INTERACTIVE EFFECTS OF SOIL-WATER REGIME WITH ABOVEGROUND CONDITIONS ON PHOTOSYNTHESIS IN WHEAT PLANTS .1. PHOTOSYNTHESIS AS AFFECTED BY SOIL-WATER REGIME AND THE ABOVE GROUND CONDITIONS CHANGING WITH TIME OF DAY

Wheat plants were grown outdoors and in an environment-controlled walk-in growth chamber under two levels of soil water. The photosynthetic rates of leaf blades (Pn) and ears (Pg) were measured at every one hour intervals in the light stronger than photosaturation point. Photosynthesis was depressed by soil water deficit during the day irrespective of above-ground conditions. The depression was much larger in leaf blades than in ear, and among the uppermost three leaves the lower the leaf position was, the larger the extent of depression was. The aspect of the diurnal variation of Pn depended on above-ground conditions. On clear days with higher air temperature and lower air humidity, Pn decreased with time in the daytime and recovered in the latter half of the afternoon. On rainy day, Pn depression was not apparent. The diurnal variation of Pg of ear was not apparent under all conditions. The extent of Pn depression in the daytime was smaller in the second leaf than in the third leaf and larger in the flag leaf than in the second leaf. Under the soil water deficient condition Pn depression was larger and Pn recovery was smaller than under the well watered condition. The pattern of diurnal variation of Pn seemed to be determined primarily by above-ground conditions and only slightly affected by the soil water regime, which suggested, in a sense, that these two factors affected Pn almost independently. However, in details, the combined effects of the soil water regime and above-ground conditions were mainly additive but partly synergistic. © 1990, CROP SCIENCE SOCIETY OF JAPAN. All rights reserved.