'Neem' (Azadirachta indica), 'Amaltas' (Cassia fistula), 'Bargad' (Ficus benghalensis), 'Pipal' (Ficus religiosa), 'Pilkhan' (Ficus virens), 'Mango' (Mangifera indica), 'Champa' (Plumeria alba) and 'Jamun' (Syzygium cumini).
The study, titled 'Seasonal variation of biochemical and morphological traits of selected tree species in polluted urban areas of Delhi City' was conducted across four zones, with Ashram marked as a polluted road stretch, Tughlaqabad as an industrial hotspot, Dwarka as a moderately polluted residential area, and the Kamla Nehru Ridge as the least polluted urban biodiversity park.
The methodology section of the study states that leaf samples were collected during monsoon (August-September) and winter (November-December) in 2023. The researchers examined biochemical indicators such as chlorophyll, proline, water content and pH, along with structural traits like thickness and dry matter content.
According to the study, the monsoon period allowed most trees to maintain higher chlorophyll levels, thicker leaves and better water content because rainfall washed dust off the leaf surface and humidity helped the leaves stay hydrated.
In contrast, winter pollution had a greatly adverse impact. The study observed that the dense particulate load in winter reduced chlorophyll more sharply, increased proline levels, made leaves thinner and lowered their water content across sites.
The researchers noted that while monsoon conditions supported recovery and healthier leaf function, winter conditions intensified stress, making biochemical and structural weakening more pronounced.
The study noted that Amaltas and Bargad were among the most pollution tolerant across sites, while Saptparni and Bael were consistently the most sensitive. Mango, Champa, Pilkhan and Jamun fell in the moderately tolerant range.
Chlorophyll enables a tree to carry out photosynthesis, which produced energy out of sunlight and releases oxygen.
Vineet Kumar Singh, an assistant professor in the college and an author of the study, told PTI that the decrease in chlorophyll reduces photosynthesis, which is directly proportional to oxygen production.
Pollutant particles settle on leaves and block the stomata, tiny pores through which leaves release oxygen, disturbing the plant's normal exchange, he said citing the study.
"When dust mixes with the oxygen coming out, the proportions change and the leaf cannot function normally," he said.
The study says winter pollution affects trees more strongly due to the high particulate load. The seasonal increase in pollution makes leaves thinner, lowers their water content and interferes with their internal balance.
The paper found that proline, which rises when a plant undergoes stress, shoots up during winter at the more polluted sites. Neem, Amaltas and Bael recorded spike in proline at Ashram and Tughlaqabad zones, while those in Kamla Nehru Ridge showed milder increase because of lower pollutant levels.
The study also examined the pH level in leaves to assess their internal environment and functions. Healthy leaves usually maintain a steady pH, while those affected by pollution became more acidic as pollutant particles disturbed normal cellular activity.
According to the study, Jamun at Ashram zone showed one of the lowest pH readings in winter, suggesting stress and altered metabolism.
Relative water content, which indicates how much water a leaf can hold, also fell in several species. The study said Bael, Saptparni and Amaltas had reduced water content at polluted sites because dust-clogged stomata interfered with their water retention.
The study also analysed leaf structure and found that leaves became noticeably thinner in winter at high-pollution locations. Neem showed one of the highest dips in leaf thickness, which, according to researchers, is a sign of stress as thinner leaves lose water faster and offer less protection from dust.
Leaf thickness dropped at polluted sites. According to the study, Saptparni recorded some of the thinnest leaves at Ashram and Dwarka in winter, indicating weakened protection against dust and moisture loss. Bargad and Champa maintained greater thickness and were less affected.
Leaf dry matter content, which reflects a leaf's sturdiness, rose in tolerant species like Mango, Amaltas and Jamun. The study says this increase helps these trees reinforce themselves against winter pollution. Species such as Champa and Saptparni, which showed low dry matter content, remained more vulnerable.
"Pollution exposure disrupts plant physiology and structure by altering chlorophyll, water content, pH and leaf form," the study read.
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