| Results Photosynthesis rate of the uppermost fully expanded leaf showed a decrease from anthesis onwards (Table 1). The leaves at different positions in the MS did not differ significantly in photosynthesis rate 23 days before anthesis (Table 2). At anthesis however, the flag leaf and penultimate leaf had higher rate of photosynthesis and the rate decreased significantly towards lower leaves. At 20 days after anthesis lower leaves senesced and only flag leaf had some activity followed by the penultimate leaf. The rate of photosynthesis was more during the forenoon and decreased towards the evening at both pre-anthesis and anthesis stages (Table 3). At 20 days after anthesis the net photosynthesis could be detected only in the initial two measurements i.e. at 10 and 11 AM. The stomatal diffusive resistance was less in the morning, increased during the mid day period and again decreased towards evening by 4 PM (Table 4). The effect of ear removal on rate of photosynthesis of the flag leaf of MS showed no significant difference ten days after anthesis (Table 5). At 20 days the photosynthesis rate was considerably more in treatments where ears were removed. It may be mentioned that flag leaf remained green for a longer period in treated plants. The MS weight and shoot weight of tillers were not significantly affected by removal of MS ears (Table 6). The ear weight of tillers however, was increased by this treatment. There was considerable increase in the shoot weight of tillers when ears of MS and tillers were removed. This appeared to be due to the formation of new tillers during the later stages of development when all the ears were removed. Discussion The present study showed diurnal changes in photosynthesis rate and stomatal diffusive resistance in wheat variety Kalyansona. The flag leaf of de-eared plants retained higher rate of photosynthesis in this cultivar. Diurnal changes in photosynthesis rate have been reported in other species (UPMEYER & KOLLER 1973, PALLAS et al. 1974, KERR et al. 1985). In peanut the diurnal changes in photosynthesis seemed to be associated with stomatal component (PALLAS et al. 1974), but not so in case of soybean (KERR et al. 1985). In the present study photosynthesis rate decreased towards evening and this pattern was not parallel with stomatal diffusive resistance. It may be mentioned here that a decline in rate of net CO2 assimilation with time of illumination in wheat has been reported (AZCON-BIETO 1983). This decline was greatest under conditions of reduced export or higher rates of photosynthesis and was found to be related to the carbohydrate concentration. The decline in photosynthesis rate later in the day in soybean was reported to be associated with leaf carbohydrate level (UPMEYER & KOLLER 1973). The changes in photosynthesis rate during the ontogeny of the plant are in line with those reported earlier (RAWSON et al. 1983, GHILDIYAL & SIROHI 1986). The photosynthesis rate of the leaves at different positions in the main shoot was similar during the pre anthesis stage. Subsequently, the upper leaves were comparatively more active (Table 2). It has been suggested that during pre-anthesis period there was enough demand for assimilates from all the leaves for the growth and differentiation of root and shoot (YOSHIDA 1972, GIFFORD & EVANS 1981). After ear emergence the ear became the major sink and root growth decreased considerably (ASANA 1968). The possibility of sink demand influencing the pattern of variation in photosynthesis rate among leaves at different positions in the MS may therefore, not be ruled out. |
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