Potential soil moisture deficit: A useful approach to save water with enhanced growth and productivity of wheat crop

Wheat is the main crop in the world ranks after rice and the largest grain source of Pakistan. Among several reasons for diminishing wheat yield in Pakistan, water stress throughout the growing season decreases crop production because of the short life span. Two years (2015–16 and 2016–17) of field experiments were conducted to assess the impact of various water regimes (full irrigation, irrigation at 45, 60, and 75 mm potential soil moisture deficit (PSMD)) on the growth and yield of wheat. Maximum crop growth rate was recorded by application of irrigation at 45 mm PSMD. Application of irrigation at 45 mm PSMD ensured maximum radiation use efficiency regarding total dry matter production and grain yield. The maximum number of productive tillers, spike length, and grain yield were recorded under 45 mm PSDM treatment. The present results show that the effect of water is more pronounced regarding the growth and productivity of wheat. Application of irrigation at 45 mm PSMD ensures higher economical yield.


INTRODUCTION
Pakistan has a good irrigation infrastructure, but climate change would result in inconsistency of the amount of water taken out at canal headworks and unpredictability in the frequency and intensity of rainfall.
Furthermore, the average precipitation during July to September (monsoon season) is 138 mm, while in 2011-12 it received 237 mm precipitation which is more than normal precipitation. However, in the winter season of 2012, the amount of precipitation was 34 mm which was 51% less than the normal 71 mm, and consequently a decline in water amount taken out at canal headworks (GOP ).
Hence, a decrease in canal water stores and unpredictability in precipitation are major threats to agriculture. The average need of water in rabi season (October to April) is 44.9 × 109 m 3 . However, severe scarcities of water will mean these requirements are not met (GOP ). Variability in precipitation is a burning issue for Pakistan, and it is a huge threat to the masses' food security in the long run. Water stress is a globally concerning issue and considered a complex hazard that directly influences the water balance of especially arid and semi-arid regions (Sur & Lunagaria ; Wang et al. ). Water shortage has a critical role in the final yield of crops (Holzman et al. ). Basal & Szabó () reported that water stress is one of the most hazardous abiotic stresses that adversely influences crop yield. One of the national and universal concerns is to articulate different plans to manage existing water resources for agricultural use (Smith ).
For optimum growth and better yield of wheat recurrent irrigations should be applied because it is a rather sensitive crop to water stress (Alderfasi & Neilsen ).
A better option or alternative is crop cultivation under a deficit irrigation scheme with less evapotranspiration to conserve water and other resources ( Jalota et al. ).
The ultimate objective of irrigation to crops is to meet water requirements and to boost the yield, in spite of the fact that water shortage is a severe issue for crop production in tropical areas owing to less or erratic rainfall patterns and climate change having a huge influence on tropical wheat production (Messmer et al. ). Irrigation water management is vital in all crops, especially in wheat because under-or overirrigation results in growth retardation of wheat and a decrease in yield. Most areas of Punjab often experience a dry weather environment, particularly during Rabi cropping season (Amin et al. ).
Keeping in view the above discussion, the present study was planned (i) to evaluate the performance of wheat crop under different potential soil moisture deficit (PSMD) strategies and (ii) to explore PSMD as a helpful approach for irrigation scheduling in wheat.
Faisalabad-Pakistan. Soaking irrigation (Rouni) was applied before the sowing of the crop to maintain soil moisture at field capacity (25% v/v) during the wheat crop growing season of 2015-16 and 2016-17. Wheat cultivar Sahar-2006 was sown using a seed rate of 100 kg ha À1 in rows 20 cm apart using a manual single row hand drill.
Physio-chemical properties of the soil at the experimental site (Table S1 in Supplementary Material) were analyzed before the crop was sown.
Recommended nutrient dose of N:P:K at 120:90:60 kg ha À1 was applied using urea, diammonium phosphate (DAP), and sulphate of potash (SOP) fertilizers as the source of nutrient. Nitrogen was applied in three equal parts, one-third of the total nitrogen was applied during seed bed preparation while the two remaining equal parts were applied at first and second irrigation. All phosphate and potash fertilizers were applied during seed bed preparation. Uniform weed management and plant protection measures were adopted throughout the experimentation cycle. Treatments included: • Full irrigation/irrigation at all growth stages (control) • Irrigation at 45 mm potential soil moisture deficit • Irrigation at 60 mm potential soil moisture deficit • Irrigation at 75 mm potential soil moisture deficit.
Before each irrigation, soil moisture percentage was determined by gravimetric method and depth of irrigation for the next irrigation was estimated as follows: where SMC ¼ soil moisture content, W w ¼ weight of moist soil (as collected from the field at 45 cm depth), and where D i ¼ depth of irrigation (cm) or crop water requirement in depth (cm), FC ¼ field capacity (% on volume basis), SMC ¼ soil moisture content (% on volume basis), BD ¼ bulk density (g cm À3 ), and D r ¼ depth of root zone (cm).
A measured quantity of irrigation was applied according to the requirement (D i ) to replenish the depleted soil moisture using the equation: For measurement of discharge, a cutthroat flume was installed in the water channel and readings were observed at every irrigation. These readings were used to derive the water flow rate. The amount (mm) and date of irrigation are presented in Table 1. Data regarding prevalent weather condition throughout the crop season were obtained from a field meteorological laboratory situated in the vicinity of the experimental site ( Figure S1).

Growth, yield and their attributes
To determine the growth parameters, each plot was divided into two sub-plots. One of them was used for destructive biomass sampling and the other half was kept intact for final grain yield measurements. From each plot, a 900 cm 2 area was harvested at ground level at intervals of every 15 days leaving appropriate borders. Fresh and dry weights of constituent fractions of the plant (leaf and stem) were determined. A sub-sample from each fraction was taken to dry in an oven to a constant weight. Crop growth rate was calculated as proposed by Hunt (): where W 1 and W 2 are the dry weights harvested at time intervals of t 1 and t 2 , respectively.
From the measurements of leaf area and dry weights the following parameters were calculated. Leaf area was measured by using a leaf area meter (Model CI-202, CID, Inc.). Five gram of leaf laminae from each experimental unit was used for the estimation of leaf area and then converted to total leaf area of harvested samples. Leaf area index (LAI) was then calculated as the ratio of leaf area to land area (Watson ): Leaf area duration (LAD) was estimated as suggested by Hunt (): Net assimilation rate (NAR) was determined according to Hunt (): The fraction of intercepted radiation (Fi) was calculated from measurements of LAI using the exponential equation suggested by Monteith & Elston (): where K ¼ extinction coefficient for total solar radiation (Monteith & Elston ). A 'k' value of 0.45 was used for wheat.
The amount of intercepted light (Sa) was determined by multiplying Fi with incident PAR (Si) during the season as: The photosynthetically active radiation (PAR) was assumed as 50% of the total incident radiation.
Radiation use efficiency for TDM (RUE-TDM) and grain yield (RUE-GY) was calculated as the ratio of total biomass and grain yield to cumulative intercepted PAR (Σ Sa): Water use efficiency (WUE) on TDM basis (WUE TDM ) and grain yield basis (WUE GY ) were calculated as the ratio of total biomass and grain yield to the total amount of irrigation applied and rainfall: Fertile tillers per unit area, randomly selected from each experimental unit, were recorded manually at maturity. Ten

Growth, yield and their attributes
Crop growth rate (CGR) was significantly (P < 0.05) influenced by different irrigation regimes (Figure 1). The results showed that maximum CGR (9.98 g m À2 d À1 ) was

CORRELATION AND REGRESSION ANALYSIS
The relationship between different growth and yield parameters is calculated and presented in Figure 3. The relationship between crop growth rate and total dry matter was recorded as linear and was strongly positive giving an R 2 value of 0.95. Regression relation of the Means sharing the same letter did not differ significantly at P ¼ 0.05.
Means sharing the same letter did not differ significantly at P ¼ 0.05.
I 1 ¼ Full irrigation/Irrigation at all stages (control), I 2 ¼ irrigation at 45 mm potential soil moisture deficit, I 3 ¼ irrigation at 60 mm potential soil moisture deficit, I 4 ¼ irrigation at 75 mm potential soil moisture deficit, Y ¼ sowing year, I ¼ irrigation treatments, Y × I ¼ interaction, ns ¼ non-significant.
Means sharing the same letter did not differ significantly at P ¼ 0.05. I 1 ¼ Full irrigation/Irrigation at all stages (control), I 2 ¼ irrigation at 45 mm potential soil moisture deficit, I 3 ¼ irrigation at 60 mm potential soil moisture deficit, I 4 ¼ irrigation at 75 mm potential soil moisture deficit, Y ¼ sowing year, The current study results revealed that deficit irrigation at 45 mm PSMD is more water-saving than conventional   and Chalabi & Rashidi (). Taiz & Zeiger () reported that measurement of biomass is the key factor to determine drought stress in plants. Therefore, biomass has a pivotal role in increasing grain yield. A positive correlation between grain yield and harvest index was observed ( Figure 3). The study of Kirigwi et al. () supports our findings as it reported a positive correlation of grain yield with biological yield and harvest index in varying water deficit levels. The current study results revealed that deficit irrigation at 45 mm PSMD is more watersaving than conventional practices of irrigation of traditional farmers. PSMD at 45 mm is also found most appropriate throughout the experimentation because it appears responsible for maximum plant height, number of grains per spike, 1,000 grain weight, productive tillers, and economic yield.

CONCLUSION
The physiological availability of water to plants is a key factor in crop production throughout the world. Data suggest that there is still huge scope to increase wheat grain yield by using deficit irrigation sensibly. Under a climate change-induced water shortage scenario, lack of adequate moisture at early growth stages has a more detrimental effect than at later growth stages. Maximum growth and yield of wheat crop in the field can be achieved by applying irrigation at 45 mm PSMD. Hence, irrigation at 45 mm PSMD is suggested for achieving high productivity.