Stress-induced carbon starvation in Rhizophora mucronata Lam. seedlings under conditions of prolonged submergence and water deficiency: Survive or succumb

The behaviour of carbohydrate metabolism in a plant, particularly its total starch content, total soluble sugar (TSS) content and their utilisation, is of great importance in coping with abiotic stress conditions. With this in mind, we studied total starch and TSS contents, survival, growth, biomass accumulation and stomatal conductance in Rhizophora mucronata under conditions of prolonged submergence and water stress for a period of 11 months. The experiment was designed in such a way as to include three replicates per each treatment level, about 1600 young mangrove plants being subjected to study in the process. Under conditions of prolonged submergence and high levels of water stress, a small number of mangrove plants survived and they were promptly exhausted due to higher starch utilisation rates (0.75-1.05% dry mass/month). Although TSS content was increased under these intense stress conditions, it was not matched by increased seedling growth or biomass production; instead, a significant reduction in growth (i.e., ~78%) and dry matter content was observed in stressed seedlings as compared to young plants in the respective controls. It follows that the intense increase of TSS content might be due to the direct conversion of starch to soluble sugars in order to produce metabolic energy for tolerance mechanisms like osmoregulation and root anatomical adaptations under stress conditions. This indicates that more energy is allocated for plant maintenance than for growth and biomass production under stress conditions, which might be a good acclimatory strategy to rescue young mangrove plants at the early phase. However, stomatal closure under stress conditions may have caused restricted photosynthesis. Therefore, stress-induced starch degradation may upsurge, which in turn might lead in the long-run to carbon starvation, a condition lethal to mangrove seedlings.