Knowledge of the upper limits of temperature tolerance is essential to understand how tropical trees will respond to global warming. We quantified leaf thermotolerance in 41 tree species growing in a seasonally dry tropical region of the Indian subcontinent to examine: (1) differences between evergreen and deciduous species; (2) relationships with leaf mass per area (LMA) and leaf size; and, (3) seasonal variation in thermotolerance. Thermotolerance ranged from 45.5 °C to 50.5 °C among species, was higher for evergreen than deciduous species, and was negatively related to a continuous estimate of deciduousness. Species with higher LMA had higher thermotolerance, but we did not detect any relationship between leaf size and thermotolerance. Seasonal changes in thermotolerance varied among species implying that species’ capacity to acclimate may differ. Thermal safety margins, the difference between thermotolerance and maximum habitat temperatures indicate that most species may be highly vulnerable to future warming. Overall our results show that deciduous, and fast growing species with low LMA are likely to be more negatively affected by global warming. This differential vulnerability may lead to directional changes in composition in dry tropical forests, and such changes could alter vegetation-atmosphere feedbacks and further exacerbate global warming.

We observed that the upper thermal limits of leaf function in 12 tree species from a seasonally dry tropical forest were perilously close to maximum temperatures in this region. Exposure to drought increased thermotolerance, and species that were more drought tolerant were also more thermotolerant. Importantly, thermotolerance was positively related to the key leaf functional trait - leaf mass per area, and negatively related to photosynthetic rates. These results indicate that tropical trees will be vulnerable to increased warming, and more productive species with lower LMA and higher photosynthetic rates may be more negatively affected by global warming and future climate change.

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Summary It has been observed that the yellowing in the flue‐cured tobacco leaf starts from tips and margins and progresses gradually towards the base and midrib. A study of the pattern of distribution of chemical constituents in the different regions of the leaf was undertaken to see how it tallied with the pattern of yellowing. Twenty leaves were cut into two halves each by removing the midrib and each half was again cut into six portions such as base, middle, tip, lower margin, middle margin and upper margin. The segments from corresponding regions were mixed. The twelve composite samples thus obtained were analysed for moisture, total nitrogen, protein nitrogen, nicotine, total sugars and starch. It was found that there was a gradient in these constituents and that the tips and margins which yellowed first contained more total nitrogen, protein nitrogen and nicotine and less moisture and starch than the areas near the base and midrib.