Germination of salt-stressed seeds as related to the ethylene biosynthesis ability in three Stylosanthes species.

Stylosanthes, a genus of tropical forage legume, is known to exhibit good persistence in saline soils, yet mechanisms for regulation of seed germination under salt stress are poorly understood. This study was carried out to evaluate the mode of action of salt stress on seed germination of Stylosanthes. 1-Aminocyclopropane-1-carboxylic acid (ACC) increased ethylene biosynthesis and germination of NaCl-inhibited seeds in a dose-dependent manner. Contents of ACC and germination of Stylosanthes humilis seeds increased following transfer from NaCl solution to deionised water, but not after transfer to l-α-(2-aminoethoxyvinyl)-glycine (AVG) solution, an inhibitor of ethylene biosynthesis. Ethylene biosynthesis was much larger in NaCl-treated seeds of Stylosanthes guianensis than in seeds of S. humilis and Stylosanthes capitata, a fact which was reflected in higher germination rates. S. guianensis seedlings also displayed higher growth and survival rates than S. humilis and S. capitata under salt stress. Moreover, smaller ACC levels, as well as reduced ethylene biosynthesis of S. capitata seeds were accompanied by lower germination under salt stress. In addition, S. capitata seedlings treated with NaCl solutions exhibited relatively lower growth and survival rates in comparison with S. humilis and S. guianensis. Thus, different abilities to synthesize ethylene by S. guianensis, S. humilis and S. capitata seeds explain the differences in tolerance to salt stress of the three species.

Physiological and biochemical abilities of robusta coffee leaves for acclimation to cope with temporal changes in light availability.

The effects of varying intensities of light on plants depend on when they occur, even if the total amount of light received is kept constant. We designed an experiment using two clones of robusta coffee (Coffea canephora) intercropped with shelter trees in such a way that allowed us to compare coffee bushes shaded in the morning (SM) with those shaded in the afternoon (SA), and then confronting both with bushes receiving full sunlight over the course of the day (FS). The SM bushes displayed better gas-exchange performance than their SA and FS counterparts, in which the capacity for CO2 fixation was mainly constrained by stomatal (SA bushes) and biochemical (FS bushes) factors. Physiological traits associated with light capture were more responsive to temporal fluctuations of light rather than to the amount of light received, although this behavior could be a clone-specific response. The activity of key antioxidant enzymes differed minimally when comparing the SM and SA clones, but was much larger in FS clones. No signs of photoinhibition or cell damage were found regardless of the light treatments. Acclimations to varying light supplies had no apparent additional cost for constructing and maintaining the leaves regardless of the light supply. Both the SM and SA individuals displayed higher return in terms of revenue streams (e.g. higher mass-based light-saturated photosynthetic rates, photosynthetic nitrogen use efficiencies and long-term water use efficiencies) than their FS counterparts. In conclusion, shading may improve the physiological performance of coffee bushes growing in harsh, tropical environments.