WATER STRESS TOLERANCE OF FODDER COWPEA AS INFLUENCED BY VARIOUS ADDED LEVELS OF POTASSIUM SULPHATE

A field experiment was carried out on sandy soil of the Agricultural Research Experimental Station, at Ismailia Governorate (latitude 30o 35ʹ 41.9ʺ N and longitude 32o 16ʹ 48.8ʺ E), Egypt, during two successive growing summer seasons; 2013 and 2014. The study aimed to investigate the effect of applying three levels of water supplied through sprinkler irrigation system representing; 100 %, 75 % and 50 % of water supply which corresponding to 2500, 1875 and 1250 m3 fed-1, on the forage yield productivity and quality of fodder cowpea. Such levels represented commonly practiced irrigation (control), moderate and severe water deficit stress for fodder cowpea, respectively. Also, to evaluate the role of four rates of potassium sulphate fertilization namely, 24, 36, 48 and 60 K2O kg fed-1 in alleviating the adverse effect of water deficit stress on cowpea plants under these conditions. Three cuts were taken throughout the growth season. The obtained results revealed that decreasing the levels of water from 100% to 75 or 50% level markedly reduced all traits under studied; plant height, leaves number per plant, leaves/stem ratio of cowpea, green and dry forage yield (kg fed-1), at the three cuts as well as the total yield of the three cuts. The reduction noticeably appeared more detrimental under 50% of water stress especially at the 3rd cut. The results showed that the total green and dry forage yield of the three cuts of plants treated with 48 kg K2O fed-1, under 50% irrigation level, reduced by 52.4 % and 57.0 %, respectively; compared with the yields obtained by plants received the recommended K rate (48 kg K2O fed-1) under commonly practiced irrigation. Increasing water deficit stress lowered carbohydrate %, crude proteins % and ash content (kg fed-1) in plants as well as K% in leaves. Also, it significantly decreased the contents of Fe, Zn and Mn in plants.      Increasing the application rates of K up to 60 K2O kg fed-1 substantially elevated all the aforementioned studied traits under each level of irrigation and the reduction resulted due to water stress, under 75% and 50% levels could be suppressed. Application of the highest K rate (60 K2O kg fed-1) not only considerably heightened all the detected nutrients; carbohydrate % and proteins %, ash and micronutrients (Fe, Zn and Mn) uptake by plants as well as K% in leaves of cowpea, but also it remarkably enhanced the water use efficiency by plants grown under 75% irrigation than all applied K rates treatments under 100% irrigation. From the obtained results, it could be inferred that under moderate water stress 75% of the commonly practiced irrigation application of 60 K2O kg fed-1 could sustain cowpea plant to grow satisfactorily.