Heat unit requirement of wheat (Triticum aestivum L.) under different thermal and moisture regimes

An experiment was conducted during 2016-17 and 2017-18 at WTC, ICAR-IARI New Delhi on wheat crop sown on three dates (15 th November, 30th November and 15th December) with five irrigation treatments. The results indicated that the number of days required for attaining different phenological stages decreased with delay in sowing.For all the phenological stages, crop sown on 15 th November consumed higher heat units and consequently resulted in higher yield and heat use efficiency (HUE) than that of other sowing. However, higher pheno-thermal index (PTI) values were observed for the late sown crop i.e. 15 th December. Further, among the irrigation treatments, five irrigations throughout the growing period showed increase in days to physiological maturity as well as heat units and HUE for grain and biological yield as compared to other treatments. Five irrigations throughout the growing period increased the grain yield by 69 per cent and biological yield by 46 per cent that that of one irrigation at CRI stage. However, there was no significant difference observed between four and five irrigations levels. The heat units or GDD had highly significant correlation with biological yield(r=0.91) as well as with the grain yield (r=0.85).

.As irrigation is one of the important factors for proper establishment of wheat and protects the crop from heat stress during reproductive stage, it becomes imperative to generate knowledge of the interactive effect of heat and moisture on growth and yield of wheat. Therefore, keeping this in view, the present study was conducted to determine agrometeorological indices at different phenological stages and to evaluate the correlation between these indices and grain yield.

Experimental details
The wheat cultivar HD-2967 was sown during rabi 2016-17 and 2017-18 in the research farm of Water Technology Centre (WTC) of ICAR-Indian Agricultural Research Institute (IARI), New Delhi, India located at 77°09' 36" E longitude and 28°37' 55" N latitude at an average elevation of 167.17 m above mean sea level. The experiment was laid out in split plot design with three replications and 15 treatment combinations consisting of 3 dates of sowing and 5 irrigation levels. The main plot comprised 3 dates of sowing (S 1 =15 th November, S 2 = 30 th November and S 3 =15 th December) while 5 irrigations levels at different stages (I 1 = CRI, I 2 = CRI+ tillering, I 3 = CRI+ tillering +jointing, I 4 = CRI+ tillering +jointing +anthesis and I 5 = CRI+ tillering +jointing +anthesis +dough) were imposed in sub-plots. A recommended common fertilizer dose of 120, 60 and 40 kg ha -1 of N, P and K, respectively was applied during both the experimental years. Full P and K and half N were applied as basal doses and remaining N dose was top dressed at tillering and booting stages in equal amounts.
Days to different phenological stages were recorded from each sub-plot when approximately 50 per cent of the plants reached that particular stage. After physiological maturity an area of 1 × 1m 2 from each sub-plot was harvested manually. Biomass yield was determined by taking the weight of above ground plant parts including the grain. After threshing, the grain weight (gm -2 ) was measured and was upscaled to kg ha -1 .

Calculation of agro-meteorological indices
Meteorological data viz. maximum and minimum temperature, bright sun shine hours and day length during the experiment period was acquired from the agrometeorological observatory located within the IARI farm. The agrometeorological indices such as heat units or growing degree days (GDD), phenol-thermal index (PTI) andheat use efficiency for biological yield (HUE BY ) as well as for grain yield (HUE GY ) were calculated using following

Statistical analysis
The data were pooled for two seasons (rabi 2016-17 and 2017-18) and statistically analyzed for different phenological stages, agro-meteorological indices and yield with the help of OPSTAT software (Sheoran et al., 1998) for estimation of analysis of variance (ANOVA) at p d" 0.05 probability level.

Attainment of different phenological stages
Effect of date of sowing and irrigation levels on number of days to attain different phenological stages shown in Table 1. Sowing dates significantly affected days to attain different phonological stages. The statistical analysis indicated that 15 th November sown crop took less days to CRI and tillering as compared to other two sowing dates. This may be due to relatively more temperature experienced at these stages by the 15 th November crop as compared to other sowing dates. But after tillering i.e. between jointing to physiological maturity stage, the 15 th November sown crop took more days to attain these stages followed by 30 th November and 15 th December sown crop. The days to physiological maturity were 131 days, 121 days and 111 days for crop sown on 15 th November, 30 th November and 15 th December, respectively. Similar results were reported by Hari Ram et al. (2012) for different variety of wheat in Punjab. Irrigation levels had no effect on days to CRI and tillering. After tillering stage, the least days to jointing, booting, anthesis and physiological maturity were observed for one irrigation (at CRI stage). Whereas the five irrigations supplied during different growth stages (I 5 ) showed the longest time to maturity i.e. 125 days as compared to other irrigation treatments. However there was no significant difference between four (I 4 ) and five irrigations (I 5 ) treatments.

Accumulated heat units
Accumulated heat units pertaining to different phenological stages of wheat varied with date of sowing and irrigation levels (    ). However, no significant difference in growing degree days was noticed between five and four irrigations treatment as well as between one and two irrigation levels.
However, no significant difference was observed between four and five irrigation treatments. Zhang et al. (2006) reported that water stress at jointing and anthesis stages reduced the grain yield of wheat by 14-25 per cent as compared to crops under full irrigation. So, it was observed that increasing irrigation levels (I 4 , I 5 ) would improve crop growth and thus enable crop to intercept more photosynthetic radiation leading to more biological and grain yield (Ali et al., 2012).

Yield and heat use efficiency (HUE)
The results indicated that sowing dates and irrigation   (Table  3). Wheat crop sown on Nov 15, recorded significantly higher grain and biological yield ever other sowing dates. Five irrigation produced significantly higher yield over other irrigation treatments. The heat use efficiency (HUE) for grain yield and biological yield were computed by using accumulated values of GDD at physiological maturity (Table  3).The highest HUE for grain yield (2.1 kg ha -1°C days) as well as for biological yield (8.2 kg ha -1°C days) recorded under 15 th November sown and the minimum values were observed for late sown i.e. 15 th December crop. However there was no significant difference observed between 15 th November and 30 th November sown crop. The higher HUE in 15 th November sown crop could be attributed to the proportionate increase in dry matter per each heat unit absorbed. The lower HUE in delayed sowing can be expected due to accumulation of comparable GDD to that of 15 th November sowing at later crop growth stages. With respect to irrigations, it was noted that the treatment with five irrigations (I 5 ) registered significantly maximum HUE for grain yield (3.0 kg ha -1°C days) as well as for biological yield (8.8 kg ha -1°C days) followed by four, three, two and one irrigations treatments. However the values were statistically at par between four and five irrigations treatment. The less heat use efficiency for water stress conditions (I 1 , I 2 and I 3 ) may be ascribed by less yield as well as growing degree days as compared to full irrigation Dar et al., (2018) also reported similar results.

Pheno-thermal index (PTI)
The pheno-thermal index (PTI) for consecutive phenological stages was also computed and is presented in Table 4. It was observed from Table 5 that after tillering stage the phenol-thermal index gradually increased from jointing to physiological maturity stage in all three dates of sowing and the highest value of PTI was observed at the physiological maturity. Among the sowing dates, the maximum PTI was observed at reproductive stages i.e. at anthesis (15), milking (17), dough (20.8) and physiological maturity (22.8) for 15 th December sown crop followed by 30 th November and 15 th November. This may be due to increase in temperature at reproductive stage reduced the days taken to reach different reproductive stages for 15 th December sown crop as compared to 30 th November and 15 th November sown crops. Gill et al. (2014), Hari Ram et al. (2012) and Dar et al. (2018) also reported that the PTI was affected by the growing environment. Among the irrigation treatments, there was no significant difference in PTI recorded from CRI to jointing stage, but after jointing stage as the number of irrigations increased, it started increasing significantly. However, there was no significant different was observed between value of PTI for four and five irrigations.
Correlation analysis was done between yield (both grain and biological) and meteorological indices (Table 5). Results indicated that GDD was positively correlated with biological as well as grain yield. The GDD showed highly significant relationship with biological yield(r=0.91) and grain yield (0.85).While PTI showed a negative relationship with both grain yield (0.13) and biological yield (-0.17) but it was not statistically significant. Dar et al. (2018) also reported the positive correlation of GDD with grain yield and biological yield.

CONCLUSIONS
The crop sown on 15 th November took maximum days to attain different phenological stages and required maximum heat units which got reduced with subsequent delay in sowing and the lowest values were noticed for crop sown on 15 th December. The 15 th November sown crop recorded the highest grain yield and HUE which was statistically at par with 30 th November, which suggested that with similar management practices the wheat cultivar HD-2967 can be sown up to 30 th November. Among the irrigations, five irrigations treatment took maximumdays to attain physiological maturity and consumed moreheat units or GDD during total crop period resulted in more yield, HUE and PTI as compared to other treatments. However there was no significant difference observed between four and five irrigations. The GDD also showed highly significant positive correlation with grain and biological yield. Therefore, it may be used for forecasting the phenological stages and yield under different sowing date and stage based irrigation scheduling for wheat.