Effects of water deficit on photosynthesis in wheat plants. V. Difference among plant parts in water relations

The mechanism of different water relations in each part of a wheat plant was examined to elucidate the physiological basis for the difference among plant parts in photosynthetic drought resistance. As the soil water deficit developed, the upper leaf blades maintained the higher turgor potential (TP) and relative water content (RWC) compared with the lower ones. The stem and the leaf sheath also maintained the similar level of Tp and RWC to the flag leaf. The ear, however, showed the highest RWC with a low TP. The mechanism for osmotic/adjustment was analyzed by separating the osmotic potential (OP) decrease into two components, the concentration effect and the solute accumulation effect. It was found that the solute accumulation effect was the highest in the ear, followed by the stem, the leaf sheath, the upper leaf blades and the lower ones in the last, and that the concentration effect showed the reverse order of the plant parts as above mentioned. In order to know the reason for this peculiar watch relations in the ear, the water relations were investigated in the glumes and the grain separately. It was found that the glumes showed a similar water relations to the flag leaf, whereas the grain maintained a extremely higher water content with much lower TP than any other plant part, even under severe soil water deficit condition. The strong maintaining ability of RWC seemed to be the main cause of the higher drought resistance in the ear.