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
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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
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Site
Weslaco
Orlando
Indio, CA
Degania Is.
Lmeira, Bz
Colombia
Latitude Altitude
26o05’ N 40 m
28o40’ N 30 m
33o40’N -10 m
32o40’N -200 m
22o30’S 700 m
4o20’N 400 m
HU
3900
3700
3900
3600
3000
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 mmol 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
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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 mmol 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
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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