Previous studies revealed that P deficiency reduces net photosynthetic rates (Brooks et al., 1988;) due to impaired production of NADPH and ATP (Lauer et al., 1989; Freeden et al., 1989) and stomatal conductance (gs) in different plant species (Li et al., 2006; He et al., 2011; Zribi et al., 2011). For instance, P deprivation has a greater effect on CO2 assimilation compare to light reactions (Lauer et al. 1989), through lowering functional Rubisco content, specific activity and efficiency (Brooks 1986; Lauer et al. 1989). P deficiency limits plant growth and yield through influencing CO2 assimilation.
Rate of photosynthesis (PN) depends to ATP-dependent regeneration of ribulose-1,5 bisphosphate in dark reactions and electron transport to photosystem (PS II) in light reactions, these processes are influenced by low P availability (Xu et al., 2007; Frydenvang et al., 2015).). Among legume crops, PN reduction due to low P was recorded highest in mungbean (Vigna radiata L.) (94%), followed by mashbean (V. aconitifolia) (65%) and soybean (Glycine max) (30%) (Chaudhary et al., 2008). Under low P treated conditions, PN was reduced 80% of sufficient P in sunflower (Helianthus annus), while stomatal co ductance (gs) and intra-cellular CO2 (Ci) decreased 35% and 21%, respectively (Jacob and Lawlor, 1991).
P deprivation significantly reduced the PN and gs of rice (Oriza sativa) while Ci increased or remain unchanged, indicating a significant non-stomatal limitation induced by P deficiency (Li et al., 2006; Xu et al. 2007).
Among the physiological processes, Photosynthesis is the most sensitive to drought in higher plants (Zhang et al. 2013). Soil moisture deficit leads to a permanent reduction in CO2 assimilation (Yordanov et al., 2003; Chaves et al., 2009). Decreased PN is result of stomatal (diffusive limitation of the supply of CO2 to the site of carboxylation) (Hu et al., 2010) and non- stomatal (biochemical metabolism) limitations (Graan and Boyer, 1990; Shangguan et al., 1999; Yordanov et al., 2000; Zlatev and Lidon, 2012). Non-stomatal limitation of PN has been conferred to reduced overall carboxylation efficiency through reducing ribulose-1,5-bisphospate regeneration (Tezara and Lawlor, 1995) and reduction in amount of functional Rubisco enzyme (Medrano et al., 2002) or through inhibiting functional PS II (Subrahmanyam et al, 2006). Several researchers reported a significant reduction in PN, transpiration rate (E) and gs in legume crops such as soybean (De Souza et al., 1997; Atti et al., 2004 ; Wang et al., 2018), Fababean (Abid et al., 2016), chickpea (Mafakheri et al., 2010), common bean (Zlatev and Yordanov, 2004; Mathobo et al., 2017) and mungbean (Baroowa et al., 2015; Nazran et al., 2019).