Plasticity in stomatal size and density of potato leaves under different irrigation and phosphorus regimes

Abstract

The morphological features of stomata including their size and density could be modulated by environmental cues; however, the underlying mechanisms remain largely elusive. Here, the effect of different irrigation and phosphorus (P) regimes on stomatal size (SS) and stomatal density (SD) of potato leaves was investigated. The plants were grown in split-root pots under two P fertilization rates (viz., 0 and 100 mg kg⁻¹ soil, denoted as P0 and P1, respectively) and subjected to full (FI), deficit (DI), and partial root-zone drying (PRD) irrigation regimes. Results showed that SS and SD were unresponsive to P but significantly affected by the irrigation treatment. FI plants had the largest SS, followed by DI, and PRD the smallest; and the reverse was the case for SD. Compared to FI and DI, PRD plants had significantly lower values of specific leaf area (SLA) and leaf carbon isotope discrimination (Δ¹³C) under P0. Midday leaf water potential (Ψ_leaf) and stomatal conductance (g_s) was similar for DI and PRD, which was significantly lower than that of FI. Leaf contents of C, N, K, Ca and Mg were higher in PRD than in DI plants, particularly under P0. When analyzed across the three irrigation regimes, it was found that the P1 plants had significantly higher leaf contents of P and Mg, but significantly lower leaf K content compared to the P0 plants. Linear correlation analyses revealed that SS was positively correlated with Ψ_leaf and Δ¹³C; whereas SD was negatively correlated with Ψ_leaf, Δ¹³C and SLA, and positively correlated with leaf C, N and Ca contents. And g_s was positively correlated with SS but negatively correlated with SD. Collectively, under low P level, the smaller and denser stomata in PRD plants may bring about a more efficient stomatal control over gas exchange, hereby potentially enhance water-use efficiency as exemplified by the lowered leaf Δ¹³C under fluctuating soil moisture conditions.

Introduction

Stomata play a pivotal role in the regulation of leaf gas exchange. Stomatal conductance (g_s) for water vapour and CO₂ is determined primarily by the size (SS) and density (SD) of stomata for a given species under a certain environment (Parlange and Waggoner, 1970). It is believed that SS and SD are controlled by both genetic and environmental factors (Lake et al., 2002, Hetherington and Woodward, 2003, Casson and Gray, 2008, Yan et al., 2012, Zhang et al., 2012). They may vary considerably across plant species and can be modulated by abiotic environmental perturbations such as changes of atmospheric CO₂ concentration, light intensity, temperature, soil water and nutritional status (Woodward, 1987, Woodward and Kelly, 1995, Lake et al., 2001, Lake and Woodward, 2008, Sekiya and Yano, 2008, Xu and Zhou, 2008, Yan et al., 2012). Due to their crucial role in regulating g_s, SS and SD are important ecophysiological traits controlling plant water use, especially in water-limited environments (Croxdale, 2000, Sack et al., 2003, Poulos et al., 2007, Xu and Zhou, 2008).

Accumulated evidence indicates that a long-distance signal generated from the mature leaves that senses the environmental cues plays an important role in the regulation of stomatal formation on newly expanding leaves (e.g. Lake et al., 2001, Lake et al., 2002, Sekiya and Yano, 2008, Lake and Woodward, 2008). Experimental studies involving exposure the mature leaves to different atmospheric stimulus, e.g. irradiation intensity, air humidity, and CO₂ concentration, have revealed that systemic signalling systems including sugar and phytohormones are implicated in the long-distance signalling pathways regulating the stomatal development in new leaves (Lake et al., 2002, Miyazawa et al., 2006, Lake and Woodward, 2008). In addition to changes of atmospheric conditions, variation of the belowground abiotic environments around the roots such as soil moisture and nutrient status has also been found to modify stomatal morphology (Xu and Zhou, 2008, Sekiya and Yano, 2008), yet the underlying mechanisms for such regulation remain largely elusive. Besides, there is no consensus in the literature about how changes of soil environments affect SD and SS. For instance, earlier findings on modifications of SD responding to soil water deficits have been contradictory. Some studies reported that SD increases in response to soil water deficit (e.g. Elias, 1995, Franks et al., 2009, Fraser et al., 2009), while others observed an opposite effect (e.g. Quarrie and Jones, 1979, Silva et al., 2009). It seems that the response of SS and SD to soil water deficits depending on the degree of severity of the stress, and SD increases when the stress is moderate, and after reaching a threshold level of soil water deficit, SD decreases as the stress becomes severer (Xu and Zhou, 2008). Moreover, studies have documented that the effects of soil water status on SS and SD are interactively influenced by soil nitrogen (N) (Yan et al., 2012) or phosphorus (P) (Sekiya and Yano, 2008), underpinning the complexity in the regulation of stomatal morphology by soil water and nutrient status. Nonetheless, both studies have shown that SD is negatively correlated with Δ¹³C of the leaves (Sekiya and Yano, 2008, Yan et al., 2012), indicating that SD is closely associated with the optimization of leaf gas exchange, such that a maximal water use efficiency (WUE) is obtained according to the availability of water and nutrient in the soil. Collectively, it is clear that both water and nutrient status in the soil may influence the stomatal morphology thereby affecting WUE.

Considering freshwater resources become scarcer in many places in the world, research on water saving irrigation strategies such as alternate partial root-zone drying (PRD) irrigation and deficit irrigation (DI) has received increased attention (Yan et al., 2012). DI is a method that irrigates the entire root zone with an amount of water less than the potential evapotranspiration, and the minor stress that develops has minimal effects on the yield (English and Raja, 1996). PRD is a further refinement of DI; it involves irrigating only part of the root zone, leaving the other part to dry to a predetermined level before the next irrigation (Davies et al., 2002). Both DI and PRD allow the induction of the abscisic acid (ABA)-based root-to-shoot chemical signals to regulate plant growth and water use hereby improving WUE (Sun et al., 2013). Distinct effects on g_s and WUE between the two irrigation regimes have frequently been noticed; however, it remains unknown whether different irrigation in combination with P-fertilization regimes affect stomatal morphologic characteristics in plants, and its implications in modulating crop WUE.

In the present study, potato plants were grown in split-root pots and the effects of different irrigation regimes and P-fertilization rates on SS and SD of the fully expanded upper canopy leaves were investigated. The objective was to analyse the associations between leaf physiochemical characteristics and stomatal morphology, which will be helpful in understanding the mechanisms by which changes of soil water and nutrient environments modify SS and SD in potatoes.