Comparative Study of Jatropha curcas Accessions Control of Photoinhibition and Photoprotective Mechanisms in Senescing Leaves in a Semi-Arid Region Botswana

Senescence in plants is the last development phase, leading ultimately to the death of organs such as leaves, sepals, petals, and fruits. During senescence, internal factors and the environment play an important role in tightly controlled alterations at the molecular, cellular, biochemical, and physiological levels. However, leaves are programmed to perform the crucial task of nutrient remobilization. Remobilization of nutrients is a life strategy to supply nutrients to plant parts, such as leaf primordia, emerging new leaves, reproductive organs, or storage organs. This study focussed on how the J. curcas accessions in Southeast Botswana compared in their control of photoinhibition and photoprotective mechanisms of their senescing leaves as a life strategy. J. curcas accessions were raised in a field located in the Department of Agricultural Research, Sebele, Botswana (25° 56′ 37′′ E 24° 3′ 40′′ S). The accessions originated from several parts of the country: Tsamaya, from the north; Tabala, from the central region; and Tlokweng, from the southeast region. One of the accessions was obtained from Ghana. Seedlings were transplanted into the 0.5 ha field with a spacing of 2 m × 2 m in December 2011. Drip irrigation supplied 5 litres of water per week. Gas exchange, chlorophyll fluorescence, photosynthetic pigments, and antioxidants were studied. The onset of senescence triggered degradation of chlorophyll and carotenoid pigments with the consequent decline of photosynthesis. Reduction in the dark adapted Fv/Fm ratio pointed to increased photoinhibition. In early senescence, carotenoid levels decreased gradually and remained functional, allowing photoprotection through their dissipation of excess energy harmlessly as heat. Increased SOD and CAT activities implied increased ROS levels. SOD and CAT activities slowed down destruction by ROS, facilitating nutrient remobilisation. In conclusion, the degradation of the photosynthetic machinery of senescing leaves increases photoinhibition and photooxidation stress. Photoinhibition was more pronounced towards the end of senescence, while photoprotection was more pronounced earlier in senescence to prevent premature death of leaves during remobilization. Ghana and Tlokweng accessions exhibited stronger photoprotection mechanisms in early senescence, allowing nutrient remobilisation compared to the Tsamaya and Tlokweng accessions. Their higher anthocyanin levels in early senescence added to the photoprotective mechanisms in early senescence.