High light decreases xylem contribution to fruit growth in tomato

Isohydric stomatal behaviour alters fruit vascular flows and minimizes fruit size reductions in drought-stressed 'Hass' avocado (Persea americana Mill.)

BACKGROUND AND AIMS

Plant water status is important for fruit development, because many fleshy fruits contain large amounts of water. However, there is no information on vascular flows of Persea americana 'Hass' avocado. The aims of this research were to explore the impact of drought stress on the water relationships of the 'Hass' avocado plant and its fruit growth.

METHODS

Well-watered and water-stressed 'Hass' avocado plants were compared. Over 4 weeks, water flows through the shoot and fruit pedicel were monitored using external sap flow gauges. Fruit diameter was monitored using linear transducers, and stomatal conductance (gs), photosynthesis (A) and leaf and stem water potentials (Ѱleaf and Ѱstem) were measured to assess the response of the plants to water supply.

KEY RESULTS

In well-watered conditions, the average water inflow to the shoot was 72 g day-1. Fruit water inflow was 2.72 g day-1, but there was water loss of 0.37 g day-1 caused by the outflow (loss back into the tree) through the vascular tissues and 1.06 g day-1 from the fruit skin. Overall, fruit volume increased by 1.4 cm3 day-1. In contrast, water flow into fruit of water-stressed plants decreased to 1.88 g day-1, with the outflow increasing to 0.61 g day-1. As a result, increases in fruit volume were reduced to 0.4 cm3 day-1. The values of A, gs and sap flow to shoots were also reduced during drought conditions. Changes in the hourly time-courses of pedicel sap flow, fruit volume and stem water potential during drought suggest that the stomatal response prevented larger increases in outflow from the fruit. Following re-watering, a substantial recovery in growth rate was observed.

CONCLUSIONS

In summary, a reduction in growth of avocado fruit was observed with induced water deficit, but the isohydric stomatal behaviour of the leaves helped to minimize negative changes in water balance. Also, there was substantial recovery after re-watering, hence the short-term water stress did not decrease avocado fruit size. Negative impacts might appear if the drought treatment were prolonged.

Impact of Climate Change on Altered Fruit Quality with Organoleptic, Health Benefit, and Nutritional Attributes

As a consequence of global climate change, acute water deficit conditions, soil salinity, and high temperature have been on the rise in their magnitude and frequency, which have been found to impact plant growth and development negatively. However, recent evidence suggests that many fruit plants that face moderate abiotic stresses can result in beneficial effects on the postharvest storage characters of the fruits. Salinity, drought, and high temperature conditions stimulate the synthesis of abscisic acid (ABA), and secondary metabolites, which are vital for fruit quality. The secondary metabolites like phenolic acids and anthocyanins that accumulate under abiotic stress conditions have antioxidant activity, and therefore, such fruits have health benefits too. It has been noticed that fruits accumulate more sugar and anthocyanins owing to upregulation of phenylpropanoid pathway enzymes. The novel information that has been generated thus far indicates that the growth environment during fruit development influences the quality components of the fruits. But the quality depends on the trade-offs between productivity, plant defense, and the frequency, duration, and intensity of stress. In this review, we capture the current knowledge of the irrigation practices for optimizing fruit production in arid and semiarid regions and enhancement in the quality of fruit with the application of exogenous ABA and identify gaps that exist in our understanding of fruit quality under abiotic stress conditions.

Vascular Tissues Distribution Affects Calcium and Calcium Oxalate Crystals in Fruits of Wild Tomato (Lycopersicon pimpinellifolium (L.) Mill.)

Tomato fruit is an excellent model for evaluating calcium regulation in plants since it expresses symptoms of either calcium deficiency or calcium excess. Aiming to evaluate the structure of the vascular system and its interactions with calcium and calcium oxalate crystals (CaOx), fruits of Lycopersicon pimpinellifolium were studied. Calcium levels were evaluated in basal, median, and distal pericarp portions, which were also analyzed under a light microscope to describe the structure. The L. pimpinellifolium pericarp shows idioblasts with calcium oxalate crystals. Vascular bundles of the basal pericarp show large transverse sections and abundant xylem vessels. The vascular bundles were smaller in the distal pericarp, and the xylem showed fewer and narrower vessels. The terminal bundles often consisted exclusively of phloem. Despite the differences observed in vascular bundle composition, the density of the vascular system was uniform in the pericarp as a consequence of bundle ramifications that occur at distal portions. The calcium concentration and crystal idioblasts decrease towards the apex of the fruit. The reduction in the xylem:phloem ratio seems to determine the low calcium concentration in the distal fruit portion.

Additive effects of light and branching on fruit size and chemical fruit quality of greenhouse tomatoes

Introduction

Greenhouse tomato growers face the challenge of balancing fruit size and chemical quality traits. This study focused on elucidating the interplay between plant branching and light management on these traits, while maintaining consistent shoot density.

Methods

We evaluated one- and two-shoot plants under varying top light intensities using high-pressure sodium lamps and light-emitting diode (LED) inter-lighting.

Results

The reduced yield in the two-shoot plants was mainly due to smaller fruit size, but not due to source strength limitations, as evaluated through leaf weight ratio (LWR), chlorophyll index, specific leaf area (SLA), leaf dry matter percentage, and stem soluble carbohydrate accumulation. Enhanced lighting improved fruit weight and various fruit traits, such as dry matter content, total soluble carbohydrate content, and phenolic content, for both one- and two-shoot plant types. Despite lower mean fruit weight, two-shoot plants exhibited higher values for chemical fruit quality traits, indicating that the fruit growth of two-shoot plants is not limited by the available carbohydrates (source strength), but by the fruit sink strength. Diurnal analysis of fruit growth showed that two-shoot plants had reduced expansion during light transitions. This drop in fruit expansion was not related to changes in root pressure (measured as xylem sap exudation from decapitated plants), but might be related to diminished xylem area in the stem joint of the two-shoot plants. The concentration of several hormones, including cytokinins, was lower in two-shoot plants, suggesting a reduced fruit sink capacity.

Discussion

The predominant impact of branching to two-shoot plants on sink capacity suggests that the fruit growth is not limited by available carbohydrates (source strength). Alongside the observation that light supplementation and branching exert independent additive effects on fruit size and chemical traits, this illuminates the potential to independently regulate these aspects in greenhouse tomato production.

Pedicel anatomy and histology in tomato vary according to genotype and water-deficit environment, affecting fruit mass

The growth and composition of fleshy fruits depend on resource acquisition and distribution in the plant. In tomato, the pedicel serves as the final connection between plant and fruit. However, very few quantitative data are available for the conducting tissues of the pedicel, nor is their genetic variability known. In the present study, a histological approach was combined with process-based modeling to evaluate the potential contribution made by the anatomy and histology of the pedicel to variations in fruit mass. Eleven genotypes were characterized and the impact of water deficit was studied for a single genotype using stress intensity and stage of application as variables. The results highlighted extensive variations in the relative proportions of the different pedicel tissues and in the absolute areas of xylem and phloem between genotypes. The model suggests that the variations in the area of the pedicel's vascular tissues induced by differences in genotype and water-deficit environments partly contributed to fruit mass variability. They therefore warrant phenotyping for use in the development of plant strains adapted to future environmental constraints. The results also demonstrated the need to develop non-invasive in vivo measurement methods to establish the number and size of active vessels and the flow rates in these vessels to improve prediction of water fluxes in plant architecture.

Reproductive water supply is prioritized during drought in tomato

Reproductive success largely defines the fitness of plant species. Understanding how heat and drought affect plant reproduction is thus key to predicting future plant fitness under rising global temperatures. Recent work suggests reproductive tissues are highly vulnerable to water stress in perennial plants where reproductive sacrifice could preserve plant survival. However, most crop species are annuals where such a strategy would theoretically reduce fitness. We examined the reproductive strategy of tomato (Solanum lycopersicum var. Rheinlands Ruhm) to determine whether water supply to fruits is prioritized above vegetative tissues during drought. Using optical methods, we mapped xylem cavitation and tissue shrinkage in vegetative and reproductive organs during dehydration to determine the priority of water flow under acute water stress. Stems and peduncles of tomato showed significantly greater xylem cavitation resistance than leaves. This maintenance of intact water supply enabled tomato fruit to continue to expand during acute water stress, utilizing xylem water made available by tissue collapse and early cavitation of leaves. Here, tomato plants prioritize water supply to reproductive tissues, maintaining fruit development under drought conditions. These results emphasize the critical role of water transport in shaping life history and suggest a broad relevance of hydraulic prioritization in plant ecology.

Water transport in fleshy fruits: Research advances, methodologies, and future directions

Fruits are reproductive organs in flowering plants and the harvested products of many agricultural crops. They play an increasingly important role in the human diet due to their nutritional values. Water is the most abundant component of most fleshy fruits, and it is essential for fruit growth and quality formation. Water is transported to the fruit via the vascular system (xylem and phloem) and lost to the air through the fruit surface due to transpiration. This minireview presents a framework for understanding water transport in fleshy fruits along with brief introductions of key methodologies used in this research field. We summarize the advances in the research on the patterns of water flow into and out of the fruit over development and under different environmental conditions and cultural practices. We review the key findings on fruit transpiration, xylem transport, phloem transport, and the coordination of water flows in maintaining fruit water balance. We also summarize research on post-vascular water transport mediated by aquaporins in fruits. More efforts are needed to elucidate the mechanisms by which different environmental conditions impact fruit water transport at the micro-level and to better understand the physiological implications of the coordination of water flows. Incorporating fruit water transport into the research area of plant hydraulics will provide new insights into water transport in the soil-plant-atmosphere continuum.

Supplemental Light-Emitting Diode Inter-Lighting Increases Tomato Fruit Growth Through Enhanced Photosynthetic Light Use Efficiency and Modulated Root Activity

We investigated the effect of supplemental LED inter-lighting (80% red, 20% blue; 70 W m-2; light period 04:00-22:00) on the productivity and physiological traits of tomato plants (Flavance F1) grown in an industrial greenhouse with high pressure sodium (HPS) lamps (235 W m-2, 420 µmol m-2 s-1 at canopy). Physiological trait measurements included diurnal photosynthesis and fruit relative growth rates, fruit weight at specific positions in the truss, root pressure, xylem sap hormone and ion compositions, and fruit quality. In the control treatment with HPS lamps alone, the ratio of far-red to red light (FR:R) was 1.2 at the top of the canopy and increased to 5.4 at the bottom. The supplemental LED inter-lighting decreased the FR:R ratio at the middle and low positions in the canopy and was associated with greener leaves and higher photosynthetic light use efficiency (PLUE) in the leaves in the lower canopy. The use of LED inter-lighting increased the biomass and yield by increasing the fruit weight and enhancing plant growth. The PLUE of plants receiving supplemental LED light decreased at the end of the light period, indicating that photosynthesis of the supplemented plants at the end of the day might be limited by sink capacity. The supplemental LED lighting increased the size of fruits in the middle and distal positions of the truss, resulting in a more even size for each fruit in the truss. Diurnal analysis of fruit growth showed that fruits grew more quickly during the night on the plants receiving LED light than on unsupplemented control plants. This faster fruit growth during the night was related to an increased root pressure. The LED treatment also increased the xylem levels of the phytohormone jasmonate. Supplemental LED inter-lighting increased tomato fruit weight without affecting the total soluble solid contents in fruits by increasing the total assimilates available for fruit growth and by enhancing root activity through an increase in root pressure and water supply to support fruit growth during the night.

Modelling grape growth in relation to whole-plant carbon and water fluxes

The growth of fleshy fruits is still poorly understood as a result of the complex integration of water and solute fluxes, cell structural properties, and the regulation of whole plant source-sink relationships. To unravel the contribution of these processes to berry growth, a biophysical grape (Vitis vinifera L.) berry growth module was developed and integrated with a whole-plant functional-structural model, and was calibrated on two varieties, Cabernet Sauvignon and Sangiovese. The model captured well the variations in growth and sugar accumulation caused by environmental conditions, changes in leaf-to-fruit ratio, plant water status, and varietal differences, with obvious future application in predicting yield and maturity under a variety of production contexts and regional climates. Our analyses illustrated that grapevines strive to maintain proper ripening by partially compensating for a reduced source-sink ratio, and that under drought an enhanced berry sucrose uptake capacity can reverse berry shrinkage. Sensitivity analysis highlighted the importance of phloem hydraulic conductance, sugar uptake, and surface transpiration on growth, while suggesting that cell wall extensibility and the turgor threshold for cell expansion had minor effects. This study demonstrates that this integrated model is a useful tool in understanding the integration and relative importance of different processes in driving fleshy fruit growth.

A 3-D functional-structural grapevine model that couples the dynamics of water transport with leaf gas exchange

Background and Aims

Predicting both plant water status and leaf gas exchange under various environmental conditions is essential for anticipating the effects of climate change on plant growth and productivity. This study developed a functional-structural grapevine model which combines a mechanistic understanding of stomatal function and photosynthesis at the leaf level (i.e. extended Farqhuhar-von Caemmerer-Berry model) and the dynamics of water transport from soil to individual leaves (i.e. Tardieu-Davies model).

Methods

The model included novel features that account for the effects of xylem embolism (fPLC) on leaf hydraulic conductance and residual stomatal conductance (g0), variable root and leaf hydraulic conductance, and the microclimate of individual organs. The model was calibrated with detailed datasets of leaf photosynthesis, leaf water potential, xylem sap abscisic acid (ABA) concentration and hourly whole-plant transpiration observed within a soil drying period, and validated with independent datasets of whole-plant transpiration under both well-watered and water-stressed conditions.

Key Results

The model well captured the effects of radiation, temperature, CO2 and vapour pressure deficit on leaf photosynthesis, transpiration, stomatal conductance and leaf water potential, and correctly reproduced the diurnal pattern and decline of water flux within the soil drying period. In silico analyses revealed that decreases in g0 with increasing fPLC were essential to avoid unrealistic drops in leaf water potential under severe water stress. Additionally, by varying the hydraulic conductance along the pathway (e.g. root and leaves) and changing the sensitivity of stomatal conductance to ABA and leaf water potential, the model can produce different water use behaviours (i.e. iso- and anisohydric).

Conclusions

The robust performance of this model allows for modelling climate effects from individual plants to fields, and for modelling plants with complex, non-homogenous canopies. In addition, the model provides a basis for future modelling efforts aimed at describing the physiology and growth of individual organs in relation to water status.

Introducing turgor-driven growth dynamics into functional-structural plant models

Background and Aims

In many scenarios the availability of assimilated carbon is not the constraining factor of plant growth. Rather, organ growth appears driven by sink activity in which water availability plays a determinant role. Current functional-structural plant models (FSPMs) mainly focus on plant-carbon relations and largely disregard the importance of plant water status in organogenesis. Consequently, incorporating a turgor-driven growth concept, coupling carbon and water dynamics in an FSPM, presents a significant improvement towards capturing plant development in a more mechanistic manner.

Methods

An existing process-based water flow and storage model served as a basis for implementing water control in FSPMs. Its concepts were adjusted to the scale of individual plant organs and interwoven with the basic principles of modelling carbon dynamics to allow evaluation of turgor pressure across the entire plant. This was then linked to plant organ growth by applying the principles of the widely used Lockhart equation.

Key results

This model successfully integrates a mechanistic understanding of plant water transport dynamics coupled with simple carbon dynamics within a dynamically developing plant architecture. It allows evaluation of turgor pressure on the scale of plant organs, resulting in clear diel and long-term patterns, directly linked to plant organ growth.

Conclusions

A conceptual sap flow and turgor-driven growth model was introduced for functional-structural plant modelling. It is applicable to any plant architecture and allows visual exploration of the diel patterns of organ water content and growth. Integrated in existing FSPMs, this new concept fosters an array of possibilities for FSPMs, as it presents a different formulation of growth in terms of local processes, influenced by local and external conditions.

Heat girdling does not affect xylem integrity: an in vivo magnetic resonance imaging study in the tomato peduncle

Heat girdling is a method to estimate the relative contribution of phloem vs xylem water flow to fruit growth. The heat girdling process is assumed to destroy all living tissues, including the phloem, without affecting xylem conductivity. However, to date, the assumption that xylem is not affected by heat girdling remains unproven. In this study, we used in vivo magnetic resonance imaging (MRI) velocimetry to test if heat girdling can cause xylem vessels to embolize or affect xylem water flow characteristics in the peduncle of tomato (Solanum lycopersicum cv Dirk). Anatomical and MRI data indicated that, at the site of girdling, all living tissues were disrupted, but that the functionality of the xylem remained unchanged. MRI velocimetry showed that the volume flow through the secondary xylem was not impeded by heat girdling in either the short or the long term (up to 91 h after girdling). This study provides support for the hypothesis that in the tomato peduncle the integrity and functionality of the xylem remain unaffected by heat girdling. It therefore confirms the validity of the heat girdling technique as a means to estimate relative contributions of xylem and phloem water flow to fruit growth.

A Modeling Approach to Quantify the Effects of Stomatal Behavior and Mesophyll Conductance on Leaf Water Use Efficiency

Water use efficiency (WUE) is considered as a determinant of yield under stress and a component of crop drought resistance. Stomatal behavior regulates both transpiration rate and net assimilation and has been suggested to be crucial for improving crop WUE. In this work, a dynamic model was used to examine the impact of dynamic properties of stomata on WUE. The model includes sub-models of stomatal conductance dynamics, solute accumulation in the mesophyll, mesophyll water content, and water flow to the mesophyll. Using the instantaneous value of stomatal conductance, photosynthesis, and transpiration rate were simulated using a biochemical model and Penman-Monteith equation, respectively. The model was parameterized for a cucumber leaf and model outputs were evaluated using climatic data. Our simulations revealed that WUE was higher on a cloudy than a sunny day. Fast stomatal reaction to light decreased WUE during the period of increasing light (e.g., in the morning) by up to 10.2% and increased WUE during the period of decreasing light (afternoon) by up to 6.25%. Sensitivity of daily WUE to stomatal parameters and mesophyll conductance to CO2 was tested for sunny and cloudy days. Increasing mesophyll conductance to CO2 was more likely to increase WUE for all climatic conditions (up to 5.5% on the sunny day) than modifications of stomatal reaction speed to light and maximum stomatal conductance.