THE INFLUENCE OF TEMPERATURE AND LIGHT ON THE

VEGETATIVE DEVELOPMENT OF CITRUS TREES

By K. Mendel The 'I olcani Institu te of Agricultural Research (N.U .LA,.),

Bet Dagan, Israel. 1968 Series, No 1315 - E.

If the influences of temperature and light on the vegetative developmt;)nt of Citrus trees are to be assessed, they have to be considered under conditions of soil and nutrition which do not hamper this development. Any comparison which does not take this into account is bound to render erroneous results. With these premises in mind, it can he stated that In all climatiC zones, whether subtropical or tropical, where citrus trees are grown, they ultimately reach more or less the same size. The exceptions are areas on the fringe of the citrus belt between 40° Nand 30° S, and at several high elevations in tropical areas. Numerous investigations have been carried out on the influences of climatic factors on fruit development, but much less is known about such influence on the trees during their development to ultimate size, it has generally been found that the rate of vegetative development is more accelerated in warmer climates than in cooler ones. TEMPERATURE The main temperature ranges for growth are: minimum, 12.S-l3°C; optimum, 23-34°C; and maximum (limiting growth), 37-39°C (1,8, 18). Heat units are calculated from temperatures above o 12.S C (55°F) for a given area, and Bain - on the strength of his comparison between California and Trinidad ­ postulated a close correlation between the sum of heat units and the growth rate of the trees in a given area (1). Table 1 shows the sum of heat units for various areas. Citrus trees are found in all areas listed in the table, but for comparison purposes the behavior of the trees in tropical regions will be of special interest. In addition to the sum of heat units, the length of the growing season also has to be taken into account. Table 1 shows that other than several regions in California and the Mediterranean area, the various localities !Jsted have no o months with an average temperature below 12.S C. How­ ever, if we note the number of months with an average less o than 17.S C (up to 5°C over the minimum), the pattern is qUite different. Especially at the higher elevations in tropical regions we arrive at an appreciable number of months with presumably reduced growth activity. Only the average July temperatures at Indio, California, ,'-- reach the maximum temperature range. Whereas the daily

maxima in many places reCl,ch these temperatures dUrIng th~ hottest months of the year, especially in the northern subtropical regions,::he hours of restricted giowth ar~ limited, LIGHT

It has long been recognized that light ha~ an aHreciabl~ influence on the rate of growth. Light intensity is a grmvth retarding factor, with tht;; growth rate decreasing with increasing light intensity (12), ~herefore, in areas with "bright" S;Jmmers, like California and the Mediterranean basin, the high light intensities tend to restrict tree growth. On the other hand, citrus trees rflact strong"y to increased day length. Under conditions of illumination for 16 hours, citrus trees produced more and longer shoots than with short-day (8 hours) illumination (4,14,17,19). The quality of the light received by the plants also has an influence on the growth rate. It has been shown (14) that in the red range of the spectirum, "far-red" light promotes the elongation of .internodes of lemon shoots whereas "fed" light has the opposite effect. A recent observation by Gael! * showed that fluorescent light - emitting largely far-red light - (14-16-hour illumination) results in an almost uninterrupted growth activity in PalestIne Sweet Lime seedlings. The effects ofU.V. radiation on growth are well known, as is the fact that at higher elevations (above l200m), it has an overall influence on plant growth. GROWTH FLUSHES Citrus shoots grow in cycles, with the numbers of growth flushes during the growing season varying from two to five (l, 8, 9, 15, 18), according to area. It is generally stated that in tropical regions no clearly defined growth flushes occur, and the trees remain more or less in a state of continuous growth, unless interrupted by drought. Drought results in the same reactions of the citrus tre~s as does cold: cessation of growth and inductIOn to flower, ing. The well known production of summer lemons ('1er delli) in Italy is based on this phenomenon. In tropical * A. Goell, personal communication (1967).

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Data on Temperature ana Annual Heat Units {above 12.5°C} for Various Citrus Growing Areas * Area and Location

Elevation (m)

Latitude

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Annual No of months with average temp.

heat Units less than less than °c 17.5°C 12.5°C

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a: Sub t r Spain (\hlencial Cal ifo;"nia (Riverside) California (Indic) israel (Deganial :srae! (Rehovot) !=!orida (Orlando) Texas (Weslaco) Brail {Umeind

0

10°40' N 10°30' N 4°20' N 3°30' N 4° 40' N

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pic a I Regions

39°30' N 30 34° 0' N 260 33°40' N -10 32:40' N -200 31 50' N 50 28°40' N 30 26°05' N 40 22°30' S 700 b; T r

ii"inidad (:'iaico A'rp.) Coiombia {Aracataca~ Colombia (Girardot) Colombia (Palmira) Colombia (LaFloridai ECuador (Sta. RossI Ecuador (Conocoto) Kenya (Mombassal Kenya (Nairobi! Uganda (Jinja) Ceylon (Mannar) Ceylon (Nuwara Elyia)

0

1600 1700 3900 3600 2600 3700 3900 3000

3 3 1 0 1 0 0 0

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6 6 4 4 4 2 2 1

pic a I Regions

10 30 400 1000 1800 3°30' S 10 0°15' S 2200 4° 0' S 20 1°20' S 1600 0°30' N 1100 9° 0' N 30 7° 0' N 1900

5000 5600 5700 3500 1700 4400 1000 5200 2500 3330 5700 1000

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regions, drought plays the same role as cold does in the subtropical regions, leading to renewal of growth and flowering with the onset of the rainy period (1). In an irrigation experiment in the arid Negev area of Israel, the long-interval treatments resulted in new growth each time irrigation was resumed, and these flushes were quite differ­ ent from the normal cycle (3). It is not clear which factors determine the number of growth flushes. It has been shown that there is no chilling requirement for the sprouting of citrus buds (16). Sprouting takes place at any time when soil temperatures fise above 12°C. In the coastal plairt of Israel, sprouting begins, irrespective of air temperature, between February 4 and 15, at which time soil temperatures are about 12°_13°C. The duration of the flushes varies, as do the intervals between them. Whereas Cooper et a1. (6) stated that the spring flush was of the longest, duration, Krishnamurti and co-workers (l0) noted that the main summer flush was the longest. It has also been shown that, under long-day condition,s (16-hours illumination), the period between flushes is shorter than under short-day (8 hours illumination) condi­

tions (14). Bertossi (2) reported the influence of the duration of illumination on a tissue culture of lemon stems. During a 12-hour illumination the same rhythm of flushe~, (five) appeared as in intact plants; after changing to continuous lighting, the growth of the tissue became an almost uninterrupted phase of proliferation. From the results of research work in our Department (5, 7, 11), it became apparent that the growth flushes are governed by a promotion - inhibition balance. The rate of accumulation of inhibiting substances ("Total Phenolic Compounds") determines the cessation of growth and, with it, the termination of a flush. During the period between flushes, the amount of the inhibiting substances decreases. In this mechanism, many details still have to be elucidated. As noted above, two to five flushes in citrus occur in subtropical regions, with 1~)Tlgest interval between the first and second flush, whereas in the tropics, growth continues almost uninterruptedly, Temperature seems to be a decisive factor. From Table 2 it is evident that temperatures between the first and second flush in the subtropical regions are markedly lower than in the tropics. We may therefore suggest that high

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