Effects of water stress on photosynthetic activity, dry mass partitioning and some associated metabolic changes in four provenances of neem (Azadirachta indica A. Juss)

Chlorophyll (Chl) content, dry mass, relative water content (RWC), leaf mass per area (LMA), proline (Pro) content, malondialdehyde (MDA) content, superoxide dismutase (SOD) and peroxidase (POD) activity, P N-PAR response curves and gas exchange were studied to determine the effects of water stress on photosynthetic activity, dry mass partitioning and metabolic changes in four provenances of neem (Azadirachta indica A. Juss). The results indicated that provenance differences existed in the adaptation response to water stress that included changes to growth strategies coupled with ecophysiological and metabolic adjustments. As water stress increased, stomatal conductance (g s), net photosynthetic rate (P N), transpiration rate (E), and leaf RWC decreased while LMA increased in all provenances. Dry mass was reduced in droughted plants and the percentage increased in dry mass allocated to roots, and enzyme activities of SOD and POD were highest in neem originating from Kalyani (KA) provenance and lowest in neem originating from New Dehli (ND) provenance. In contrast, water stress increased MDA content least in KA and most in ND. Furthermore, neem originating from ND also had the greatest decrease in Chl a/b ratio while the ratio was least affected in neem originating from KA. These findings suggest neem originating from KA may have more drought resistance than neem originating from ND. The data from P N-PAR response curves are less clear. While these curves showed that drought stress increased compensation irradiance (I c) and dark respiration (R D) and decreased saturation irradiance (I s) and maximum net photosynthetic rate (P max), the extent of decline in P max was provenance dependent. P max under non-waterlimiting conditions was higher in neem originating from Jodhpur (MA) (about 14 μmol m⁻² s⁻¹) than in the other three provenances (all about 10 μmol m⁻² s⁻¹), but mild water stress had minimal effect on P max of these three provenances whereas P max of MA provenance declined to 10 μmol m⁻² s⁻¹, i.e. a similar value. However, under severe water stress P max of MA and KA provenances had declined to 40% of non-stressed values (about 6 and 4 μmol m⁻² s⁻¹, respectively) whereas the decline in P max of neem originating from Kulapachta (KU) and ND provenances was about 50% of nonstressed values (about 5 μmol m⁻² s⁻¹). These data suggest the P N responses of KU and ND provenances are most tolerant, and KA and MA least tolerant to increasing water stress, but also suggest MA provenance could be the most desired under both non-water-limiting and water-limiting conditions due to highest P max in all conditions.