Effect of different irrigation intervals on some morphophysiological characteristics of different genotypes of Guar (Cyamopsis tetragonoloba)

Guar (cluster bean) Cyamopsis Tetragonaloba is one year plant belongs to the leguminous family. Guar has high nutritional value for humans. Global demand for guar has increased dramatically in recent years, which has led to the introduction of the plant in several countries. Water stress is the most important factor limiting planting and reducing food crops, especially in arid and semi-arid regions of the world. Therefore, identification of resistant plants and mechanisms that lead to optimal water use and high crop yields are critical. Given the limited water resources in the country, the evaluation of guar stands under drought stress is of particular importance. 

The experiment was carried out at the Research Farm of Iranshahr Agricultural and Natural Resources Faculty in 2017-18. The experiment was conducted as a split-plot in a randomized complete block design with three replications. Three levels of irrigation interval (7, 5, and 9 days) and six guar stands (Sarbaz, Saravan, Iranshahr, RGC1033, Indian, and Grembit) were considered as main and sub-factors, respectively. After planting in the 5-leaf stage, the stress operation began and continued until the seeds were harvested. At the end of the growing season, samples were transferred to the laboratory and shoot length and root length, fresh and dry weight of shoot, fresh and dry root weight, number of branches, 100 seed weight were measured. Leaf chlorophyll index was measured using Spad 502 Minolta chlorophyll meter. The chlorophyll fluorescence was measured using a Handy-PEA portable fluorometer (Hansatech Instruments). Finally, statistical analysis was performed using SAS software and mean comparisons were made using the LSD test at 5% probability level.

Increasing irrigation intervals significantly decreased plant height in Sarbaz and Saravan. The results showed that at 9-day irrigation period, the highest plant height (77.5 cm) was related to RGC1033 which did not show any significant difference with Saravan. The lowest plant height (42.03cm) was related to Sarbaz ecotype. Results showed that the 9-day irrigation interval increased root length in RGC1033, Iranshahr and Sarbaz and significantly reduced root length in Indian ecotype. The results showed that in 9-day irrigation period the number of branches in Iranshahr and Grambit decreased but increased in Saravan and RGC1033 genotypes. While the irrigation interval had no effect on Sarbaz ecotype. Most of the branches were 5.7 in RGC1033 ecotype. The Indian ecotype lacked a branch at all irrigation intervals. At 9-day irrigation period, Saravan and Iranshahr had the highest leaf dry weight. In Iranshahr and RGC1033 genotype, fresh and dry weight increased. Increasing irrigation interval resulted in a significant decrease in the dry weight of Indian root. The irrigation period had no significant effect on Saravan and Grambit ecotypes. Increasing irrigation intervals did not have a significant effect on the Sarbaz, Grambit, and Indians. With increasing irrigation intervals in RGC1033, Saravan and Iranshahr dry weights increased. The simple effect of ecotype type on seed weight showed that Saravan, Indian, and Iranshahr had the highest seed weight, respectively. Whereas, the SG, RGC1033, and Grambit had the lowest seed weight without any significant differences together. Saravan ecotype at all irrigation levels showed more biological function than other ecotypes. Results showed that the harvest index decreased with increasing irrigation intervals. In the 5-day irrigation treatment, the Grambit and Saravan ecotypes had the highest and the lowest harvest index, respectively. The 9-day irrigation period significantly reduced Fv/Fm in Sarbaz, Grambit, and Iranshahr. RGC1033 and Indian populations had the highest Fv/Fm in leaves and Saravan and Iranshahr had the lowest Fv/Fm in leaves at 7 days irrigation period, respectively. The number of pods per plant, number of lateral branches, 100 seed weight and plant height have a direct and positive effect on guar yield. Under drought stress, plant height and leaf number decrease, resulting in lower shoot dry weight. Some researchers showed that decreasing plant height under water stress is associated with loss of protoplasmic water, resulting in reduced cell division and development. The harvest index decreased with decreasing number of pods, thereby reducing economic yield. Differences between susceptible and resistant ecotypes appear in high-stress conditions and high Fv/Fm ratio ecotypes have higher photosynthetic efficiency under severe stress conditions. Chlorophyll fluorescence is considered as a criterion for assessing the integrity of the thylakoid membrane in the relative efficiency of electron transfer from Photosystem II to Photosystem I. 

Mean root length, the number of branches, fresh and dry weight of RGC1033 were not affected by a 9-day irrigation interval. In the 9-day irrigation period, the highest mean dry weight of leaf, main stem weight and seed weight were related to Saravan ecotype. The highest biological yield was observed in the irrigation period of 7-day and Saravan ecotype. In general, the results showed that irrigation had different effects on different guar populations. RGC1033 and Saravan ecotypes showed the lowest percentage of stress-affected variations.