Effect of Potassium and Zinc Foliar Application on Growth Physiological Indices, Chlorophyll Fluorescence Parameters and Yield of Two Bread Wheat Cultivars under Late Planting Date

Wheat (Triticum aestivum L.) is one of the most important sources of plant food for human among the main crops globally. High temperature resulting from delay in planting is one major environmental factor limiting growth and production of wheat, especially in tropical regions. Most of the Iranian soils have high pH and calcareous nature, so absorption of nutrients is limited in these soils. Mineral nutrition of plants plays a critical role in increasing plant resistance to environmental stresses. Among the mineral nutrients, potassium plays a crucial role in the survival of crop plants under environmental stress conditions. Potassium is essential for many physiological processes, such as photosynthesis, maintenance of turgidity and activation of enzymes under stress conditions. Zinc is a ubiquitous micronutrient. It is required as a structural and functional component of many enzymes and proteins. To study the effect of potassium and zinc foliar application on growth physiological indices, chlorophyll fluorescence parameters and yield of two bread wheat cultivars under late planting date, an experiment was conducted as split-split plot based on randomized complete blocks design with 16 treatments and three replications in Ramhormoz city during 2015-2016. The experimental factors were included planting date in two levels optimum (November 21) and late (January 5) as the main factor, nutrients foliar application in four levels (water as a control, potassium, zinc and combination potassium + zinc (each 3 l.ha-1)) as the sub factor and two cultivars of bread wheat Pishtaz and Chamran 2 as the sub-sub factor. Solutions for foliar application were prepared by using potassium (21%) and zinc-chelate (7.5%). Traits measured were included leaf area index (LAI), leaf area duration (LAD), crop growth rate (CGR), relative growth rate (RGR), net assimilation ratio (NAR), maximal quantum yield of PSII in the dark-adapted state (Fv/Fm), effective quantum yield of PSII photochemistry (ΦPSII), maximal quantum yield of PSII in the light-adapted state (Fv'/Fm'), nonphotochemical quenching (NPQ) and grain yield. To determine LAI, LAD, CGR, RGR, NAR, Fv/Fm, ΦPSII, Fv'/Fm' and NPQ were used from equations 1-9, respectively.
 LAI= Pla×den /10000
 LAD= 1/2 (LA2+LA1) (T2-T1) 
 CGR= [(W2 – W1)/(T2 – T1)] × (1/GA) × 100
 RGR= [(lnW2 – lnW1)/( T2 – T1)] × 100
 NAR= CGR/LAI
 Fv/Fm= (Fm – F0)/Fm
 ΦPSII= (Fm – Ft)/Fm'
 Fv'/Fm'= (Fm' –F0')/Fm'
 NPQ= (Fm – Fm')/Fm'
where, Pla is average leaf area per plant (cm2); den is real density (plant m-2); LA1 is primary leaf area (cm2); LA2 is secondary leaf area (cm2); T1 is first sampling time (day); T2 is second sampling time (day); W1 is primary dry weight (g); W2 is secondary dry weight (g); GA is ground area (m2); lnW2 – lnW1 is natural logarithm difference of dry weight; Fm is maximal fluorescence yield of the dark-adapted state; F0 is minimal fluorescence yield of the dark-adapted state; Fv is variable fluorescence (dark) (Fm-F0); Ft is fluorescence emitted by the leaves adapted to lighting; Fm' is maximal fluorescence yield of the light-adapted state; F0'is minimal fluorescence yield of the light-adapted state, and Fv' is variable fluorescence (light) (Fm'-F0'). The grain yield was determined at maturity stage and through the harvest of all spikes from the level of 1 m2 per plot and after removing 0.5 m from the beginning and end respective planting rows (rows 5 and 6).Analysis of variance was performed using general linear model (GLM) procedure of statistical analysis system (SAS version: 9.3). The means were analyzed using the least significant difference (LSD) method at P=0.05 (LSD 0.05). The results showed that late planting date due to terminal heat stress significantly decreased LAI (30.0%), LAD (48.4%), CGR (21.8%), RGR (13.7%), NAR (22.4%), Fv/Fm (3.2%), ΦPSII (4.4%), Fv'/Fm' (3.2%) and grain yield (27.0%) of two cultivars of bread wheat Pishtaz and Chamran 2, but increased significantly the NPQ (22.7%). Potassium and zinc foliar application improved significantly traits the LAI (17.5%), LAD (17.62%), CGR (33.5%), RGR (12.0%), NAR (37.6%), Fv/Fm (3.1%), ΦPSII (7.4%), Fv'/Fm' (6.6%) and grain yield (17.30%) of two cultivars of bread wheat Pishtaz and Chamran 2 under late planting date except for the NPQ. Among the interactions of nutrients foliar application and bread wheat cultivars, the response of Pishtaz and Chamran 2 cultivars were more suitable to potassium foliar application, and zinc and zinc + potassium foliar application, respectively. As well as, among wheat cultivars cultivated under the conditions of non-application of potassium and zinc (control), Chamran 2 cultivar had a comparative advantage in all measured traits under both optimum and late planting dates compared to Pishtaz cultivar. In general, it can be used from timely planting date, potassium and zinc foliar application and suitable wheat cultivar such as Chamran 2 as three management strategies to reduce the harmful effects of terminal heat stress caused by late planting date in Ramhormoz city.