INTRODUCTION

The quality of wine is highly dependent on the quality of the fruit produced from grapevines (Watson 2003, Coombe and Dry 2004, Conde et al. 2007). Fruit quality for wine is characterized by an optimal balance between different compounds in the berries as they ripen (Coombe and Dry 2004). The main compounds are acids (malic and tartaric acid), phenolic compounds (give colour and aroma to wine e.g. anthocyanins) and sugar (carbohydrates) (Coombe and McCarthy 2000). Sugar in the berry will drive the fermentation process during winemaking (Jackson 2014). The sugar acts as a food source for the yeast which metabolizes the sugar into ethanol and carbon dioxide (Jackson 2014). Sugar in a grapevine is produced by the biochemical reaction photosynthesis (Liakopoulos et al. 2007, Orr et al. 2017). Photosynthesis is not only important for grape berry development but also plays an important role for general growth of a grapevine and carbohydrate storage (Liakopoulos et al. 2007, Keller 2015). 

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Grapevine photosynthesis is the process by which the grapevine uses sunlight, carbon dioxide and water to produce carbohydrates in the form of sugar as well as oxygen as a by-product (Orr et al. 2017) (Figure 1). Water (taken up by the roots and transported to the leaves via the xylem vessel) and sunlight in the visible spectrum (400 to 700nm) are absorbed by chlorophyll harboured in chloroplasts in the mesophyll cells of the leaves (Smart and Robinson 1991, Orr et al. 2017). Carbon dioxide enters through openings (known as stomata) on the underside of the leaf (Orr et al. 2017). The sunlight provides the energy, while the water provides the necessary energy molecules, to drive the fixation of carbon dioxide (Calvin cycle-driven by the enzyme Rubisco) to produce sugar and oxygen (Orr et al. 2017).

Optimal photosynthetic reactions rely on a certain set of climatic and environmental conditions. Photoreceptors in the chloroplasts respond to sunlight intensity and sunlight in the visible spectrum (400 to 700nm) (Smart and Robinson 1991). In viticulture shaded leaves within canopies receive low intensities of sunlight and have reduced levels of photosynthesis (Smart and Robinson 1991). Photosynthesis can occur in a wide range of temperatures (20 to 30°C) but the reaction will be inhibited by temperature below 10°C and above 30°C (Smart and Robinson 1991). Wind affects stomatal behaviour which can reduce the uptake of carbon dioxide and transpiration levels (Kobriger et al. 1984). Direct negative effects of wind on photosynthesis is known to only occur in grapevines exposed to long periods of extremely windy conditions (Kobriger et al. 1984). Indirectly, long term windy conditions can lead to asymmetrical grapevine canopies which may lead to reduced sunlight penetration and photosynthetic levels (Tarara et al. 2005). Water stressed vines are known to close their stomata (to reduce water loss via transpiration) but are known to reduce photosynthetic reactions due to low carbon dioxide levels (Liakopoulos et al. 2007). As a result over a long term of drought, no soil water availability and very low humidity, the vine will start to shut down and will eventually die (Liakopoulos et al. 2007).   

In this report the main factors that affect photosynthesis and the management thereof will be discussed in reference to Marlborough Pinot Noir grown on 2 cane vertical shoot positioning (VSP). Marlborough as a region consists out of various soil types but mainly consists out of free draining alluvial and stony gravels washed down in the rivers via snow melt form eroded ancient glaciers (Wine Marlborough 2019). Marlborough has one of the sunniest and driest climates in New Zealand but temperatures are kept low by easterly sea breezes that cools the vineyards during the day (Wine Marlborough 2019). These climatic conditions in combination with soil type are suited for the growth of Pinot Noir in this region (Wine Marlborough 2019). Pinot Noir fruit has very thin skins and are highly sensitive to high temperatures, high humidity and high rainfall (induces rot and disease in the fruit) (Wine Marlborough 2019).

FACTORS THAT COULD LIMIT PHOTOSYNTHESIS OF MARLBOROUGH PINOT NOIR ON 2 CANE VSP

In this scenario the main factors that could limit photosynthesis are light quantity and quality, water availability in the soil, carbon dioxide concentration in the atmosphere, temperature, humidity, mineral nutrition and wind. From an ecological perspective it is always difficult to point out single factors that might affect physiological and biochemical processes in plants. Environmental factors act together in a complex system that will influence photosynthesis in a grapevine. With regards to the scenario I believe that the three main factors that could limit photosynthesis and that should be considered are light quantity and quality, mineral nutrition and water availability in the soil. The reasoning and potential affects will be discussed below.

Light quantity and quality

In this scenario Pinot Noir is grown on 2 cane VSP. Two cane VSP is well suited for varieties that naturally carry a low yield (ideal for Pinot Noir) but 2 cane VSP canopies can become dense, shaded and congested if not managed properly (Smart and Robinson 1991). Dense shaded canopies will dramatically reduce the quality and quantity of light that will reach the leaves (Figure 2) (Smart and Robinson 1991). A dense shaded canopy will have much lower rates of photosynthesis compared to an open less dense canopy (Smart and Robinson 1991). Leaves that receive high quality light, which are not shaded by other leaves, have maximum carbon dioxide uptake via the stomata, optimal Rubisco activity and overall high photosynthetic rates compared to shaded leaves (Archer and Strauss 1990, Liakopoulos et al. 2007). The reduction in photosynthesis, due to dense canopies, have been shown to lead to low quality fruit with reduced sugar levels which is not ideal for wine production (Smart 1987, Dokoozlian and Kliewer 1996, Coombe and Dry 2004).  

Mineral nutrition

The grape growing regions in Marlborough mainly consist out of rocky and stony soils with variable amounts of clay, silt and wind-blown loess (Gibbs and Vucetich 1962). Due to the nature of the soil (mainly stony which drains well) the nutrients are variable among the different grape growing regions in Marlborough with most regions having issues with nutrient leaching (Gibbs and Vucetich 1962). Key nutrients that have been shown to directly affect Vitis photosynthesis are nitrogen, phosphorus and the trace element iron (Chen and Chang 2003, Chen et al. 2004, Keller 2005). Nitrogen deficiency leads to a reduction in chlorophyll concentration as well as the activity of Rubisco (Figure 3) (Chen and Chang 2003, Keller 2005). As a result nitrogen deficient grapevines will have a reduction in carbon dioxide fixation and will start to synthesize secondary carbon based metabolites including anthocyanins (Chen and Chang 2003). This will lead to bleached fruit of red grape varieties (anthocyanins provide colour to the grape berries) which is unacceptable for red wine production (Keller 2005). Grapevines that are phosphorus deficient will also start to metabolize secondary metabolites and have been shown to have a reduction in stomatal conductance leading to reduced water transport from the roots (water potential levels are reduced) and carbon dioxide uptake via the stomata (Keller 2005). Iron deficient grapevines have reduced levels of carbon dioxide fixation due to the reduction in the activity of Rubisco (Chen et al. 2004).  

Water availability in the soil

Soils in the Marlborough wine region mainly consist out of rocky and stony architecture and are known to be mainly free draining soils (Gibbs and Vucetich 1962). Marlborough soil structure in combination with the relatively low rainfall can limit the water availability to the grapevines in the soil (Wine Marlborough 2019). A reduction in water availability in the soil will lead to the closure of the stomata on the grapevine leaves (Chaves et al. 2002). The roots send a hormonal signal (abscisic acid) to the stomata during a drought which will lead to the closure of the stomata (Correia et al. 1995). This will lead to a reduction in water loss via transpiration but will also lead to a reduction in carbon dioxide uptake (Maroco et al. 2002, Bertamini et al. 2006). Over a long drought, photosynthesis will start to slow down due to reduction in carbon dioxide availability (Maroco et al. 2002, Bertamini et al. 2006). Furthermore the activity of enzymes (e.g. Rubisco) is also reduced during periods of drought decreasing the overall photosynthetic activity of the plant (Figure 4) (Maroco et al. 2002, Bertamini et al. 2006).   

MANAGEMENT PLAN TO ACHIEVE OPTIMAL PHOTOSYNTHETIC LEVELS

Light quantity and quality

For optimal photosynthesis a canopy should be open and uncongested so that most of the leaves will receive optimal amounts of light and most of the photosynthetically active light (400 to 700nm). The management of a canopy all starts with the pruning process. The extent of the pruning conducted will govern how much active shoot growth will occur and will setup the canopy condition for the season. One way to determine the optimal number of nodes to leave on a cane is to weigh the pruned shoots from the previous season and relate the weight of the prunings to the yield weight from that vine (Smart and Robinson 1991). The ideal ratio should be in the range of 5 to 10 and the number of nodes laid down per cane should be adjusted accordingly. A second method to determine the optimal number of nodes to leave on a cane is by conducting active shoot counts. Active should counts are done after leaf drop has occurred and will determine how many active shoots the vine can produce in a season. When pruning it is preferred to leave a hand spacing between the final nodes on canes between vines as well as final nodes on canes between vines and posts (Smart and Robinson 1991). Good pruning practices will provide a good starting point for the growing season and will reduce further canopy management practices.

At the start of the growing season the vine will push for additional shoots. These shoot usually grow from the stem and are known as water shoots. These shoots will grow into the canopy and will create a shading effect if not managed properly (Goldammer 2018). Furthermore some nodes on the canes may house more than one fruitful bud and can produce more than one active shoot per node (Goldammer 2018). It is important to conduct a shoot thinning process early in the growing season (E-L 12) to remove water shoots from the stem as well as active shoot doubles from the canes (Smart and Robinson 1991). As the growing season progresses some shoots will produce lateral shoots which are unfruitful and will lead to severe shading of the canopy (Smart and Robinson 1991). These laterals should be removed just after flowering (E-L 26) (Smart and Robinson 1991).

Other forms of canopy management include trimming and leaf removal. Trimming is done soon after flowering and the process removes active growing shoot tips and helps to maintain a non-congested canopy. Leaf removal is ideally done one month before veraison (Smart and Robinson 1991, Tardaguila et al. 2008, Tardaguila et al. 2010). If done too early bunches can be subjected to sunburn and the whole process will have to be repeated again as lateral leaves will grow in the fruit zone (Smart and Robinson 1991) which will lead to excess shading on the fruit and other leaves.   

Mineral nutrition

As a baseline and management check petiole testing should be done during the growing season. Petioles are more responsive to nutrient changes compared to leaf blades and is a more accurate test compared to soil analyses (White 2003). Soil analyses does not give a direct and true interpretation of the vine’s nutrient ability uptake (White 2003). Petiole testing is usually done at 80% cap fall (E-L 25) and the petioles collected are opposite form the most basal bunches on the shoot (White 2003). It is important to understand the nutrient balance in the vineyard before starting a nutrition program (White 2003). Too much of a nutrient can be just as detrimental to the grapevine as too little of a specific nutrient (White 2003).

Nutrient balance can be achieved in a vineyard using a fertilizer program but there are ways to keep nutrient balance in a vineyard using minimal fertilizer additions (Goldammer 2018). During pruning the pruned shoots can be left in the inter-rows and can be mulched back into the soil to return stored nutrients in the shoots back into the soil (Goldammer 2018). During winter, sheep can be used to graze the grass in the vineyard and in return will restore nitrogen back into the soil (Goldammer 2018). Cover crops can be used to aid in nutrient recycling in the vineyard, e.g. legume species (Carlsson and Huss-Danell 2003). Legume species (e.g. clover species and peas) have a symbiotic relationship with nitrogen fixing bacteria and will aid in providing useable nitrogen in the soil for grapevines (Carlsson and Huss-Danell 2003). Compost can also be used to return nutrients to the soil (Goldammer 2018). Some wineries recycle the skins and stalks post-harvest and use this material to produce their own compost on site for use in the vineyard.   

Water availability in the soil

Irrigation lines will be setup in this particular scenario due to the free draining soils and relatively low rainfall in the Marlborough region. Preferably the irrigation line should be installed below the surface (around 400mm deep) to encourage the vine roots to grow downwards in search of the water supplied to them. The subsurface irrigation will also eliminate competition with aggressive weed species. The irrigation schedule for watering the vines should be based on readings done by a pressure chamber and not a soil moisture probe (Scholander et al. 1965). The pressure chamber directly measures water stress of a vines as opposed to a soil moisture probe that gives information about moisture content in a specific part of the soil (Scholander et al. 1965). The pressure chamber measures the water potential of the vine and the higher the water stress the more negative the pressure will be (Scholander et al. 1965). Readings are taken from E-L 17 up until harvest every 2 to 3 days from 11:00 to 14:00 (Chone et al. 2001). The pressure chamber will allow the manager to exactly know the level of stress in the vine and when to irrigate and not to irrigate.

The factors that affect photosynthesis act together in a complex system. The management of the three factors singled out in this assignment will act together in synergy to optimize photosynthesis. The water availability in the soil will be retained better by the cover crops planted for the nutrient recycling in the soil. Cover crops planted for nutrient recycling will improve the soil structure of the free draining soils that are present in Marlborough and could potentially increase the water retention in the soil after a rainfall event (Goldammer 2018). Furthermore by managing the canopy to remain open and uncongested the water use by the vine will be more efficient. The interaction of these factors acting together will optimize the photosynthetic activity of Marlborough Pinot Noir grown on 2 cane VSP.

FIGURES

Figure 1: Illustration of chloroplast and associative photosynthetic reactions. (A) Sunlight and water is captured by the thylakoid membranes in the chloroplast & (B) The energy and associative electrons from the sunlight and water drives the Calvin cycle (Rubisco enzyme-Carbon dioxide fixation) which produces sugar and oxygen (Pearson Education Inc. 2011, Orr et al. 2017). 

Figure 2: Comparison of light quality received by leaves on the outside of a canopy (ambient) compared to shaded leaves on the inside of the canopy. Optimal light for photosynthesis (400 to 700nm) is received at a more optimal level by ambient leaves compared to shaded leaves (Smart and Robinson 1991).  

Figure 3: Effect of nitrogen content (x-axis) on leaf chlorophyll levels (y-axis). Chlorophyll levels increase proportionally with nitrogen levels (Keller 2005).

Figure 4: Reduction in the activity of enzymes involved in carbon dioxide fixation (Calvin cycle) during drought stress. The clear bars represent watered grapevines and the black bars are drought stressed grapevines, P<0.05 indicates significant differences (Maroco et al. 2002).

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