effects of supplementary irrigation at phenological stages on some growth indices of lentil (lens culinaris medik.) cultivars in mashhad region

introduction lentil with about 28 percent protein occupies the second after soybean. it is one of the major crops in developing countries as a complement to cereals and is an excellent source of protein and amino acids in the diet. results of studies shown that for most crops, including lentil, the occurrence of drought stress in some phenology stages cause irreparable damages to yield. therefore, understanding the critical stages of plant to drought stress in order to provide the required humidity at the time plays an important role in yield performance and efficient use of water and soil resources. supplementary irrigation at critical stages of water requirements of lentil is one of the most effective methods to achieve sustainable production in arid and semi-arid regions. materials & methods in order to study the effects of supplementary irrigation on growth characteristics of three lentil cultivars, an experiment was carried out as a strip block based on a randomized complete block design with three replications at research field, college of agriculture, ferdowsi university of mashhad during 2008-9 growing season. treatments were supplementary irrigated (full irrigation; one irrigation at each stage of branching, flowering, podding, seed setting (with an incidence of phenological stage at least 50 percent of plants a plot), and no irrigation) as main plots, and three lentil cultivars (robat, & kalpoosh (the local population in north khorasan with the registration number of mlc245, and mlc183, respectively), and gachsaran) as subplots. lentil disinfected seeds were sown at depth of 2-3 cm and density of 200 plants/m2 in the second half of march. the size of experimental plots were 5×3.75 m and each plot had 10 rows with spacing of 37.5 cm. all treatments were irrigated once after planting to ensure the emergence of uniform seeds. next irrigation was followed according to the treatments (ibpgr, 1985). destructive sampling were performed to calculate the growth indices (such as tdw, lai, cgr, rgr and nar) from 10 plants chosen randomly from competing plants regarding the marginal effects from two weeks after plants emergence to final maturity (every 7 days; 10 steps). at the end of the growing season, seed yield was determined from 7.5 m2. the growth degree days (gdd) was used instead of calendar time to calculate growth indices using equation (1). the growth indices of lai, cgr, rgr and nar were calculated using equations (2-5). growth indices and linear regression curves were fitted using excel 2007 software. gdd = (equation 1) where gdd is the growth degree days, tmax and tmin are maximum and minimum daily temperature during test, respectively. tb is plant base temperature (= 5 °c for lentil). lai= (1/ga)[(la2+la1)/2] (equation 2) cgr= (1/ga)[(w2–w1)/(t2–t1)] (equation 3) rgr= (lnw2–lnw1)/(t2–t1) (equation 4) nar= [(w2–w1)/(t2–t1)] [(lnla2–lnla1)/(la2–la1)] (equation 5) where ga is ground area (m2), la is leaf area (m2), w is shoot dry weigh (g) and t is gdd. results & discussions results indicated that supplementary irrigation in flowering stage increased total dry weight (tdw), leaf area index (lai), crop growth rate (cgr) and net assimilation rate (nar) compared to other supplementary irrigations. complete irrigation showed the highest growth characteristics during the growing season. maximum values of total dry weight (507.4 gm-2), leaf area index (3.6), crop growth rate (1.35 gm-2 gdd-1), relative growth rate (0.04 gg-1 gdd-1), net assimilation rate (1.75 of gm-2 gdd-1) and seed yield (1213 kg ha-1) were obtained from irrigation at flowering stage following full irrigation. amiri deh ahmadi et al. (2011) concluded that drought stress at flowering stage, reduced peas dry matter to minimum values. singh (1995) also reported that water stress in all stages of growth, decreased leaf area index of beans, but stress before flowering had the greatest impact. other researchers showed that in terms of drought, crop growth rate decreased due to the decline in photosynthesis and respiration rate. other researchers concluded that stress and lack of moisture reduced plant relative growth rate. the amount of net photosynthesis and aging leaves reduced over time and this reduction was intensified in difficult environmental conditions, especially drought. robat was the best of three cultivars in growth indices. moreover, the positive and significant correlation between grain yield and total dry weight (0.93**), leaf area index (0.92**), crop growth rate (0.90**), relative growth rate (0.89*) and net assimilation rate (0.92**), have highlighted the importance of growth indices to predict the economic performance of lentils. conclusion in general, it seems in water scarcity conditions, supplementary irrigation at flowering stage could supply necessary moisture for plants and improve their crop yield.