الفهرس 
INTRODUCTION
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Abstract
The present research was concerned with studying the physiological response of two sorghum cultivars to different rates and splitting of NPK. This study was carried out in the Experimental Agricultural Farm of Agriculture Faculty in Assiut Univ, Assuit Governorate, Egypt; during the two growing seasons of 2012 and 2013. The field experiment was carried out in a randomized complete block design (RCBD) using split-split plot arrangement with three replications. The cultivars (Giza-15 and Dorado cultivar) were a signed in the main plot, while the NPK rates (75, 100 and 125% of the recommended NPK fertilizers/fed.) were allotted in the sub plot and different split doses (1, 2, 3 and 4 times) were allotted in the sub-sub plots. The experimental unit area was 12 m2 (3×4) including 6 ridge of 60 cm apart at spacing 20 cm between hills.
The first variable was cultivars. Sorghum cultivars were planted on July 2nd and 1st in the two growing seasons of 2012 and 2013, respectively. After three weeks from planting, plants were thinned into two plants per hill.
The second variable was the NPK rates. Three rates of NPK were studied as follows:
1- 75% of the recommended NPK fertilizers (Q1), (75, 23.25 and 18 unit/fed. for N, P and K, respectively).
2- 100% of the recommended NPK fertilizers (Q2), (100, 31 and 24 unit/fed. for N, P and K, respectively).
3- 125% of the recommended NPK fertilizers (Q3), (125, 38.75 and 30 unit/fed. for N, P and K, respectively).
The NPK fertilization rates were applied in the form of Urea (46.5% N), Calcium super phosphate (15.5% P2O5) and potassium sulfate (48% K2O), as a source of nitrogen, phosphorus and potassium, respectively.
The third variable was the splitting of NPK. The treatments in this respect were as follows:
1. The fertilization of NPK i.e. 75: 23.25:18, 100:31:24, 125:38.75:30 unit/fed for N: P2O5: K2O, respectively. These doses were applied before the second irrigation (Sp1).
2. Splitting the fertilizers doses into two equal parts before the second and third irrigations (Sp2).
3. Splitting the fertilizers doses into three equal parts before the second, the third and the fourth irrigation (Sp3).
4. Splitting the fertilizers doses into four equal parts before the second, third, fourth and fifth irrigations (Sp4).
All other agricultural practices were carried out as recommended for grain sorghum in both seasons. The preceding crop was wheat in both seasons.
Data were recorded by using competitive plants from each sub-sub plot (12 m2). A plant was considered competitive when it was guarded from four sides, i.e. two sides on the same ridge and the other two sides on the adjacent ridges. Random samples of five plants were chosen from the six inner rows of every sub-sub plot. This sampling was done in three replications of each experiment at the harvest time; these characters were recorded on October 22 and 25 in 2012 and 2013 seasons, respectively. The following characters were recorded. Plant height (cm), Panicle weight (g), Seed index (g), Grain yield /fed., Nitrogen Use Efficiency (NUE), Phosphorus Use Efficiency (PUE) and Potassium Use Efficiency (KUE).
All collected data were analyzed with analysis of variance (ANOVA) procedures using the MSTAT-C Statistical Software Package (Michigan State university, 1983). Means of any trait studied were compared using LSR at 5% probability according to Fisher (1964).
The results obtained in this study could be summarized as follows:
• Plant height (cm):
Plant height was significantly affected by cultivars in both seasons. Giza-15 cultivar surpassed Dorado one in this respect .The tallest plants were obtained from Giza-15 cultivar (318.1 and 296.2 cm in 2012 and 2013 seasons, respectively), while the shortest plants were obtained from Dorado cultivar (114.0 and 102.8 cm in the 2012 and 2013 seasons, respectively). Here too, the application of NPK fertilizers exerted a significant influence on plant height in both seasons. Applying 100 and 125% NPK/fed. of the recommended fertilizers were more effective compared with other NPK rates in 2012 and 2013 seasons, respectively.
The effect of splitting number on plant height was significant in both seasons. Splitting NPK into four equal doses resulted in the highest value of plant height (218.1 and 202.0 cm in 2012 and 2013 seasons, respectively). The shortest plants were observed when application the fertilizers in one full dose after thinning. The first order interaction of cultivars × NPK rates, cultivars × splitting number, NPK rates × splitting number and the second order interaction of cultivars × NPK rates × splitting number were significant in this respect through the two growing seasons.
• Panicle characters and seed index:
Panicle length, panicle width, the ratio of panicle length/panicle width, panicle weight and seed index were affected significantly by studied cultivars in both seasons. Giza-15 cultivar surpassed Dorado one in this respect and produced the highest mean values of panicle width, panicle weight and seed index (8.2 cm, 123.5 g and 55.9 g in first season, respectively being 9.1 cm, 136.4 g and 58.6 g in the second season in the same order), whereas Dorado cultivar produced the highest mean values of panicle length and the ratio of panicle length/panicle width (22.7 and 3.3 cm in first season, respectively being 22.1 and 3.0 cm in the second season in the same order).
NPK fertilization turned to be significant on the above mentioned traits in the both seasons, except panicle width and the ratio of panicle length/panicle in the second season were not significant. The maximum mean values of panicle length, panicle weight and seed index were obtained when the highest rate of NPK was applied (125% of the recommended NPK fertilizers/fed) in both seasons while, the highest mean values of panicle width and the ratio of panicle length/panicle width in first season reacted to 100 and 75% of the recommended NPK fertilizers/fed, respectively. Here too, the above mentioned traits reacted significantly to NPK fertilizer splitting in the two growing seasons except the ratio of panicle length/panicle was not significant in both seasons. Splitting NPK into three equal doses resulted in the highest mean values of panicle weight (122.1 and 135.3 g) and seed index (44.4 and 46.1 g) in both seasons and panicle length (21.8 cm) in 2012 season. While in 2013 season the highest mean value of panicle length (21.5 cm) and panicle width (8.6 cm) were obtained from splitting NPK into two equal doses of application. However, the highest mean value of panicle width (7.8 cm) was obtained from one full dose after thinning in 2012 season.
The first order interaction of cultivars × NPK rates, cultivars × splitting number, NPK rates × splitting number and the second order interaction of cultivars × NPK rates × splitting number were significant in this respect through the two growing seasons except the first order interaction of NPK rates × splitting number in the second season with regard to the ratio of panicle length / panicle width.
• Grain yield/fed. (ardab):
The effect of cultivars on grain yields/fed. (ard) were significant in both seasons. Giza-15 cultivar was significantly superior in grain yield as compared to Dorado one. Giza-15 cultivar produced the highest mean values of grain yield (18.5 and 18.3 ard/fed in the first and second seasons, respectively). The lowest grain yield was recorded from Dorado cultivar (14.9 and 14.4 ard/fed. in the 2012 and 2013 seasons, respectively).
Grain yield /fed. increased significantly by increasing of NPK rates in both seasons. Applying 125% of the recommended NPK fertilizers /fed were more effective compared with the other NPK rates in both seasons. The recorded grain yield was 17.8 ard/fed. in both seasons. Here too, splitting NPK exerted a significant influence on grain yield. Splitting NPK into three equal doses resulted to produce the highest mean value of grain yield/fed. (18.0 and 18.3 ard/fed. in 2012 and 2013 seasons, respectively). The first order interaction of cultivars × NPK rates, cultivars × splitting number, NPK rates × splitting number and the second order interaction of cultivars × NPK rates × splitting number were significant in this respect through the two growing seasons. In general, the maximum values of grain yield (21.2 and 23.1 ard/fed. in 2012 and 2013 seasons, respectively) were obtained from Giza-15 cultivar when received the highest NPK rate (125% of the recommended NPK fertilizers/fed.) applied at three equal doses.
• Nitrogen, Phosphorus and Potassium Use Efficiency:
N, P and K use efficiency were affected significantly by tested cultivars in both seasons. Giza-15 cultivar surpassed Dorado one in this respect. The application of NPK fertilizers exerted a significant influence on N, P and K use efficiency in the two growing seasons. Increasing nitrogen, phosphorus and potassium rates decreased N, P and K use efficiency values although higher N, P and K rates generally led to higher yield. Applying 75% of the recommended NPK fertilizers/fed were more effective in this respect compared with other NPK rates in both seasons. Here too, splitting numbers had a significant influence on N, P and K use efficiency in the two growing seasons. NUE, PUE and KUE were increased by increasing splitting numbers. Splitting NPK into three equal doses resulted in the highest mean values of N, P and K use efficiency. The lowest mean values of N, P and K use efficiency were observed when application the fertilizers in one full dose after thinning. The first order interaction of cultivars × NPK rates, cultivars × splitting number, NPK rates × splitting number and the second order interaction of cultivars × NPK rates × splitting number were significant in this respect through the two growing seasons.