Effect of canopy temperature on physiological processes of grass pea as influenced by seed priming and foliar fertilization

Variations in atmospheric temperature have become a major concern with respect to agricultural production. Plants being poikilothermic in nature, any change in temperature summons change in canopy temperature, inevitably affecting their growth and productivity (Basu et al., 2014). Measurement of canopy temperature is necessary to understand the plant-water status at a particular phenophase of a crop. Interestingly, plant-water status has a direct role in controlling stomatal activity of leaves. In fact, stomata are the site for moisture and gaseous exchange in plants. Thus, the photosynthesis and transpirational characteristics are strongly influenced by stomatal behaviour (Li et al., 2017). However, moisture status of a crop can be explained more prominently with the estimation of stress degree day index (SDDI), derived from the difference between canopy temperature and atmospheric temperature. The higher the value of SDDI, greater is the stress. Canopy temperature beyond optimum often drops the potential for physiological processes of crop plants with special reference to pulse crops, hampering their production (Tzudir et al., 2018).

The rich nutritional composition, minimum requirements of inputs and outstanding tolerance to environmental adversities have popularized the cultivation of grass pea (Lathyrus sativus L.) among the resource poor Indian farmers (Dixit et al., 2016). To achieve better growth and production of pulse crops through need based and timely supply of nutrients, seed priming and foliar fertilization are two incredible cost effective interventions (Banerjee et al., 2019). This experiment was framed to address the lacuna of systematic research on impact of canopy temperature on rate of photosynthesis and transpiration in grass pea in lights of seed priming and foliar spray.
The field experiment was conducted during winter seasons of 2017-18 and 2018-19 at District Seed Farm, Bidhan Chandra Krishi Viswavidyalaya, West Bengal in factorial RBD comprising two seed priming levels (S 1 : No priming and S 2 : Seed priming with Ammonium molybdate @ 0.5 g kg -1 seed) and five foliar spray levels ( Rice crop was harvested 7-10 days after sowing of grass pea in both the years under investigation. Application of basal dose of fertilizers and irrigation were excluded. Rate of photosynthesis and transpiration were recorded with the help of Cl-340 hand-held photosynthesis system (ClD Company, Camas, USA) during 1130 to 1230 hours on sunny days. Canopy temperature was measured at the same hours with Infrared tele-thermometer (AG-42 Telatemp infrared thermometer, Australia). Stress Degree Day Index (SDDI) was computed with the following formula (Idso et al., 1977) : Where, T c = Canopy temperature at mid-day and T a = Air temperature at mid-day The overall statistical analysis was done by using SPSS 7.5 software, (SPSS 7.5 copyright, 1997 by SPSS Inc., USA Base 7.5 Application guide).

Effect of seed priming and foliar nutrition on rate of photosynthesis and transpiration
The rate of photosynthesis and transpiration in the above-ground parts of relay grass pea grown

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progressively increased upto 60 DAS and afterwards a gradually declined (Table 1 and 2). This increment in rate of photosynthesis from 30 DAS to 60 DAS might be due to greater availability of photosynthesizing site through leaves/canopy enlargement. Pooled analysis revealed that highest rate of photosynthesis was attained under the treatments S 2 and F 5 i.e., seed priming (16.27 and 6.13 µmol m -2 sec -1 ) and twice foliar spray of NPK (19:19:19) (18.25 and 7.06 µmol m -2 sec -1 ) at 60 and 90 DAS, respectively. This trend was clearly identified by the advances in study of leaf photosynthesis which revealed that maximum photosynthetic efficiency was positively correlated with leaf nitrogen and phosphorus and potassium content (Walker et al., 2014). The crop again attained the maximum rate of transpiration at 60 and 90 DAS, respectively in S 2 (3.78 and 2.15 mmol m -2 sec -1 ) and F 5 (3.67 and 2.27 mmol m -2 sec -1 ). Most of the cases exhibited non-significant interactions. The role of potassium in modulating the internal water balance was supposed to lower down canopy temperature by maintaining transpirational cooling even under moisture stress.

Effect of canopy temperature and SDDI on physiological parameters and yield
In all the cases, pooled values of these physiological parameters were found to be polynomial functions of canopy temperatures at respective phenophases (Fig. 1). About 84.74, 92.31 and 81.21 per cent variations in rate of photosynthesis and 92.5, 96.02 and 83.52 per cent variations in rate of transpiration could be contributed to the variation in mean canopy temperature at 30, 60 and 90 DAS respectively. Both the canopy temperature and SDDI were negatively correlated with the rate of photosynthesis and transpiration (Table 3). Evidently, the transpiration of grass pea exhibited increasing rate from 30 DAS to 60 DAS simultaneously with the rise in canopy temperature. This increment might be explained with the theory of Baligar et al. (2012) describing the incremental rate of transpiration with escalated leaf temperature as the leaf diffusive resistance increased. However, the declining trend in rate of photosynthesis and transpiration after 60 DAS might be linked with the non-stop rise in ambient and canopy temperature beyond optimum level which compelled the stomata to close partially with greater stomatal diffusion resistance due to moisture deficit, curtailing the photosynthetic efficiency and transpiration (Basu et al., 2013).   to canopy temperature (*significant at 5 % level). Lower canopy temperature and SDDI have been reported to enhance the seed yield in chickpea (Summy et al., 2015), in groundnut (Chakravarti et al., 2010) and in mungbean (Tzudir et al., 2018).
The study revealed that seed priming with Ammonium molybdate @ 0.5 g kg -1 seed along with twice spraying of 0.5 per cent NPK (19:19:19) spray were effective in modulating canopy temperature and physiological attributes. Canopy temperature significantly but negatively influenced the rate of photosynthesis and transpiration at different phenophases as well as seed yield of grass pea.

Conflict of Interest Statement :
The author(s) declare (s) that there is no conflict of interest.

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