ABSTRACT

Greenhouse and field experiments were carried out with canola cv. Master in order to study its performance under water deficiency. Greenhouse experiments were sown on July 14 and August 26. Plastic pots filled with sandy loam soil were used. The field experiment was carried out at Tres Picos (38°30’ S, 62° 38’ W). The treatments applied were as follows: Irrigated plants (I), were watered daily (greenhouse experiment) or were watered when the plants started to show signs of water deficiency (field experiment), and Non-irrigated plants (NI), where water was withheld until the plants wilted, when they were watered once and a new drought cycle was started again (greenhouse experiments) or were not watered after stage 4.2 (Harper and Berkenkamp, 1975) (field experiment). Relative water content (RWC), leaf stomatal conductance (LSC) and soil moisture content (SMC) were measured periodically. In the greenhouse, the duration of selected leaves was measured during the growing season. In the field, leaf area, leaf dry and specific weight and yield were determined. Mineral nutrient concentration (N, P, K, S, Ca, Mg and Fe) was analized. In the greenhouse there were no significant differences in the RWC between I and NI plants despite the fact that there were reductions of more than 80% in the LSC and of 50% in the SMC. The leaf duration was 128 days for the first sowing date and 92 days the second. In the field, the leaf area per plant and the leaf weight of the I plants were 32% and 9% higher, respectively, than those in NI plants. There were reductions in the K and Fe concentrations of the NI plants compared to the I plants. Yield in the field was 16.8% higher in I plants.

INTRODUCTION.

Even though canola is one of the world’s principal oil crops its diffusion in Argentina has been very limited. Canola is a profitable alternative crop which fits into wheat rotations it matures earlier than wheat and eventually offers a potential for double cropping. In addition to the economic benefit given by the crop, one should also consider the break-crop benefit that increases the grain yield and protein levels of the wheat crop grown the following year (Angus et al, 1991; Heenan, 1995) as well as its contribution to disrupting the life cycles of wheat diseases.

Canola is generally considered to be more susceptible to drought than wheat. However, water deficiency produces different consequences according to the stage of the crop cycle and the magnitude of the deficiency. The yield is mainly affected by water shortages which occur during the stage from flowering to the end of the seed set. Dembinska (1970) demonstrated that water deficiency later in the growing cycle reduced the seed yield by 15%. When the water deficiency extends to 2 weeks, after anthesis, with the soil at 50% of field capacity, seed yields were reduced by 20% (Mingeau, 1974). As the crop approaches the end of its cycle, the need for water decreases and if the water deficiency is relieved before ripening there is some compensation.

Soils in the region near Bahía Blanca have a low water retention capacity and the subsoil restricts root development. Thus, the possibility of water deficiency during the crop cycle is relatively high. Some studies have mentioned significant increases in seed yield in response to an irrigation input (Krogman and Hobbs, 1975; Mendham et al., 1984; Wright et al., 1988), but there are no data describing the effects of irrigation in Argentina.

The yield and biochemical composition of the seeds are subject to large changes depending on growth conditions (Mailer and Cornish, 1987; Sang et al, 1986). These properties are markedly affected by water availability (Mingeau, 1974). The most pronounced effects are observed when the water shortage occurs during the flowering period or pod-filling stages (Dembinski, 1970; Richards and Thurling, 1978). Nutall (1973) found that moisture stress increased the protein content of rape. Good and Zaplachinski (1994) found that drought stress conditions caused a small decrease of protein synthesis in leaves followed by a resumption of synthesis upon rehydration. A water shortage applied at flowering or at early stages of vegetative growth significantly increased seed protein concentration (Bouchereau et al, 1996).

Canola leaves are the major source of photosynthates from their emergence until the middle of the flowering period. Although the leaves may not contribute directly to seed development, they influence the development of the sink capacity. In order to quantify canola leaf developement, not only is the maximum leaf area important, but also the leaf area duration. Even mild water deficient periods result in an inhibition of the leaf expansion, which can occur before changes in leaf water status can be detected.

The canola crop in the subhumid and semiarid regions of Argentina can be exposed to water deficiency which may limit seed yield so our objective was to study the behaviour of canola under rainfed and irrigated conditions, with regard to the effects on growth, water relations, mineral nutrition and leaf area throughout the crop cycle.

MATERIALS AND METHODS

Canola cv. Master seeds were used in all the experiments.

Field experiment: This was carried out in Tres Picos (38° 30’ S, 62° 38’W). Seed was sown on July 15 in a sandy loam soil. Each plot comprised of eight 10m-long rows spaced at 0.3m, with a plant density of 80 plants m^-2. Diseases and weeds were controlled. The plants were well watered until the treatments were imposed. The treatments applied from stage 4.2 (Harper and Berkenkamp, 1975) were: Irrigated plants (I) were watered when the plants started to show signs of water deficiency, and Non-irrigated plants (NI) were not irrigated after the stage 4.2. Leaf area per plant, leaf dry weight and leaf specific weight were determined during crop growth. To measure seed yield 6m² of the central rows was harvested. Each treatment was replicated 4 times.

Greenhouse experiments: The greenhouse experiments were sown on July 14 and August 26 in Bahía Blanca (38° 45’ S, 62° 11’W). The seeds were planted in 12 L plastic containers, filled with a sandy loam soil. Seedlings were thinned to 4 per pot. The plants were watered to field capacity every two days until the treatments were imposed. The treatments applied were : Irrigated plants (I) were watered daily to field soil capacity, and water was withheld from the Non-irrigated plants (NI) until they were wilted when they were watered once to field soil capacity and a new drought cycle restarted for up to nine times (first sowing date) and eight times (second sowing date). For the first sowing date treatments were applied from the 4.2 stage, whilst they were applied from the 3.2 stage for the second sowing date . Each treatment was replicated 8 times. The leaf duration of selected leaves was followed throughout the growing season. Each leaf selected was labeled and its drop was registered once per week.

In both experiments the water status of the plants was evaluated through the leaf relative water content (RWC) and leaf stomatal conductance (LSC). Soil cores were taken periodically in order to quantify the soil moisture content (SMC). LSC was measured at noon, with a diffusive Delta T AP4 porometer. The concentrations of K, Ca, Mg, Fe, P and S were analysed with a Shimadzu 1000-III plasma spectrophotometer (ICP-AES), and the total nitrogen content was determined by semi-micro Kjeldahl.

The experimental lay-out of the experiments was a completely randomized design.

RESULTS

The precipitation during the life cycle of the crop is shown in Figure 1. In general, the relative leaf water content and the leaf diffusive conductance were also higher in the irrigated plants (Table 1). A similar trend was observed in the greenhouse experiments (Table 2, only first sowing data is shown), but the differences between the two treatments were more evident than in the field experiment.

Figure 1 - Daily rainfall at Tres Picos.

Table 1 - Components of the water relationships (soil moisture content (SMC), leaf relative water content (RWC) and leaf diffusive conductance (LDC)) under irrigated (I) and non-irrigated (NI) treatments. Field experiment.

	SMC		RWC		LDC
Date	I	NI	I	NI	I	NI
	%		%		cm s-1
Oct 28	10.0 a*	06.2 b	85.0 a	75.9 b	0.56 a	0.28 b
Nov 04			91.2 a	86.6 a
Nov 17	16.6 a*	20.6 a	93.3 a	83.9 b	0.83 a	0.76 a
Nov 24	16.3 a*	13.0 b	88.6 a	84.8 a	0.37 a	0.48 a
Dec 12	13.6 a*	05.8 b	81.9 a	78.6 a	0.37 a	0.12 b
Nov 19			85.4 a	77.2 b

* Means for each water parameter, within a row, followed by different letters are significantly different (p<0.05).

Table 2 - Components of the water relationships (soil moisture content (SMC), leaf relative water content (RWC) and leaf diffusive conductance (LDC)) under irrigated (I) and non-irrigated (NI) treatments. Greenhouse experiment - 1^st sowing date.

	SMC		RWC		LDC
Date	I	NI	I	NI	I	NI
	%		%		cm s-1
Oct 23	16.6 a*	5.0 b	78.9 a	83.6 a	0.25 a	0.11 a
Oct 29	11.3 a*	2.9 b	94.3 a	81.4 b	0.89 a	0.03 b
Nov 05	16.3 a*	7.2 b	94.6 a	87.1 b	1.71 a	0.06 b
Nov 13	14.4 a*	6.5 b	96.4 a	82.1 b	0.66 a	0.51 a
Nov 19	21.6 a*	5.3 b	95.6 a	91.6 a	0.79 a	0.22 b
Nov 26	13.1 a*	6.3 b	90.2 a	84.9 a	1.16 a	0.09 b
Dec 15	10.2 a*	8.6 a	91.7 a	93.1 a	0.75 a	0.72 a

* Means for each water parameter, within a row, followed by different letters are significantly different (p<0.05).

Leaf area was higher (32%) in the irrigated plants compared to the non-irrigated plants in the field experiment, during the treatment period, and the differences were significant (Table 3). The specific leaf weight was higher in the non-irrigated than in the irrigated plants, though the differences were significant only for the last harvest (Table 3). An increase in specific leaf weight is a commonly observed response to water deficit, tending to increase the water use efficiency by lowering leaf area relative to photosynthetic capacity (Wright et al, 1996).

Table 3 - Leaf area and specific leaf weight. Field experiment.

	Leaf
	Area		Specific weight
Date	I	NI	I	NI
	cm2 plant-1		mg cm-2
Oct 28	1180.8 a*	1172.2 a	4.2 a	14.7 a
Nov 17	1523.1 a*	1225.8 b	5.5 a	16.0 a
Nov 24	1359.5 a*	1092.7 a	6.3 a	19.7 a
Dec 12	1547.2 a*	1288.1 b	6.5 a	10.1 b

* Means for each leaf parameter, within a row, followed by different letters are significantly different (p<0.05).

Plants irrigated and non-irrigated did not significantly differ in leaf area duration (data not shown). However, at the first sowing date leaf area duration was 1.42 times longer (128 days) than that of the plants of the second sowing date. The field experiment showed an increased yield of 308 kg ha^-1 (16.8 %) for the irrigated plants and the seed nitrogen level was higher than in the irrigated plants (Table 4).

Table 4 - Seed yield (t ha^-1) and nitrogen percentage in seeds (%) under irrigated (I) and non-irrigated (NI) treatments. Field experiment.

	I	NI
Seed yield (t ha-1)	2134 a*	1827 b
Seed N (%)	*2.67 a*	*2.35 b

* Means within a row followed by different letters are significantly different (p<0.05).

There was an increase in the K and Fe concentrations of the stems and fruits of the irrigated plants to the non-irrigated (Table 5). The differences for all the other mineral nutrients analyzed were not significant.

Table 5 - Nutrient concentration (%) in different plant organs under irrigated (I) and non-irrigated (NI) treatments. Field experiment.

		P	K	S	Ca	Mg	Fe
Leaf	I	0.28 a*	3.24 a	1.94 a	3.41 a	1.39 a	0.071 a
	NI	0.25 a*	2.88 a	1.70 a	3.26 a	1.62 a	0.073 a
Stem	I	0.07 a*	1.93 a	0.35 a	0.49 a	0.23 a	0.030 a
	NI	0.07 a*	0.42 b	0.31 a	0.49 a	0.28 a	0.013 b
Siliqua	I	0.43 a*	1.25 a	0.73 a	1.32 a	0.62 a	0.054 a
	NI	0.45 a*	0.84 b	0.81 a	1.09 a	0.71 a	0.029 b

* Means for each organ, within a column, followed by different letters are significantly different (p<0.05).

Water deficiency decreased the growth of the plant and seed yield. The differences in the growth parameters of the irrigated plants were higher than the differences in the relative leaf water content. When the plants were irrigated, some mineral nutrient concentrations increased (K, Fe, N).

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