Evaluation of forage yield, water-use efficiency and drought tolerance of Persian clover genotypes

Clover is considered as one of the most important forage crops of the leguminous family which is appropriate for temperate and humid areas. Insufficient water can severely affect the production of forage legumes leading to a reduction in yield based on the severity and duration of drought stress. Due to the relative sensitivity of clover to drought stress, the yield of this crop is affected by water scarcity, especially in arid and semi-arid regions. One of the effective solutions in the field of plant breeding for drought resistance is to identify stress-tolerant genotypes. This study aimed to compare the forage yield and water productivity of Persian clover genotypes under full irrigation and drought stress conditions, as well as evaluate their drought tolerance. The experiment was conducted as a split plot based on a randomized complete block design with three replications at the Seed and Plant Improvement Institute, Karaj, Iran, during the 2017-18 and 2018-19 cropping seasons. Irrigation regimes at two levels (full irrigation and deficit irrigation with supply 100% and 75% of clover water requirement, respectively) as the main factor and Persian clover genotypes at five levels (cultivar Pars, and ecotypes Zabol, Aleshtar, Eghlid, Harati) as sub-factor were evaluated. Each subplot consisted of four rows with a length of five meters and 50 cm between rows. The planting operation was carried out manually on September 11 of each year with 20 kg ha-1 seeding rate. The volume of irrigation water was determined by the Penman–Monteith method and the counter meter was used to accurately measure and control the amount of applied water. In the first and second cropping seasons, the total volume of irrigation water used were equal to 10125 and 11985 for full irrigation and 7594 and 8989 for deficit irrigation conditions, respectively. In this study, the fresh forage and dry matter yields, irrigation water efficiency for production of fresh forage and dry matter, the days to flowering, and drought tolerance indices were investigated. For all three cuts, clover cultivars were harvested at the 10% flowering stage. To determine the fresh forage yield, the middle four rows of each plot were harvested and weighed. To determine the dry matter yield, fresh samples (2 kg plot-1) were randomly selected and dried in a forced-air oven at 65 °C. Irrigation water-use efficiency was calculated from the ratio of yield to the amount of water consumed. The combined analysis of variance was performed using SAS9.1 and the means were compared by the LSD method at P < 0.05. The fresh forage and dry matter yields in the first, second and third cuttings, as well as their total and the irrigation water-use efficiency for forage and dry matter production were significantly affected by the year × irrigation × genotype interaction. Ecotype Harati had the highest total fresh forage and dry matter yields during three cuts (91.19 and 13.02 ton ha-1, respectively) in full irrigation conditions, whereas, under deficit irrigation conditions, the maximum total fresh forage and dry matter yields (68.07 and 10.79 ton ha-1, respectively) were obtained by ecotype Zabol. The maximum water-use efficiency (1.455 kg dry matter m-3) was obtained in the second year and by ecotype Zabul under deficit irrigation conditions, whereas the minimum efficiency (0.899 kg dry matter m-3) was recorded in the first year under full irrigation conditions and in the ecotype Zabol. Drought stress treatment in genotypes Pars, Aleshtar, Eghlid, Harati and Zabol compared to full irrigation caused a decrease in dry matter yield by 24.0, 21.7, 23.4, 23.8 and 11.9%, respectively. Among the drought tolerance indices, tolerance index (TOL), stress sensitivity index (SSI), yield stability index (YSI) and relative stress index (RSI) had the highest correlation with yield under full and defecit irrigation conditions. Based on the mentioned indices and principal components analysis (PCA), the ecotype Zabul, followed by ecotype Eleshtar were identified as the most tolerant genotypes to the defecit irrigation stress. Overall, the results of this experiment demonstrated that in the irrigation water availability conditions, the ecotype Harati and in the water limitation conditions, the ecotype Zabol are recommended for cultivation in the semi-arid regions of the country.