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Environmental Constraints
Text Chapter 3
Subtropical Regions
• Most commercial citrus production
–23.5 - 40o NS Latitudes
• Minimum temperatures > -7oC (19.6oF
• Examples
–Rio Grande Valley ~ 26o NL
–Orlando, Florida ~ 28o NL
– Indio, California ~ 33o NL
Tropical - Subtropical Comparison
• Tropical between 23.5o NL SL
• Subtropical 100 tonnes ha-1
• Tropical 15 tonnes ha-1
–Minimum 0oC (32oF)
• Small fluctuations in DL 10o NL SL
• Small fluctuations in temp at low to mid elevations (-3oF/1000 Ft rule)
Lowland Tropical Regions
• Altitude 0 - 500 m
• Highest average temperatures
• Highest heat units
• Distinct wet-dry cycles
–Wet season onset of flowering
High Elevation Tropical Regions
• Fog reduces light intensity (Day)
–Lowers temp & CO2 assimilation
• Fog (Night) decreases radiation
– Increases temp
• Ultraviolet radiation increased
–Due to reduced particulate matter
–Leaf distortion & reduced growth
Limiting Factors
• Flower initiation av. < 24oC
• Vegetative growth > 12.5oC
• Heat units > 5000
– increased respiration
– < fruit solids & acids
Heat Unit Calculation
12.5C + 24.5C = 18.5
2
18.5 - 12.5 = 6
6 x 30 days =180 heat units / month
180 x 12 = 2160 heat units / year
Examples
Site Latitude Altitude HU• Weslaco 26o05’ N 40 m 3900 • Orlando 28o40’ N 30 m 3700• Indio, CA33o40’N -10 m 3900• Degania Is. 32o40’N -200 m 3600• Lmeira, Bz 22o30’S 700 m 3000• Colombia 4o20’N 400 m 5700
Diurnal Fluctuations
• Greater in subtropical regions
• Mean annual temps from 15 to 18oC
• But many exposed to -0oC
–Minimum of -10oC (14oF) Florida
• Freeze damage in US, Europe, Mexico, Australia, central China.
• Rarely in Brazil
Far East
• Minimal damage
–Large bodies of water
–Satsuma mandarin / Trifoliate orange
–Extended periods of cool temps
•Maximum freeze hardiness
•Not true in Texas and Florida
The Juvenile Plant
• Thorny (modified leaves)
• Upright unbranched growth habit
• Apical meristem abscises
– lateral buds break
–new shoots from lateral buds
• Process repeated Zig-zag growth (sympodial) Determinate habit
Duration of Juvenility
• Inversely related to
– tree vigor
–heat unit accumulation
• Example
–Lemons and limes < 2 years
–Others - 5 to 13 years
• Air layers ‘Tahiti’ lime - fruit in 1 year
Hardiness
• Seedling citrus > hardy than same cultivar budded to a root stock.
Influence of Climate
• Lowland tropical areas with high RF
–Shorter juvenile period
• Arid subtropical area
–Longer juvenile period
• Calamondin & ‘Key’ lime - 18 months
–30oC days, 25oC nights, 16 h DL
•No flushes (continuous growth)
The Budded Plant (Budling)
• Adult - so earlier production.
• Budling -
– Riverside, CA 30 mo. Market sz
–Mannar, Sri Lanka 15 mo.
• Difference?
–Riverside 1700 heat units
–Mannar 5700 heat units
Net CO2 Assimilation
• Linear photosynthetic photon flux (PPF) up to 700 mol m-2 s-1 (Fig. 3.3)
• PPF of 2000 m-2 s-1 represents full sunlight
• Thus max CO2 assimilation at 30% full sunlight
• Max growth when DL 12 h
Influence of High Radiation
• High light intensity may decrease CO2 assimilation
–High light increase leaf temp 10oC
–Optimum range 28 - 30oC
• > 35oC limits RuBisCo inc VPD Mid day stomatal closure poor
growth
Shoot and Leaf Growth
• Shoot growth - 2 to 5 distinct flushes
• Tropical - may be continuous
• Growth begins at >12.5oC (55oF)
• Spring flush
–March - April NL
–September - October SL
• Mean temps 12 - 20oC in this period
Other Flushes
• Summer flushes
–June - July NL
–January - February SL
• Fewer growing points
–Longer internodes
• Tropical - continuous
• Same annual growth - distribution?
Leaf Area
• 3 year old tree - 16,000 leaves (34 m2)
• 29 yr tree - 173,000 (203 m2)
• Interior PPF down to 50 mol m-2 s-1
–What should be done?
Leaf Function
• Leaves C importers ~ 6 wks post bloom
• Stomata on abaxial surface take time to develop - poor transpiration contr.
• CO2 assimilation stabilizes in ~ 7 mo
• Healthy citrus leaves may for 3 yrs.
Root Growth
• Temp regulates root growth, water, and nutrient uptake
• Root growth begins at temp > 7oC
–May - June and August - Sept.
• Increases from 17 to 30oC
• Water & nutrient uptake inc 10 - 30oC
• Winter chlorosis
Water and Root Growth
• Root growth decreases significantly
–When soil moisture < 45% of field capacity.
• Excessive water for only a few days due to H2S produced by soil-bore bacteria caused by lack of oxygen.
Flower Induction and Differentiation
• Flower bud differentiation occurs during winter rest in subtropics
• Or during dry season in tropics
• Temperatures < 25oC rest
• Drought > 30 days induce flower bud
• Italy - water withheld from lemons - irrigated - blooms - control harvest
Anthesis
• Events leading to anthesis (Fig. 3.5)
• Min. threshold 9.4oC (48oF)
• Low to moderate temps during bloom (<20oC) protracted bloom
• Higher temps (25-30oC) shorter bloom
Factors Associated with Flowering
• Carbohydrates
–Girdling
–Alternate bearing mandarins
• Hormones - GA3 influence not clear
• Nutrition -
Carbohydrate / Nitrogen Ratio
• I II III IV
• C/NNNN CC/NNN CCC/NN CCCC/N
• No fruit No Yes No
• Age • Pruning
• Irrigation • Nitrogen fertilizer
Pollination and Fruit Set
• Bee activity minimal temp < 12.5oC
• Pollen germination best at 25 - 30oC
• Pollen tube growth through stylar canals - 2 days to 4 wks
–Depending on cultivar & temp.
Initial Fruit Set
• 100,000 - 200,000 flowers / tree
–Only 1 - 2 % produce fruit
• 1st drop - 3 - 4 weeks post bloom
–Defective flowers or no pollination
• High temps (>40oC) (104oF) may cause fruit drop in Navel Oranges.
Physiological Drop
• 2nd drop - May - June NL
– ( Nov-Dec SL)
–Fruitlets 0.5 - 2.0 cm
• Caused - competition for metabolites
–High temps. H2O stress stomatal closure assim CO2
–Neg. C balance fruit abscission
Longevity
• Freezes primary cause of short life.
• Aside from freezes:
–Humid subtrop - fruit in 15-20 yrs
–Med. Climates - peak 20-25 yrs
• Humid subtrop - life 100 years
• Mediterranean - life 200 years
Fruit Yields
• FL av grapefruit 41 max 120 Mg ha-1
–Oranges 31 Mg max 100 Mg ha-1
• Brazil and China 1/2 - lack irrigation
• Semiarid or arid - < hu < tree size < yield.
• % marketable fruit > in semiarid area
• < pests, < blemishes & > peel color
Fruit Growth
• Sigmoid pattern (Fig. 3.7)
• Phase I - Cell div. (all cells formed)
–1 - 1.5 mo - determines fruit size
• Phase II - Cell differentiation
• Phase III - Cell enlargement (6 mo)
Maturation
• Phase IV - Peel color from green to yellow or orange
–Slight increase in TSS
–Rapid decrease in TA
• Time 6 - 16 months for ‘Valencia’
– Influenced by water and temp.
External Quality
• Peel color (air & soil temps < 15oC)
–Chloroplasts Chromoplasts
•Carotenoids (grapefruit)
• Lycopenes (red grapefruit)
•Anthocyanin (blood oranges)
• Low vigor more color
• Excessive N poor color
Fruit Shape
• High temperatures sheepnosed fruit
–Excessive cell division in albedo near stem end.
Fruit Size
• Citrus 85 - 90% water by weight
–Fruit size function of water
–Leaves on detached fruitless twig wilt in few hrs
–Leaves on detached fruited twig remain turgid for many hours
• WHY?
• Water translocated to leaves is stored in fruit peel
• Irrigation scheduling
Internal Quality
• Carbohydrates 75 - 80% of TSS
–Regulates internal quality
• TSS decrease > rapid - low tropical than in sub-tropical areas (Fig. 3.8)
• TA - same relation (Fig. 3.9)
–Function of heat units
•Organic acids in Citric acid cycle
TSS:TA Ratio
• Determines edible quality
• ‘Valencia’ reach 9:1 in 7 mo in tropics
–Coastal subtropics - up to 16 mo
– (Fig. 3.10)
THE END