The present study demonstrated that photosynthetic performance in tomato was significantly affected by the atmospheric evaporative demand. The proportion of individual limitation components including stomatal conductance, mesophyll conductance and biochemical carboxylation inside chloroplast were not constant with varying VPD. The relative contribution of stomatal, mesophyll resistance and biochemical carboxylation imposed on photosynthesis varied under contrasting VPD condition. Thereby, the key rate-limiting step for photosynthetic performance varied with rise in VPD: under low VPD condition, stomatal and mesophyll conductance was sufficiently high for efficiently CO2 transport, which facilitated high CO2 availability inside chloroplast for carbon fixation; diffusion limitation of stomatal and mesophyll conductance accounted for a low fraction of total photosynthetic limitation under low VPD condition, biochemical carboxylation was the key rate-limiting step for raising photosynthetic potential of tomato. With VPD increasing, stomatal and mesophyll conductance for CO2 transport declined. Stomatal and mesophyll limitation on photosynthesis increased gradually with rise in VPD. Consequently, the low chloroplast CO2 concentration substantially constrained photosynthesis under high VPD condition. Thereby, CO2 diffusion limitation in series of stomatal and mesophyll resistance was key rate-limiting step for photosynthesis under high VPD condition.
Three steps were involved in the potential mechanism accounting for the increased significant limitation of stomatal and mesophyll conductance imposed on photosynthesis in tomato with rise in VPD, which was illustrated in Fig.6: (Ⅰ) rise in VPD caused plant water stress via disrupting the mass balance between soil water supply and atmospheric evaporative demand. (Ⅱ) plant water stress with rise in VPD triggered stomatal closure and reduced stomatal conductance for CO2 uptake. (Ⅲ) leaf anatomical acclimation to atmospheric drought modulated mesophyll conductance for CO2 transport within leaf.
Rising in VPD triggered plant water stress via disrupting the mass balance between soil water supply and atmospheric evaporative demand
Passive water movement was driven by the gradually declined free energy along soil-plant-atmospheric continuum, which can be quantified as the gradient in water potential in liquid phase. Water movement at leaf-air boundary in gas phase was driven by the difference in VPD. Based on physical principles, excessive air desiccation triggered high VPD and great negative air water potential. Δψleaf-air was substantial greater than Δψsoil-leaf, which pulled water out of plant. The substantial difference between Δψleaf-air and Δψsoil-leaf was logarithmically enlarged with rise in VPD. Quantitatively, the atmospheric driving force at leaf-air boundary can be more than a hundredfold larger than soil-leaf component under high VPD condition. The great asymmetric between atmospheric evaporative demand and soil water supply triggered disruption in water balance despite plants were well irrigated. Root water uptake and supply was inadequate to keep pace with the great atmospheric driving force under high VPD condition, which consequently triggered leaf dehydration and declines in water potential. Thereby, VPD was crucial external stimulations pulling water out of soil and affect water balance. VPD fluctuated dramatically over the diurnal course in crop production, especially for greenhouse cultivation. Soil moisture was relatively stable over short term, with a minor variation compared with atmospheric evaporative demand . Plant-water relations was regulated to a greater extent by VPD, and to a less extent modulated by soil moisture. Similar as soil drought, VPD induced atmospheric drought and plant water stress was also important factors triggering depression in photosynthesis.
Plant water stress with rise in VPD triggered stomatal closure and reduced stomatal conductance for CO2 uptake
Stomata was the “gatekeepers” for exchange of water vapour and CO2. Guard cells surrounding the stomatal pore respond to perturbations of soil-plant-atmospheric hydraulic continuum, which was putatively transduced into stomatal movements by feedback and feedforward mechanisms [27-29]. Stomatal control of transpired water loss was critical for sustaining physiological processes, such as leaf water status and photosynthetic CO2 uptake. It has been recognized that plant respond to drought by closing guard cells to reduce excessive water loss and prevent the development of water deficit in plant tissues . In the present study, atmospheric driving force was an order of magnitude greater than water supply, which lead to a great symmetry between water supply and evaporative demand. The water supply-evaporative demand symmetry triggered declines in leaf water potential and stomatal closure. However, the mechanism of VPD-triggered stomatal closure was still uncertain, which was‘black box’ . Some hypotheses hold that stomatal closure under high VPD condition was a passive process triggered by leaf dehydration and turgor loss. However, large evidences were provided that high-VPD triggered stomatal closure was probably more than a passive process . Some proposed hypothesis hold that high-VPD triggered stomatal closure was an active process rather than passive, since the plant stress hormone of abscisic acid (ABA) was continuously produced and delivered with transpiration stream to guard cell [33, 34]. However, it is not clarified whether the ABA mediated active process also participated in VPD-induced stomatal regulation in tomato.
Despite stomatal closure prevented excess water loss to maintain physiological process by passive or active mechanisms, the closed “gatekeepers” simultaneously increased the stomatal resistance for photosynthetic CO2 uptake from air to intercellular. Intercellular CO2 concentration was gradually reduced with rise in VPD. Consequently, stomatal limitation imposed on photosynthesis increased with rise in VPD. The declines in leaf water potential and stomatal conductance with rise in VPD was less marked in high-VPD grown plants in this research. The distinct response to VPD may can be attribute to the physiological acclimation to growth condition. Long-term acclimation to high VPD condition enhanced water stress tolerant, which prevent the dramatic declines in leaf water potential, stomatal conductance and photosynthetic parameters when subjected to atmospheric drought.
Anatomical determination for mesophyll conductance of CO2 transport within leaf under contrasting VPD condition
In additional to the first barrier of stomata, CO2 transported from intercellular to carboxylation site was constrained by a comparable resistance with stomata. The present study demonstrated that mesophyll resistance was a significant component of diffusion resistance from air to Rubisco in tomato. A strong positive correlation between mesophyll and stomatal conductance was observed among treatments. Similar as stomatal conductance, mesophyll conductance of tomato was also linearly reduced with rise in VPD. Under low VPD condition, stomatal conductance in coupled with mesophyll conductance was sufficiently high for efficient CO2 transport to carboxylation site within chloroplasts. High diffusion conductance in series of stomatal and mesophyll facilitated high chloroplast CO2 concentration for carbon fixation. With rise in VPD, CO2 concentration drawdown along “air- substomatal cavity- chloroplasts” was enlarged. Consequently, CO2 concentration inside chloroplasts was substantially reduced under high VPD condition. Limitation of mesophyll conductance imposed on photosynthesis gradually dominated with rise in VPD.
Unlike the rapid and sensitive stomatal response to external environment, mesophyll conductance from substomatal cavity to carbon fixation site was determined to a large degree by leaf anatomical traits [35-39]. Leaf dry mass area (LMA) was a composite of underlying traits affecting mesophyll conductance, such as lamina thickness, mesophyll thickness, cell wall thickness, cell shape and bulk leaf density . LMA determined the upper limit on mesophyll conductance. Meanwhile, LMA was closely linked to abiotic stress tolerance [40, 41]. Generally, higher LMA was a good indicator of greater stress tolerant. In the present study, LMA of high-VPD grown plants was higher than low-VPD plants. Higher LMA of tomato was an ecological strategy in response to atmospheric drought under high VPD condition. As aforementioned, root water uptake and supply were inadequate to keep pace with the great atmospheric driving force under high VPD condition. A higher LMA indicated dense structural traits, which buffered cellular transpired water loss and prevent leaf tissue dehydration under high VPD condition. However, CO2 and water transport shared pathway through the mesophyll cell walls and perhaps plasma membranes within leaves [42-45]. Despite the dense structural traits improved drought tolerance, the resistance for CO2 diffusion through substomatal cavity to chloroplasts was simultaneously increased. LMA was negatively correlated with mesophyll conductance in the present study, which was consistent with previous studies .