Abstract

Jatropha curcas is an oilseed-bearing shrub with high potential for biodiesel production in arid regions, where it is also subject to abiotic stress such as high light and high temperatures. As a plant, being sessile, its survival therefore depends on the development and efficient activation of resistance responses to these stressors. The photosynthetic process is vital for plant growth and survival and the leaves are the major photosynthetic organs. The life cycle of a leaf is made of different stages viz., the emerging leaf, the mature leaf and the last stage, the senescing leaf. The aim of the study was two-fold:
(i) assessing the degree of photoinhibition from the emerging leaf stage (young leaf) through their development to mature leaf stage (older leaves) and
(ii) investigating their photoprotective mechanism(s).
The plants were raised in a field located at Sebele, Botswana The field was established in 2011 from seedlings planted from the seeds of various J. curcas accessions collected from different areas of Botswana (Tsamaya from the north, Tabala from the central region and Tlokweng from the Southeast region). The fourth accession was obtained from Ghana. Gas exchange, chlorophyll fluorescence, photosynthetic pigments and antioxidants were studied. The younger leaves of all accessions (7 days old) exhibited lower photosynthetic rates and lower photosynthetic pigments than the older leaves (35 days old). Ghana and Tlokweng accessions appeared to exhibit higher photosynthetic rates than the Tsamaya and Tabala accessions in all leafs developmental stages. All accessions generally displayed lower photosynthetic rates and dark-adapted F_v/F_m ratios at 13:00hrs than at 07:00hrs. The decline in dark-adapted F_v/F_m ratio and photosynthetic rate exhibited by the accessions at midday can be attributed to higher midday photosynthetic active radiation and temperature. Since the accession exhibited full recovery of their dark-adapted F_v/F_m ratio and photosynthetic rate by late afternoon, the decrease in these two parameters can be associated with high carotenoid levels. Ghana and Tlokweng accessions exhibited higher levels of antioxidants (SOD and CAT) and carotenoids than Tabala and Tsamaya accessions which protect them from effects of high photosynthetic radiation and high temperatures. It is therefore, concluded that the Tlokweng and Ghanaian accessions can be recommended for growing in Botswana (semi-arid country), as valuable feedstocks to produce biodiesel, which is environmentally friendly

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