| Resumo: |
Water deficit alters biochemical and physiological processes ranging from photosynthesis to protein synthesis and solute accumulation in the different organs of the plant. The objectives of this study were to determine the effect of water deficit on photosynthetic responses of leaves at different ages and the accumulation of soluble sugar and starch in the stems and leaves of sunflowers (Helianthus annuus L.). Two sunflower cultivars (Sanbro and TR-3080) were used under two water regimes, well watered (70 % irrigation) and water deficit (20 % irrigation) according to the field capacity of soil under greenhouse conditions. The experimental design was randomized complete block with five replicates. The transpiration rate (E), the ratio of intercellular air space and atmospheric CO2 molar fractions (Ci/Ca), the stomatal conductance (g s), and the net CO2 assimilation rate (A) were used to determine leaf gas-exchange parameters. These data were used to evaluate stomatal limitations to photosynthesis in relation to water deficit. It was found that water deficit (WD) reduced the rate of photosynthesis to a different level in young, fully developed and old leaves in both sunflower cultivars. Under water deficit condition, the sunflower leaves of varying ages showed different responses to water deficit in terms of their resistanance to dehydration. Especially, as the net CO2 assimilation rate was decreased among old leaves, transpiration rate was increased in mature leaves under water deficit. The percentage of soluble sugar was approximately 2-fold higher in the stem of water deficit plants as compared to well watered ones. This study suggests that the cv. Sanbro is likely to be more tolerant due to its high photosynthetic activity, high soluble sugar and starch accumulation and low relative membrane permability (RMP) as compared to the cv. TR-3080.
|
